Lake Tarpon Water Quality Management Plan. Final. May Submitted to: Prepared by:

Transcription

1 Lake Tarpon Water Quality Management Plan Final May 2017 Submitted to: Prepared by:

2 This project is a cooperative funding project between Pinellas County and the Southwest Florida Water Management District to develop a Water Quality Management Plan for Lake Tarpon. Lake Tarpon is a District Surface Water Improvement and Management (SWIM) Priority Waterbody. Inclusion of proposed projects, corrective actions, best management practices and programs does not indicate that the projects are permittable, or would be eligible for funding through the District's cooperative funding program. Proposed projects are subject to regulatory review and permitting as appropriate. Requests for funding assistance must meet the requirements of funding programs and will be subject to the District's Governing Board approval of cooperative funding requests.

3 Table of contents Chapter List of Acronyms Executive Summary Pages 1. Introduction Project Background History 5 2. Data Inventory Surface water data Groundwater data Aquatic Vegetation Internal Cycling Processes Modeling Component Hydrologic/Hydraulic (H & H) Model Pollutant Loading Model Components Historical and recent conditions in the lake and watershed Lake and watershed description, including alterations to hydrology Regulatory implications Previous Pollutant Load Estimates External Pollutant Loads Identify relationships that may affect lake conditions Ambient water quality Identification of nutrient of concern External nutrient load relevance to water quality Influence of lake elevation and residence time on water quality Submerged Aquatic Vegetation Proposed actions to protect and/or improve lake condition Approaches to managing stormwater and groundwater Approaches to managing lake levels Approaches to managing submerged aquatic vegetation (SAV) General overview of project types recommended Project Descriptions Stormwater Infiltration Areas / Raingardens Groundwater characterization and protection Managing lake levels Managing aquatic vegetation Development of site-specific water quality criteria for Lake Tarpon References Cited 82 iv v Atkins Lake Tarpon Water Quality Management Plan Final May 2017 i

4 Appendices 86 Appendix A. Lake Tarpon Watershed and Water Quality Evaluation Report Appendix B. Surface Water Resource Assessment Appendix C. Modeling Plan Appendix D. Monthly Pollutant Loads and Water Quality Results Appendix E. Non-parametric analysis summary tables Appendix F. Monthly Water Quality Data Analysis Memo Appendix G. SAV Assessment Appendix H. Lake Tarpon Modeling Scenarios Tables Table 1. Surface Water EMC Values Table 2. Groundwater Typical Values Table 3. Impairment designations within Lake Tarpon and Brooker Creek WBIDs Table 4. Input and output components of the Lake Tarpon water and nutrient budget (from 1998 Study) Table 5. Brooker Creek Annual Average Load Comparison ( ) Table 6. Annual Average Nutrient Load Comparison (2006 and 2016 Studies) Table 7. Annual Average TN Loads by Source, Compared to TN Loads Leaving Lake Tarpon (2016 Study) Table 8. Annual Average TP Loads by Source, Compared to TP Loads Leaving Lake Tarpon Table Study Lake Tarpon Input/Output Components for the Water and Nutrient Budgets Table 10. Results of NNC evaluation for Lake Tarpon (WBID 1486A), Table 11. Spearman Correlation Analysis of Rainfall or Total Nitrogen Annual Loads with Lake Tarpon Ambient Total Nitrogen or Chlorophyll-a concentrations Table 12. Spearman Correlation Analysis of Rainfall or Total Phosphorus Annual Loads with Lake Tarpon Ambient Total Phosphorus or Chlorophyll-a concentrations Table 13. Summary statistics generated from Lake Tarpon light attenuation coefficient Figures Figure 1. Lake Tarpon and contributing watershed Figure 2. Lake Tarpon outfall canal structure Figure 3. Photograph of Lake Tarpon and disconnected sinkholes along western shoreline Figure 4. Mean chlorophyll-a concentrations, mean annual TN concentration, and cumulative annual rainfall for (Figure 2-2 from Atkins 1998) Figure 5. Lake Tarpon TSI values for (from Pinellas County, 2006) Figure 6. The Lake Tarpon watershed prior watershed modeling efforts Figure 7. Plot of mean annual chlorophyll-a concentrations vs. cumulative total area of hydrilla Figure 8. Acres of hydrilla treated in Lake Tarpon by year from as reported by SWFWMD and FFWCC PMARS Figure 9. Fish biomass data from shoreline blocknets in Lake Tarpon, Figure 10. Fish community balance in Lake Tarpon, (from Atkins, 1998) Figure 11. Lake Tarpon, Brooker Creek and South Creek watershed boundaries Figure 12. Influence of reduced external phosphorus reductions on phosphorus in water column (after Reddy et al. 1999) Figure 13. Brooker Creek Water Quality Sampling Locations Figure 14. Brooker Creek Total Nitrogen and Total Phosphorus Concentrations Figure 15. Lake Tarpon bathymetry Figure 16. Water Body Identification (WBID) Numbers Atkins Lake Tarpon Water Quality Management Plan Final May 2017 ii

5 Figure 17. Plots of mean annual chlorophyll-a concentrations vs mean annual TN concentrations Figure 18. Annual Total Nitrogen Load to Lake Tarpon (2016 Study) Figure 19. Annual Total Phosphorus Load to Lake Tarpon (2016 Study) Figure 20. Percent Contribution of Total Nitrogen to Lake Tarpon by Source During 1997 to 2004 (2016 Study compared to 2006 Study) Figure 21. Percent Contribution of Total Phosphorus to Lake Tarpon by Source During 1997 to 2002 (2016 Study compared to 2006 Study) Figure 22. Lake Tarpon annual geometric chlorophyll-a mean compared to FDEP chlorophyll-a Figure 23. Lake Tarpon annual geometric TN and TP mean compared to FDEP NNC for colored Figure 24. Total Nitrogen:Total Phosphorus ratio over time in Lake Tarpon Figure 25. Correlation between annual geometric TN and Chlorophyll-a in Lake Tarpon ambient water quality over the period of (p= ). Graphic provides a visual representation of the degree of correlation Figure 26. Correlation between annual geometric TP and Chlorophyll-a in Lake Tarpon ambient water quality over the period of (p=>0.05). Graphic provides a visual representation of the degree of correlation Figure 27. Trend in acres of land classified as Orange Bearing in Florida and as Orchards in the County (data from FDACS) Figure 28. Historical aerial photography from 1951 with scattered characteristic features of citrus related agriculture adjacent to the west side of Lake Tarpon Figure 29. Significant inverse correlation between annual average water elevation and chlorophyll-a concentrations (p=0.044) Figure 30. Significant direct correlation between the annual coefficient of variation in water elevation and chlorophyll-a concentrations (p=0.004) Figure 31. Annual Average Chlorophyll-a Concentrations VS Acreage of hydrilla Treated in the Same Year (1994 to 2002) Figure 32. Period of Record data depicting hydrilla treatment and coverage associated with corresponding annual total nitrogen, total phosphorus, chlorophyll-a, rainfall and water elevation Figure 33. Incremental subsurface irradiance based on water depth under multiple light attenuation scenarios for Lake Tarpon. Portion of lake (acres) represented by each one-foot water depth increment Figure 34. Incremental subsurface irradiance using median light attenuation coefficient. The existing Lake Tarpon SAV deep edge is referenced (USF 2015) Figure 35. Lake Tarpon Water Elevation (ft NGVD 29) at the time of the SAV and bathymetry mapping effort performed by USF (2015) Figure 36. Spatial distribution of Type A soils in the Lake Tarpon watershed Figure 37. Photos of two constructed SIAs / raingarden projects in City of Winter Haven Figure 38. Photo of constructed SIA / raingarden project in City of Winter Haven incorporating existing inlet to storm sewer system (at slightly raised elevation) Figure 39. Land use type within areas with Type A soils in the Lake Tarpon watershed Figure 40. Photos of constructed raingarden project in City of Winter Haven constructed as a community amenity, along with associated educational signage Atkins Lake Tarpon Water Quality Management Plan Final May 2017 iii

6 List of Acronyms Term Acronym Term Acronym Average AVG Pollutant Load Reduction Goal PLRG Best Management Practices BMP Quality Assurance/ Quality Control QA/QC Biological Oxygen Demand BOD Submerged Aquatic Vegetation SAV Pinellas County County Stormwater Infiltration Area SIA Clean Water Act CWA Site Specific Alternative Criteria SSAC Lake Tarpon Drainage Basin DBMP Surface Water Ambient Water SWARM Management Monitoring Program Dissolved Oxygen DO Southwest Florida Water SWFWMD Management District Emergent Aquatic Vegetation EAV Surface Water Improvement SWIM Management Plan Event Mean Concentration EMC Storm Water Management Model SWMM Environmental Protection Agency EPA Technical Advisory Committee TAC Environmental Resource ERP Tampa Bay Estuary Program TBEP Florida Department of Environmental FDEP Total Maximum Daily Load TMDL Protection Florida Department of Transportation FDOT Total Nitrogen TN Florida Fish and Wildlife Conservation FFWCC Total Phosphorus TP Commission Hydrologic and Hydraulic H&H Trophic State Index TSI Interconnect Channel and Pond ICPRv4 Total Suspended Solids TSS Routing Model Version 4 Impaired Waters Rule IWR U. S. Army Corps of Engineers USACE Municipal Separate Storm Sewer MS4 University of South Florida USF System North American Vertical Datum NAVD United States Geologic Survey USGS National Geodetic Vertical Datum NGVD Water Quality Assessment WASP Program Numeric Nutrient Criteria NNC Waterbody Identification WBID Nitrate NO3 Watershed Management Area WMA Nutrient of Concern NOC Water Quality Management Plan WQMP National Pollutant Discharge NPDES Elimination System National Water Information System NWIS Percent Area Covered PAC Platinum Cobalt Units PCU Atkins Lake Tarpon Water Quality Management Plan Final May 2017 iv

7 Executive Summary Background In 1998, the Lake Tarpon Drainage Basin Management Plan (DBMP) made several conclusions about the factors influencing water quality in Lake Tarpon. Three years later, the findings in the DBMP were incorporated into the Surface Water Improvement and Management (SWIM) Plan for Lake Tarpon (SWFWMD 2001). The DBMP identified water level management as one of the most important components affecting both water quality and the health of the emergent aquatic vegetation communities in Lake Tarpon. The DBMP hypothesized that declines in chlorophyll-a concentrations had occurred in response to the accidental release of water from the lake in the spring of 1990, which lowered the lake levels by approximately one foot, discharging algae-laden waters to Old Tampa Bay. Within a year, however, chlorophyll-a concentrations had returned to levels seen prior to the accidental drawdown of the spring of A subsequent decrease in chlorophyll-a concentrations in 1992 was thought to be related to a dramatic expansion in the coverage of hydrilla during this time period. The hydrilla expansion occurred during a period of time in which funding constraints had precluded maintenance of nuisance submerged aquatic vegetation (SAV). As those funding constraints lessened, a widespread (ca. 500 acres) chemical treatment of nuisance SAV occurred, which seemed to have resulted in a large and multi-year increase in the concentration of chlorophyll-a, starting in Project Description and Purpose With this history as a background, this Water Quality Management Plan (WQMP) was developed. The main project components include the following: A review and synopsis of existing information and reports, An inventory of available water quality, hydrologic and land use data for the lake and contributing watershed, The development of a linked groundwater and surface water model to quantify hydrologic and nutrient budgets for Lake Tarpon, and The development of an empirically derived statistical water quality model for the lake, to examine the influences on water quality of external nutrient loads, lake elevations, fluctuations in lake elevations, residence time within the lake, SAV abundance, and SAV management events, as well as other factors, including rainfall, water temperature and the potential influence of wetlands and their associated tannins. Findings General findings of the empirical water quality model include: As found in earlier studies, the abundance of phytoplankton in the lake correlates with the availability of nitrogen, rather than phosphorus. Existing water quality standards for chlorophyll-a (a measure of phytoplankton abundance) are not met in Lake Tarpon, as outlined in the State of Florida s Numeric Nutrient Concentration (NNC) criteria (FAC ). Atkins Lake Tarpon Water Quality Management Plan Final May 2017 v

8 Water quality standards for the nutrients Total Nitrogen (TN) and Total Phosphorus (TP) are in compliance in Lake Tarpon, as outlined in NNC criteria. The finding of exceedance of NNC criteria for chlorophyll-a, but not for TN or TP, can be interpreted in two ways: 1) the chlorophyll-a criteria might be overly restrictive and not locally appropriate, or 2) the nutrient criteria for TN and TP are not restrictive enough to protect what is a reasonable chlorophyll-a criteria. Over the past 20 years, water quality has been variable from year to year, but there are no long-term trends in the concentrations of either TN or chlorophyll-a, suggesting that the water quality in the lake is neither improving nor degrading. Year to year variation in external nutrient loads (whether from gaged portions of Brooker Creek, ungaged portions of the watershed, groundwater inflow or atmospheric deposition) are inversely correlated with water quality, such that wet years (with higher nutrient loads) are associated with better, not worse, water quality in the lake. Chlorophyll-a concentrations are inversely correlated with lake levels, with highest concentrations at low lake elevations and lowest concentrations at high lake elevations. Chlorophyll-a concentrations are positively correlated with annual variation in lake elevations, with highest concentrations in years with greater variation in lake levels, and lowest concentrations in years with reduced variation in lake elevations. Chlorophyll-a concentrations are also positively correlated with residence time on an annual basis, such that years with longer residence times (i.e., older water ) are typically years with worse water quality. Combined, the findings related to lake elevation and residence times suggest that years with higher rainfall, and thus shorter residence times and higher (on average) lake levels, are also years with lower than average values for chlorophyll-a. Chlorophyll-a concentrations appear to be influenced by the abundance of SAV in two ways: 1) during years when SAV abundance is high, chlorophyll-a concentrations are typically lower, and 2) during years when large-scale chemical treatment of nuisance SAV have occurred, and in immediately following years, chlorophyll-a concentrations are typically higher. Based on a 2015 SAV mapping effort conducted by the Florida Fish and Wildlife Conservation Commission (FFWCC): o SAV in Lake Tarpon is currently restricted to depths less than 6 to 7 feet of water, with SAV covering approximately 21 percent of the lake bottom, and o Nuisance SAV species comprise less than 5 percent of the total SAV acreage in the lake. Currently, the population and size structure of the largemouth bass (Micropterus salmoides) population in Lake Tarpon is characterized as excellent by the FFWCC, and Lake Tarpon is classified as one of Florida s top-ten bass-fishing lakes ( Atkins Lake Tarpon Water Quality Management Plan Final May 2017 vi

9 Summary and Recommendations Based on the above findings, Lake Tarpon can be described as a waterbody with excellent sport fishing and a healthy SAV population. Lake Tarpon is also a waterbody with no evidence of deteriorating water quality, at least over the past 20 years. However, water quality criteria in Lake Tarpon are both met (for nutrients) and not met (for chlorophyll-a) which can be interpreted as indicating that either the standard for chlorophyll-a is overly restrictive, or that nutrient criteria are not restrictive enough. Even if the water quality is determined to be both appropriate and non-degrading, management actions should be enacted to ensure that future population growth in the watershed does not result in an apparently healthy lake becoming impaired as a result of continued urbanization. Regarding the hydrology of the lake, and it s influence on water quality, Lake Tarpon should be maintained at water elevations similar to what it has experienced over the past five to ten years, and that frequent variations to lake levels, while sometimes necessary for flood protection, are no longer recommended as a restorative practice related to water quality. Lowered lake levels allow for increased light penetration to portions of the lake that currently have too little light for establishment of SAV and while responses may vary, the first recruits into such areas could be nuisance species such as hydrilla. Establishment of hydrilla could lead to substantial expansions in coverage, even if lake levels were to be raised afterwards, and expanded coverage of nuisance SAV is often accompanied by pressure to reduce coverage via chemical treatment. In the past, chemical treatments of nuisance SAV have led to subsequent phytoplankton blooms. Model results show that the current lake level management practices do not compromise flood protection in the watershed. Despite the findings of adequate water quality to allow for an extensive and mostly native SAV community, and the documentation of a healthy recreational fishery in the lake, the water quality of Lake Tarpon would be classified as impaired for chlorophyll-a but not for nitrogen or phosphorus. Consequently, it is recommended that the County perform a paleolimnological study to better assess the validity of the existing chlorophyll-a standard for Lake Tarpon. In addition to the need for assessments of the long-term trends in nutrients and/or chlorophyll-a through the use of paleolimnological techniques, additional studies are appropriate. While the largemouth bass community of Lake Tarpon has been characterized by FFWCC as excellent, not as much information is available for some of the less abundant fish species. Therefore, it is recommended that a fish community structure study be conducted for Lake Tarpon. In addition, it is recommended that not only the SAV community, but the abundance and diversity of emergent aquatic vegetation (EAV) should be monitored on a regular basis to determine if ongoing and/or potential future management programs focused on the restoration of native EAV communities along the shoreline should be modified. As is the case for Tampa Bay, the management of nutrient loads to Lake Tarpon should be considered to be a hold the line situation, to ensure that continued development in the watershed does not cause a future imbalance to water quality. Lake Tarpon has been designated a nutrient-sensitive watershed, thus ensuring that stormwater runoff from future development and redevelopment activities achieve Outstanding Florida Waters (OFW) treatment standards. Additionally, the continuation and potential expansion of Atkins Lake Tarpon Water Quality Management Plan Final May 2017 vii

10 preventative stormwater management strategies (e.g., street sweeping and regularlyscheduled cleaning out of conveyance and treatment infrastructure) are cost-effective techniques to minimize the potential for future water quality problems in the lake. Such projects and programs could be pursued at much lower costs than the construction of regional stormwater treatment systems. Lastly, projects enhancing infiltration of stormwater runoff into the surficial aquifer have been shown to be a useful stormwater treatment and lake management strategy for the City of Winter Haven, as such projects are associated with higher nutrient removal benefits (compared to typical wet detention ponds) and they also produce at least some amount of recharge of the surficial aquifer, which could help maintain higher water levels in the lake during the dry season. The soil types on the western shoreline of Lake Tarpon are ideal for the use of stormwater infiltration areas (SIAs; aka raingardens ) and such systems should be evaluated as a stormwater treatment option in areas with appropriate soil types where impervious areas have resulted in direct stormwater runoff to the lake. Atkins Lake Tarpon Water Quality Management Plan Final May 2017 viii

11 1. Introduction Lake Tarpon is the largest lake in Pinellas County (County) with a surface area of over four square miles (Figure 1). The lake s historically excellent water quality and healthy fish and wildlife populations made it an important recreational resource for many years. Figure 1. Lake Tarpon and contributing watershed. Atkins Lake Tarpon Water Quality Management Plan Final May

12 Lake Tarpon and its watershed have been substantially altered from natural conditions. The lake historically had no outfall other than a tidally influenced sinkhole on the northwest shore of the lake. The saltwater tidal influence resulted in wide fluctuations of the lake level and occasional pulses of saline water and estuarine organisms. In response to several flood events, the U.S. Army Corps of Engineers (USACE) constructed an outfall canal and control structure at the south end of the lake in 1967 (Figure 2). The outfall canal crossed a natural basin divide, discharging water to Old Tampa Bay in the vicinity of Safety Harbor. Two years later, the Southwest Florida Water Management District (SWFWMD) built an earthen dike around the sinkhole leaving the outfall canal as the only point of discharge, and eliminating the tidal and salt-water influence (Figure 3). Concurrent with these structural modifications was urban development of the lake s shoreline and watershed. Construction of the outfall canal and the sinkhole dike has largely reduced the number of flooding events around the lake; however, combined with rapid watershed urbanization they may have contributed to the observed ecological degradation. Figure 2. Lake Tarpon outfall canal structure. Atkins Lake Tarpon Water Quality Management Plan Final May

13 Figure 3. Photograph of Lake Tarpon and disconnected sinkholes along western shoreline Project Background Anthropogenic eutrophication of the lake, first evident in the 1980s, increased rapidly in the 1990s. In response, the County and SWFWMD staff have been sampling the lake for selected water quality parameters since May 1988 following a lake-wide algae bloom in In addition, declining water quality led to the formation of a multi-agency committee in 1988 to address lake water quality issues. A decade of diagnostic and feasibility studies on the lake culminated in the completion of the Lake Tarpon Drainage Basin Management Plan (DBMP Atkins (formerly PBS&J), 1998), which was adopted by the Pinellas County Board of County Commissioners in October The DBMP recommended goals and measurable objectives as well as a range of capital projects and non-structural programs addressing the improvement of water quality, aquatic vegetation, habitat, and fisheries. Since the adoption of the DBMP many of the recommended projects and programs called out in the report, including the following, have been implemented: Partial conversion of on-site wastewater treatment systems (septic tanks) in the Lake Tarpon basin to central sewer systems was performed Construction and maintenance of an enhanced stormwater treatment facility in Area 6 Atkins Lake Tarpon Water Quality Management Plan Final May

14 Adopted a resolution designating the Lake Tarpon Basin as a "Nutrient Sensitive Watershed." Proposed a change to local ordinances to strengthen the regulation of stormwater treatment for redevelopment in the Lake Tarpon Basin so that new development or redevelopment would have stormwater treatment sufficient to meet requirements for Outstanding Florida Waters (OFWs). Established a Lake Tarpon Watershed Management Area (WMA) via amendments to the County and City of Tarpon Springs comprehensive plans which is denoted by signs adjacent to the WMA. Inventoried permitted Management and Storage of Surface Waters (MSSW) facilities and developed improvement plans Currently installing additional buoy markers for, and post the allowable activities within, the 200' perimeter restricted speed zone. Performing ongoing operation and maintenance activities for recommended structural and management components of the plan. Implemented a County-wide wet season (June 1 to September 30) fertilizer application ban. Installed stage and flow measurement devices at the Lake Tarpon Outfall Canal control structure. Despite these projects, water quality conditions in the lake remain a concern, even though water quality does not seem to be trending in any adverse direction over the past few years (see later sections of the report). In addition, the State of Florida adopted Numeric Nutrient Criteria (NNC) applicable to lakes in 2012, and these new criteria may affect the regulatory status of Lake Tarpon. In 1999, the State of Florida passed the Florida Watershed Restoration Act ( ; Florida Statutes). The Act defined the process through which the State of Florida collects, analyzes, and interprets water quality data to determine if water bodies meet the water quality standards associated with their designated use. The Act also established the mechanisms through which the Florida Department of Environmental Protection (FDEP) develops and implements the Total Maximum Daily Load (TMDL) program. The TMDL program is structured so that a science-based effort is applied to develop estimates of the amount of pollutant loads required for impaired water bodies to meet their water quality goals. After these load reduction estimates are developed, the Act requires FDEP to develop a collaborative process to ensure that pollutant load reduction quantities are implemented via an equitable allocation among various pollutant loading sources. The Act also states that implementation of pollutant load reductions shall involve a costeffective approach coordinated between contributing point and nonpoint sources of pollution and that implementation of load reductions may include allocation through non regulatory and incentive-based programs. For these reasons, the County and SWFWMD jointly funded the development of a Lake Tarpon Water Quality Management Plan (WQMP) with the goal of answering the following questions: Are the nutrient targets for Lake Tarpon developed using Trophic State Index (TSI) consistent with empirically-derived nutrient targets for Lake Tarpon? Are empirically-derived nutrient targets for Lake Tarpon consistent with NNC guidance? Atkins Lake Tarpon Water Quality Management Plan Final May

15 Are other factors, such as restoration of hydrologic functions, more likely to benefit water quality than stormwater retrofits alone? What projects will be most feasible and cost-effective in managing the water quality and natural resources of Lake Tarpon, within the constraints of compliance with NNC guidance? What are the estimated costs of these projects? To address these questions, scientifically-defensible water quality goals and targets, as well as appropriate projects to meet the goals and targets, were developed for the WQMP. The approach is consistent with methods used to develop the current water quality standards. Rather than comparison of existing conditions to default state-wide TSI values, algal biomass (quantified as chlorophyll-a) were compared to current nutrient concentrations, as well as factors such as lake levels, water residence times, water tannin concentrations, and acres of submerged aquatic vegetation (SAV) (treated or not treated). An updated hydrologic/hydraulic (H&H) model for the Lake Tarpon watershed was developed to provide the hydrologic framework for the WQMP and to evaluate various lake management projects and scenarios. Deterministic models were also developed and applied to establish appropriate and scientifically-defensible water quality targets. Finally, best scientific and engineering literature and judgment were used to develop a suite of projects and Best Management Practices (BMPs) that when implemented could lead to the attainment of the defined water quality targets History A thorough examination of available information related to the water quality status and trends within Lake Tarpon was performed as part of this work effort. A successful WQMP relies on the knowledge from previous work efforts. A literature review and summary of prior applicable studies for Lake Tarpon were completed which included relevant information from the below reports in the subsequent sections and can be found in more detail as included in Task 2.1- Lake Tarpon Watershed and Water Quality Evaluation Report (see Appendix A): The Lake Tarpon Drainage Basin Management Plan (Atkins 1998) The Lake Tarpon Drainage Basin Management Plan Update (Deitche and Hicks 2006) Final Comprehensive Report: Lake Tarpon Diagnostic/Feasibility Studies (KEA 1992) Lake Tarpon Groundwater Nutrient Study (Leggette et al. 2004) Lake Tarpon Surface Water Improvement and Management (SWIM) Plan (SWFWMD 2001) Lake Tarpon Groundwater Nutrient Study (Upchurch 1998) Effects of Water Level Fluctuations on the Fisheries of Lake Tarpon (Allen et al. 2003) Lake Tarpon Fisheries Investigations (Champeau 1992 and 1996) Previous surface water studies/plans Although water quality data from Lake Tarpon prior to the late 1980s are sparse, earlier diagnostic feasibility studies on Lake Tarpon (Huber et al., 1983; KEA, 1992) concluded that Lake Tarpon did not historically exhibit trophic related water quality issues. The same studies demonstrated that Atkins Lake Tarpon Water Quality Management Plan Final May

16 Lake Tarpon was historically: 1) nutrient balanced, with a bias for nitrogen limitation; and, 2) a mesotrophic lake with a TSI value consistently below 60, depending on the TSI index used. In the summer of 1987, a major bloom of blue-green algae (Anabaena circinalis) in Lake Tarpon covered 80 percent of the lake. The bloom persisted for much of the summer and severely impacted lake activities during the prime recreational season. Extremely low dissolved oxygen (DO) concentrations were noted in several residential canals and minor fish kills were reported. The lake surface was covered with algae and pigments from lysed algal cells and the associated unpleasant geosmin odor (distinct earthy aroma produced by a type of Actinobacteria). In response to the bloom, SWFWMD and then the County began monitoring water quality in Lake Tarpon in 1987, and the County has continued monitoring since then. The DBMP was the first document to provide a comprehensive status and trends analysis of the Lake Tarpon water quality monitoring data. The plot below was taken from the DBMP and shows mean annual chlorophyll-a concentrations in Lake Tarpon for the period of 1988 through 1996 (more recent data are included in Section 5) in relation to mean annual Total Nitrogen (TN) concentrations, and cumulative annual rainfall volume (Figure 4). Based on information presented in the DBMP, the trophic status of Lake Tarpon, based on measured nutrient and chlorophyll-a concentrations, appears to have transitioned from a mesotrophic condition in the 1980s to a borderline eutrophic condition beginning in the early 1990s, where it has remained despite the implementation of several capital projects aimed at reducing external nutrient loads to the lake from the surrounding watershed. The DBMP presented conclusions relating observed water quality conditions with significant lake management events during the period of record, including an accidental lake drawdown and a major Hydrilla verticillata (hydrilla) treatment. Overall, water level management has been suggested as one of the components affecting water quality in Lake Tarpon. Findings and conclusions presented in the DBMP are listed below. Figure 4. Mean chlorophyll-a concentrations, mean annual TN concentration, and cumulative annual rainfall for (Figure 2-2 from Atkins 1998). Atkins Lake Tarpon Water Quality Management Plan Final May

17 Chlorophyll-a concentrations were relatively low and stable in 1988 and 1989 following the major algae bloom of Chlorophyll-a concentrations decreased in Best available information indicates that this decrease was a response to the accidental release of water over the outfall structure in March of 1990, which lowered the lake levels by approximately one foot. This release of water had the effect of flushing the lake of excess nutrients, and eventually diluting the lake water with relatively nutrient-poor rainwater. In addition, best available information indicates that groundwater seepage from the surficial aquifer resulted in a reduced ph which may in turn have suppressed algae growth during the summer of Chlorophyll-a concentrations increased to pre-drawdown levels in During the summer of 1991, the ph levels in the lake returned to normal conditions. Chlorophyll-a concentrations decreased again in Although rainfall amounts were slightly higher in 1991 and 1992, it is hypothesized that the observed reduction in chlorophyll-a was a lake response to the dramatic expansion in the coverage of hydrilla during this time period. The hydrilla expansion occurred during a hiatus in the chemical treatment of the lake by the FDEP due to funding constraints. The expansion of hydrilla across the bottom of the lake may have reduced the rate of nutrient exchange between the sediments and the water column, and the hydrilla plants and associated epiphytic algae may have been competing more effectively with phytoplankton for the available water column nutrients. Chlorophyll-a concentrations increased substantially in 1993 and remained relatively high through It should be noted that the observed chlorophyll-a increases during 1993 and 1994 occurred during a period of reduced rainfall. Therefore, increased nonpoint source loadings cannot be attributed to this trend. The most plausible explanation for this trend involves the large-scale chemical treatment of hydrilla. During late 1992 and early 1993, over 500 acres of dense hydrilla were chemically treated resulting in a major hydrilla die-off. As this dead plant biomass decomposed, the nutrients contained within the hundreds of acres of plant biomass would have been released into the water column, thus stimulating phytoplankton growth. Afterwards, the reduction in overall hydrilla coverage would reduce the amount of nutrient uptake that could have been assimilated by those plants, which would then be available for phytoplankton uptake. Like chlorophyll-a, TN concentrations have also increased in the lake since The TN values observed (<1.4 mg/l) are not indicative of nitrogen-fixation (Atkins 2014) and are similar in range to current conditions. The cause of this increase is not known, however, the relationship of this trend to the 1993 hydrilla die-off is intuitive. It is also consistent with what has been observed in other Florida lakes where large-scale hydrilla treatment has been implemented (e.g., Lake Seminole). No substantial land use changes and associated nutrient loadings have occurred in the study area during the early 1990 s to account for the observed trends. The DBMP also included the calculation of a multi-parameter TSI of for Lake Tarpon using data from , and concluded that the lake was bordering a eutrophic classification indicative of degrading water quality compared to historic water quality conditions. The Lake Tarpon Drainage Basin Management Plan Update (DBMP Update; Pinellas County, 2006) provided an extended summary of ambient water quality monitoring data. Lake surface water quality data were collected from a series of fixed sampling locations from In 2002 the open water fixed station sampling program was discontinued, and then re-designed and Atkins Lake Tarpon Water Quality Management Plan Final May

18 implemented as a stratified random monitoring program in An average annual multiparametric TSI value was calculated for Lake Tarpon for each year from 1988 to 2004, the last complete year of data the authors of that report examined. The plot below is from the DBMP Update and shows the average annual TSI values from 1988 through 2004 (Figure 5). The overall mean of annual TSI values over this time period was Figure 5. Lake Tarpon TSI values for (from Pinellas County, 2006) Previous groundwater studies/plans Groundwater data collection and studies conducted prior to 1998 in the Lake Tarpon basin were summarized and synthesized in the Lake Tarpon Groundwater Nutrient Study (Upchurch, 1998). The report, which was prepared for SWFWMD to address data gaps identified in the DBMP, provided a review of available data and literature, a characterization of groundwater quality in the basin, and estimates of nutrient loadings to the lake from both the surficial and Floridan aquifers with new field measurements. With regard to groundwater quality in the Lake Tarpon basin, key findings provided in the executive summary of the report are provided below. The bedrock in the area is fractured and karstic. Lake Tarpon is superimposed on two major photolineaments (potential subsurface fractures) which determine the alignments of the northern and southern halves of the lake. The lake is directly connected to the surficial aquifer, but possible sinkholes in the bottom and along the northern and northwestern shore of the lake may allow some interaction with the Floridan aquifer. Substantial portions of the surficial aquifer within the Brooker Creek drainage basin can drain to area lakes, streams, and drainage ditches within a time frame of less than 50 years. Once any nutrient-contaminated water reaches one of these features, the feature is threatened and Brooker Creek can transport the nutrients to Lake Tarpon within a very short time frame. Water quality in the surficial aquifer reflects a combination of processes including (1) rapid recharge in the eastern half of the Lake Tarpon basin and along the eastern side of Lake Tarpon; (2) mixing of rainfall-derived water with water from the Floridan aquifer by irrigation; and (3) mixing with somewhat saline Floridan aquifer water as a result of upconing or irrigation. Atkins Lake Tarpon Water Quality Management Plan Final May

19 Floridan aquifer water in the eastern and middle thirds of the Lake Tarpon basin is dominated by water quality developed through interaction with the limestone of the aquifer. The western third of the basin, including the immediate vicinity of Lake Tarpon, is characterized by the presence of the salt-water/fresh-water transition zone. Land use in the Lake Tarpon basin has undergone significant changes within the last 50 years, with the most dramatic changes in the last 30 years. Growth between 1970 and 1990 resulted in rapid expansion of urban areas near Lake Tarpon. Waste disposal methods were in transition at the same time. Thus, potentials for ground-water contamination have evolved and contamination found in ground water is likely to result from former land-use or waste-disposal practices, as well as modern practices. Ammonia is widespread in low concentrations throughout the surficial aquifer in the basin. High ammonia concentrations were found at two locations in the Brooker Creek watershed. Both reflect local waste-water sources (septic tanks or animals). Ammonia concentrations are high in the vicinity of the lake, which reflects application of fertilizers and waste-waters. The distribution of ammonia in the Floridan aquifer is more uniform than in the surficial aquifer, and concentrations are typically low. Nitrate concentrations in the surficial aquifer are highly variable. High concentrations, which reflect septic tanks or animal wastes, were found in isolated plumes in the eastern half of the basin. Areas of elevated nitrate were also found near the lake, where golf courses, wastewater reuse facilities, and suburban development are predominant. Virtually no nitrate was detected in the Floridan aquifer. Nitrogen compounds in the isolated plumes in the eastern half of the basin constitute local hazards to surface-water bodies, especially lakes. They also constitute potential hazards to consumers of surficial aquifer water for drinking purposes. Similar hazards exist near Lake Tarpon. Nutrient loading estimates were made for the land uses prevalent in 1990, and these land uses were compared to the nitrate distributions in the surficial aquifer. Pastures and croplands represent the highest potential nitrogen load as a result of high acreage. Rainfall, because of the high volume and basin-wide application, is potentially second in TN loading. Medium and low density residences also produce a high TN load owing to acreages involved and use of both septic tanks and turf fertilization. Golf courses and areas with waste-water reuse are locally important nutrient sources, but low acreages devoted to these land uses reduce the total load relative to other, more extensive land uses. Comparison of effluent from the Northwest Pinellas Wastewater Treatment Plant, which is used for reuse, with ground-water quality and isotopic data suggested that this source is less important than previous reuse technologies that used less efficient waste-water treatment. It was determined that the flux of water and nutrients directly into Lake Tarpon basin can drain to local surface waters (e.g., lakes, streams and drainage ditches) within a time frame of less than 50 years. Once nutrient-laden surficial groundwater reaches area surface waters, those waters are at risk for nutrient enrichment and eutrophication. Brooker Creek in particular can transport intercepted nutrient rich groundwater to Lake Tarpon in a very short time. In addition to characterizing groundwater quality in the Lake Tarpon basin, the Lake Tarpon Groundwater Study provided groundwater nutrient flux estimates for Lake Tarpon. Flux rates from both the surficial and the Floridan aquifer to the lake were estimated using Darcian flow concepts (i.e. flow through a porous medium). Lowering of lake levels or raising ground water levels will accelerate flow to the lake. The time required for the flow system to adjust to head changes is high relative to the timing of seasonal lake and ground-water levels and to Atkins Lake Tarpon Water Quality Management Plan Final May

20 management-induced fluctuations in the lake. Therefore, short-term fluctuations do not result in rapid changes in water or nutrient flux from the aquifers. The flux of water from the surficial aquifer to the lake is estimated to be about 1.24 mgd, while Floridan aquifer flux is estimated to be about 0.55 mgd. Therefore, approximately 2-3 times as much water is thought to enter the lake from the surficial aquifer than from the Floridan aquifer. Given these low flux rates, it is clear that short-term draw down of the lake for management purposes will not result in short-term replacement of water in the lake from ground water flow. Flux estimates for nutrients to the lake are limited because there is a lack of adequate monitoring wells to form a tightly controlled fence around the lake for detection of nutrient flux. Some of the wells used to estimate nutrient flux to the lake are not near enough to the lake to provide accurate estimates. Some of these wells were found to have high nitrate concentrations, which introduces uncertainty in the flux estimates. Assumption that the nitrate concentration found in this well reaches the lake, results in an inflated estimate of flux to the lake. It is estimated that approximately ten times as much nitrogen enters the lake through the surficial aquifer than through the Floridan as a result of the high water flux rate and concentrations of nitrogen compounds in the surficial aquifer system. Using preferred modeling scenarios, it is estimated that approximately 2.35 tons/year of TN enter the lake via the surficial aquifer. This load consists of approximately equal amounts of ammonia and nitrate nitrogen. About 0.3 tons of nitrogen enters the lake from the Floridan aquifer each year. There is about 25 times as much ammonia as nitrate in this load. Annual nitrogen loads entering the lake from ground water are low relative to estimates for surface water of about 54 tons/year. To develop more locally-specific and spatially discrete estimates of nutrient inputs from the surficial aquifer to Lake Tarpon (as recommended in Upchurch, 1998) the County and SWFWMD entered into a cooperative funding agreement to conduct the Lake Tarpon Ground Water Nutrient Study (Leggette, Brashears and Graham, Inc., 2004). The specific objectives of the study were to: Establish a shallow groundwater monitoring network around the lake capable of providing longterm monitoring of surficial aquifer nutrient flux to the lake. Develop a groundwater flow net and nutrient flux model to provide updated nutrient flux estimates to the lake. Assess the nutrient load from existing septic tanks and evaluate the potential load reduction to the lake by replacing septic tanks with a central sewer system. Evaluate surficial aquifer water quality in the following geographic areas: 1) Highland Lakes Golf Club; 2) west and northwest regions of the lake; and 3) east and northeast regions of the lake. The installation of two monitoring wells planned for the Highland Lakes Golf Course was cancelled when site access was denied. As recommended by Upchurch (1998), the County established shallow groundwater monitoring stations at 24 new monitoring wells, and continued ongoing sampling at seven existing wells. The 31 monitoring wells were sampled during the period of May 16, 2002 to June 4, 2002, representing dry season conditions; and the period of October 22, 2002 to October 28, 2002, representing wet season conditions. Additional samples from 17 wells were taken in October for nitrogen isotope Atkins Lake Tarpon Water Quality Management Plan Final May

21 analysis to determine potential sources of nitrogen loads to the lake (e.g., septic tanks, animal waste, inorganic fertilizer). The estimated TN discharge into Lake Tarpon during the dry season was 22.6 pounds/day. The majority of loading occurred in the southwest quadrant of the lake (19.03 pounds/day) and the northeast quadrant of the lake (4.57 pounds/day). The estimated TN discharge into Lake Tarpon during the wet season was 28.1 pounds per day. The majority of loading occurred in the southwest quadrant (14.11 pounds/day) of the lake. The estimated total phosphorus (TP) discharge into Lake Tarpon during the dry season was 1.4 pounds/day. The majority of loading occurred in the southeast quadrant (0.575 pounds/day) and southwest quadrant (0.523 pounds/day). Similarly, TP discharge into Lake Tarpon during the wet season was 1.59 pounds/day. The majority of loading occurred in the southwest (0.486 pounds/day) and southeast quadrants (0.481 pounds/day). With regard to sources, the study estimated that septic tanks contribute 0.97 tons per year of TN and 0.05 tons per year of TP to Lake Tarpon, and whole fertilizer applications contribute 3.63 tons per year of TN and 0.21 tons per year of TP. The study goes on to conclude that loading from septic tanks could be removed by conversion to sanitary sewer; however, the treatment or removal of fertilizer loadings is more problematic. In summary, available groundwater studies indicate that the surficial aquifer is much more important than the semi-confined Floridan aquifer to the hydrology and water quality of Lake Tarpon. Furthermore, nitrogen loads from the surficial aquifer were estimated to range from approximately 2.5 to greater than 5 tons/year. The sources of the nitrogen loads included organic nitrogen from wetlands and decaying plant material, as well as inorganic nitrogen from human waste from septic tanks, domestic animal waste, and lawn and limited crop fertilizer applications Summary of prior applicable modeling efforts A Lake Tarpon watershed model was created for the 1998 DBMP using the Storm Water Management Model (SWMM) software. As part of the development of the WQMP, a more detailed Lake Tarpon watershed H&H model was developed using Interconnect Channel and Pond Routing software Version 4 (ICPRv4) for the following three areas in the watershed (Figure 6). Two watershed models are available for the Brooker Creek watershed. The first model was created by URS in 2009 for SWFWMD. This model incorporates both the Hillsborough County and Pinellas County portions of Brooker Creek and was completed using the HC-SWMM modeling software. The second model encompasses only the Hillsborough County portion of Brooker Creek using the SWMM 5.1 modeling software. At the time of the writing of this report, the second model was still under development, however Hillsborough County provided a draft version of the model in January These two models represent the best available information for Brooker Creek and key model elements such as culvert crossings and watershed extents were extracted for incorporation into the Lake Tarpon Model. The South Creek watershed does not have an existing watershed model. Similar to the Lake Tarpon watershed, this model was created as a part of this project utilizing the data collected during the Task 1-1: Surface Water Resource Assessment Inventory (Appendix B). As the South Creek watershed only drains to the outfall canal, and not the lake itself, this watershed was created in a simplified format so that flows from the watershed can be accounted for in the calibration at the control structure in the Lake Tarpon outfall canal Natural systems This section presents a summary and discussion on previous work assessing aquatic vegetation and fisheries in Lake Tarpon. This information is directly relevant to the development of the Lake Atkins Lake Tarpon Water Quality Management Plan Final May

22 Tarpon WQMP in that the status of aquatic vegetation and fisheries often reflect, and can directly affect, the trophic status of a lake. Figure 6. The Lake Tarpon watershed prior watershed modeling efforts. Emergent aquatic vegetation The only quantitative study of the emergent aquatic vegetation (EAV) communities of Lake Tarpon was conducted as part of the Final Comprehensive Report: Lake Tarpon Diagnostic/Feasibility Studies (KEA, 1992). The KEA report included a habitat change analysis based on historical and current aerial photographs, combined with field observations. It was concluded that cattail (Typha spp.) was historically a relatively minor component of the emergent plant community in Lake Tarpon. Since the implementation of the water level fluctuation schedule in 1972, however, the coverage of cattail expanded from less than 20-acres to approximately 120-acres. The cause of this expansion was attributed primarily to the relatively constant water levels in the lake which has allowed for the competitive dominance of cattail over other native species. Atkins Lake Tarpon Water Quality Management Plan Final May

23 As reported in the DBMP, cattail may form what are essentially monocultures of densely growing plants along the lake shoreline. While cattail is a native species, problems occur when these plants expand at the expense of native species and habitat. Cattail dominance can result in a litter buildup disproportionately high in comparison to most other aquatic plants, and reportedly become so dense that fish are restricted to the fringes rather than the interior of these stands. A more diverse assemblage of aquatic plant species is preferred since it provides a greater number of ecological niches. Increasing species diversity is equated with increasing environmental health. Periodic and seasonal lake fluctuations, particularly for seasonal high water levels (Kercher and Zelder 2004) limit the expansion of cattail into deeper water. Lake level stabilization, therefore, tends to promote the expansion of cattail. While enhanced fluctuation would control cattail stand development, urban development in the watershed, and especially into the historic floodplain, has limited the vertical range over which Lake Tarpon can fluctuate. The effectiveness of increasing the upper range of water level fluctuation, even by a minor amount, as a means of controlling cattail stands was observed in Lake Tarpon during 1995 and 1996 when cattail coverage decreased by approximately 15 percent due to increased rainfall amounts and the associated higher lake levels. Currently, cattails are essentially managed on a piecemeal basis via the issuance of individual permits by the Florida Fish and Wildlife Conservation Commission (FFWCC) for their removal along private waterfronts. Typically, applicants are required to replant their waterfronts with other desirable aquatic plants. No comprehensive program to improve the diversity of emergent aquatics in the littoral zone has yet been developed for Lake Tarpon. The removal and replacement of cattails with more desirable native species has occurred only on a limited basis through the State Environmental Resource Permit (ERP) program on private properties and as publicly funded habitat restoration projects. The largest such program was implemented by the SWFWMD Surface Water Improvement and Management (SWIM) Department in 1997 where cattails were harvested from a total of 9.3 acres at five sites in Lake Tarpon. The five sites were then revegetated with a more diverse mix of desirable native species. The success of these revegetation efforts, however, was generally poor due to the uprooting and erosion of the replanted areas by wave energy. Other smaller test revegetation projects have been successfully implemented by the County and FFWCC where bulrush (Schoenoplectus tabernaemontani) was successfully established in areas previously dominated by cattail. Successful examples of bulrush restoration projects are seen along the eastern shoreline of the lake by John Chesnut Sr. Park. The DBMP concluded that cattail harvesting followed by revegetation with a more diverse assemblage of desirable emergent aquatics would likely provide significant ecological and aesthetic benefits; but that the benefits to the littoral plant community from this effort would likely be secondary compared to the greater lakewide benefits derived from an increased lake level fluctuation range. Following adoption of the DBMP in 1998, the County began conducting annual surveys of both emergent and SAV, as recommended in the DBMP. Results of emergent vegetation surveys conducted in the early 1990s were subsequently reported in the DBMP Update (Pinellas County, 2006). Due to funding constraints, annual vegetation surveys in Lake Tarpon were discontinued by the County after However, the FFWCC has conducted quantitative/qualitative presence/absence of species through a lake-wide survey annually since the 1980s. However, collected data were insufficiently detailed to derive an estimate of the Lake Vegetative Index (LVI), a monitoring unit used to track the level of impact to lake vegetation due to human disturbance. Pinellas County began collecting additional data in 2014 to estimate the LVI in Lake Tarpon. The first study was performed on September 12 th, This data collection effort resulted in a LVI score of 34, with the dominant species in the lake being Potamogeton illinoensis. An average of 26 Atkins Lake Tarpon Water Quality Management Plan Final May

24 species was found at each LVI test site, with means of 65-percent being native plants and 29- percent being FLEPPCC Cat 1 taxa. A second study was performed by the County on August 28 th, This data collection effort resulted in a LVI score of 41, with the dominant species in the lake being Potamogeton illinoensis at three (3) sites and Ceratophyllum demersum at one (1) site. An average of 29 species was found at each LVI test site, with means of 67-percent being native plants and 27-percent being FLEPPCC Cat 1 taxa. Submerged aquatic vegetation (SAV) The composition, abundance and distribution of SAV communities in Lake Tarpon prior to the construction of the Lake Tarpon Outfall Canal have not been documented. The exotic and invasive hydrilla was probably introduced into Lake Tarpon sometime during the 1970s, but did not become a major management concern until the early 1990s. During this time period, the Bureau of Aquatic Plant Management (formerly under the Florida Department of Natural Resources - later merged under FDEP) was responsible for managing hydrilla in waterbodies state-wide. Historically, the Bureau had sufficient funding to effectively maintain submerged and floating nuisance aquatics in the lake and manmade canals of Lake Tarpon at insignificant levels. However, budgetary constraints after 1986 limited the control of hydrilla to only within the natural lake, and within an arbitrarily determined management range of up to six percent coverage - a level of coverage considered to be acceptable from the sport fishing and budgetary perspective (Atkins 1998). As reported in the DBMP (Atkins, 1998), hydrilla became a serious management issue in the early 1990s. During the summer of 1992, hydrilla began spreading and establishing along the entire western shoreline covering more area than had previously been observed in Lake Tarpon. By the fall 1992, it was estimated that dense hydrilla extending from the bottom to the surface affected over 500 acres of the lake surface area, and that the entire lake bottom was covered by at least some hydrilla (Atkins, 1998). To effectively treat the extensive hydrilla coverage it was necessary to implement several Sonar TM herbicide applications during March and May Although these applications essentially resulted in a "whole lake" treatment, the treatments did not take effect until June and July 1993 when large floating mats of dead hydrilla were observed and persisted through the summer. No attempt was made to harvest these floating mats prior to their decomposition in the lake. These treatments were ultimately very successful in reducing Hydrilla coverage back to a "maintenance" control level, as no follow up treatment was required in 1994, and only one treatment of 105 acres was required in However, the resulting massive die off of macrophytic plant tissue provided a potential pool of nutrients to be released into the overlying water column. The 2001 SWIM Plan for Lake Tarpon states that "Management of hydrilla has been correlated with degraded water quality as a result of nutrients released into the water column from degrading plant tissue." (SWFWMD 2001). A notable degradation in water quality and increase in the trophic state of the lake were observed after the extensive hydrilla treatments in 1994 and Although a clear cause and effect relationship was not established at the time, it is likely that the rapid release of nutrients organically bound up in this large mass of macrophytic plant tissue into the water column contributed significantly to the sharp increases in TN and chlorophyll-a concentrations observed in the lake during subsequent years. In addition to nutrient release from the treated hydrilla, there would also be less SAV (since hydrilla is a type of SAV) around to assimilate nutrients from the water column. Based on hydrilla nutrient content data from Lake Okeechobee (Gremillion et al., 1988), the chemical treatment of approximately 500 acres of hydrilla potentially released as much as 2.4 tons of TP and 20 tons of TN back into the water column to be subsequently taken up by phytoplankton. Figure 7 from the DBMP shows this relationship as a plot of the cumulative annual treatment acreage of hydrilla versus mean annual chlorophyll-a concentrations. Atkins Lake Tarpon Water Quality Management Plan Final May

25 Trends in the coverage of desirable, native SAV, such as coontail (Ceratophyllum spp.) and eelgrass (Vallisneria americana), have been difficult to assess due to the lack of a consistently applied quantitative monitoring program. An apparent reduction in eelgrass cover occurred in the 1980s with the expansion of hydrilla in the lake (KEA, 1992); however, since the large scale eradication of hydrilla in 1993, coontail and eelgrass appeared to expand their coverage in the lake (Atkins, 1998). Beginning in 1992, SWFWMD assumed responsibility for treatment and removal of nuisance and invasive vegetation on the lake proper and the Lake Tarpon Outfall Canal, while the County assumed responsibility for maintaining nuisance vegetation in the private canal system of the lake. Although quantitative SAV survey data from Lake Tarpon are not available after 2005, SWFWMD does report the acreage of hydrilla treated annually. Figure 8 shows a time series plot of acres of hydrilla treated by year from , based on SWFWMD and FFWCC reported data. Minimal hydrilla treatments have occurred since 2008 as extensive acreage of hydrilla have not been documented within the lake. Figure 7. Plot of mean annual chlorophyll-a concentrations vs. cumulative total area of hydrilla chemical treatment for the period (Figure 2-8 from Atkins, 1998). Atkins Lake Tarpon Water Quality Management Plan Final May

26 Figure 8. Acres of hydrilla treated in Lake Tarpon by year from as reported by SWFWMD and FFWCC PMARS. As discussed extensively in the DBMP, the abundance and distribution of SAV in Lake Tarpon is critically important to the trajectory of its trophic status. There is strong anecdotal evidence that chemical treatment of large volumes of SAV, and the subsequent decay of that plant material, impacts algal production and water quality as measured by chlorophyll-a concentrations and water clarity. However, prior to this study there was no existing comprehensive mapping of SAV abundance and distribution in Lake Tarpon. Understanding both the spatial distribution and nutrient dynamics associated with SAV communities in Lake Tarpon are considered to be critical data gaps in the development of this Lake Tarpon WQMP. Fisheries Lake Tarpon has been a popular fishing destination for many decades, and was formally designated as a State Fish Management Area by a special Resolution of the Pinellas County Board of County Commissioners in June1963. This designation, per Section of the Florida Statutes, sets forth the special regulations of state Fish Management Areas and designates the FFWCC as the State resource management agency with primary responsibility for sport fishery management. Pursuant to their statutory responsibility, the FFWCC has performed fisheries monitoring and management activities in Lake Tarpon on a periodic basis beginning in the 1970s. Partly in response to anecdotal evidence of declining water quality conditions, the FFWCC conducted detailed fisheries investigations on Lake Tarpon from July 1987 to June During this time the FFWCC used a number of techniques to assess the fishery of the lake including aerial and boat surveys to estimate aquatic plant coverage, block netting and rotenone sampling in littoral sites, night electrofishing, and a creel survey. This combination of techniques allowed the FFWCC to Atkins Lake Tarpon Water Quality Management Plan Final May

27 assess fish population structure, the relationship between the fishery and macrophyte (aquatic plant) coverage, fishery utilization of differing habitats, and angler success and preferences. Summary data presented by Champeau (1992) indicated an excellent sport fishery in Lake Tarpon. Lake Tarpon, historically mesotrophic, had a lower biomass when compared with eutrophic (i.e., productive) Florida lakes; however, the population structure of the sport fishery has remained consistently within the preferred ranges. Sport fish are proportionately abundant with good percentages of harvestable and angler preferred sizes. Although most fishing effort in Lake Tarpon is focused on largemouth bass, the crappie population expanded in the early 1980s and became an excellent secondary sport fishery. Data further indicated good reproductive success, recruitment (survival) and rapid growth. Champeau (1992) did, however, suggest that the fishery could be enhanced through habitat management. He noted that two factors, lake level stabilization and the occurrence of exotic aquatic vegetation, necessitated a more active management role with respect to aquatic vegetation. The FFWCC again performed rotenone block netting and electrofishing in the summer of 1995 and spring 1996 to assess densities and standing crops of all species present, and to determine the relative abundance and population structure of the largemouth bass population. Champeau (1996) reported results that indicate that the electrofishing catch rates per unit effort, and the population age and size structure, for largemouth bass were comparable to data obtained during the study period. In addition, the fish community balance was good with a forage biomass to carnivore biomass (f/c) ratio of 3.2. However, as shown in Figure 9 from the DBMP, the overall fish standing crop (biomass) increased by approximately 500 percent between 1990 and This dramatic increase in fish biomass was due to an increase in both sport fish and rough fish abundance, and was considered by Champeau to be an indicator of increasing eutrophication that could threaten the future integrity of sport fishing in the lake if not addressed. Using a proprietary index of fish community balance, Champeau (1996) concluded that the sport fishery in Lake Tarpon had been stable, or on a slight decline, since the 1970s. Figure 10 from the DBMP shows this trend. Champeau (1996) further concluded that while cultural eutrophication may have benefited the fisheries of Lake Tarpon by increasing productivity (as measured in fish standing crops), increasing nutrient enrichment beyond current trophic levels may have future negative consequences. As a result, the FFWCC recommended that strategies to abate significant nutrient sources to Lake Tarpon be implemented (Champeau, 1996). Atkins Lake Tarpon Water Quality Management Plan Final May

28 Figure 9. Fish biomass data from shoreline blocknets in Lake Tarpon, (from Atkins, 1998). Figure 10. Fish community balance in Lake Tarpon, (from Atkins, 1998). In support of management recommendations contained in the DBMP, the County contracted with the University of Florida Department of Fisheries and Aquatic Sciences to conduct a study with the goal of assessing the relationship between lake-level fluctuation, both man-made and natural, and fish populations. This study included a review of the available fisheries data from the previous FFWCC work, as well as additional field sampling conducted during the period of The resulting data were also used to generally evaluate fishery health in the lake. The researchers utilized a variety of sampling methods, including: block nets, electrofishing, and otter trawls to compile data on abundance and recruitment. The final report on this effort was released in September, 2003 (Allen et al., 2003). Key findings from this study were summarized in the DBMP Update (Pinellas County, 2006), excerpted below. Atkins Lake Tarpon Water Quality Management Plan Final May

29 Largemouth bass support a popular recreational fishery and Lake Tarpon is often included in the list of top ten bass lakes in Florida. Anglers advocate catch and release with few fish being pulled. The results of the study indicate that bluegill, redear, and largemouth bass made up 85 percent of the total fish biomass with bluegill representing about 50 percent of the total biomass. Largemouth bass recruitment was stable. But researchers found a negative relationship between spring and summer low water level and bass recruitment, as a drought in 2000 resulted in lower than usual water levels during bass spawning season. The abundance of black crappie, another recreationally important species, was also low in Lake Tarpon. Poor recruitment to adulthood has been found in many south-eastern lakes and is currently a topic of research by several universities. The low abundance, on the other hand, allows rapid growth in the surviving fish. Lake Tarpon crappies have some of the highest growth rates in the state, reaching harvestable size in one year. In summary, previous efforts indicated a general decline in the sport fisheries on the lake due to increasing cultural eutrophication and vegetation management issues While there have been no published reports on the status of Lake Tarpon fisheries since 2006, the current status of fisheries within Lake Tarpon was described by FFWCC in the following manner: Although the largemouth bass population and size structure is excellent, fishing pressure is relatively low. Florida Fish and Wildlife Conservation Commission (FWC) fisheries biologists regularly sample bass during electrofishing surveys on this lake. In fact, Lake Tarpon is rated one of the Top 10 bass lakes in the state of Florida by FWC fisheries biologists. ( Atkins Lake Tarpon Water Quality Management Plan Final May

30 2. Data Inventory A compilation and inventory of relevant water quality, water quantity, and any additional data required for the development of the linked hydrologic and pollutant loading models (using ICPRv4) and empirically-derived water quality models was performed. The hydrologic output from the ICPRv4 model, combined with nutrient concentrations, provided external pollutant loads to Lake Tarpon. These pollutants loads were incorporated into an empirically-derived water quality model to identify external and/or internal pollutant sources of concern. In addition to the external pollutant loads, the extensive data sets specific to Lake Tarpon and supplemented as appropriate from other Florida lakes, were used to develop linked regressions that could be used to identify and assess both external load reductions and in-lake management actions that would be expected to result in improved water quality. The text below provides a summary of the extensive data inventory included in the Task 1.1- Surface Water Resource Assessment Inventory report (see Appendix B). Graphics displaying water quality and quantity station locations can be found within Appendix B Surface water data Ambient data for the WQMP and modeling efforts include surface water quality and water quantity data which are presented below. Surface water quality and quantity data were retrieved for the Lake Tarpon, Brooker Creek and South Creek watersheds (Figure 11). Data were obtained from the County, SWFWMD, the FDEP Impaired Waters dataset (IWR Run 50) and the WaterAtlas ( The hydrologic output from the ICPRv4 model, combined with nutrient concentrations, were used to quantify external pollutant loads to Lake Tarpon. The relationships between watershed pollutant loadings and lake water quality were incorporated into an empirically-derived water quality model as part of the WQMP Surface water quality There is an extensive surface water quality dataset available for Lake Tarpon, the majority of which was acquired through the County monitoring effort. The County monitoring program began in 1991 with five fixed sampling locations. The data collected included, but is not limited to, chlorophyll-a and nutrients. In 2003, a randomized sampling design was implemented wherein the lake was divided into forty hexagons. Four sampling sites are randomly selected each sampling period from four of the established hexagons. Eight sampling periods were identified (four dry season and four wet season periods). A total of 32 random sampling sites are collected each year. Prior to 1991, a robust data collection effort by the United States Geologic Survey (USGS) is available within the Lake Tarpon watershed; however, there is very little information on concentrations of chlorophyll-a. The earliest recorded chlorophyll-a concentrations are reported by the Environmental Protection Agency (EPA) in SWFWMD reports data from Lake Tarpon over the period 1988 to 1991, which includes both chlorophyll-a and nutrient concentrations. There are several water quality sampling stations within the Brooker Creek watershed. However, there are only three sampling locations available in the western portion of the Brooker Creek watershed which directly discharges into the Lake Tarpon watershed. The USGS has one sampling location whose period of record extends from 1964 to 2011 but that data set has only limited data on concentrations of chlorophyll-a. The County reports data for two locations, both of which include chlorophyll-a concentrations. One station, 04-03, is still active. Atkins Lake Tarpon Water Quality Management Plan Final May

31 Within the South Creek watershed, the County, SWFWMD and USGS have collected surface water quality data from the Lake Tarpon outfall canal which discharges from Lake Tarpon to Old Tampa Bay. The County station (06-04) provides the most robust dataset ( ) which includes both nutrients and chlorophyll-a concentrations. No data beyond 2013 was utilized in this WQMP due to the project timeframe and data availability. Figure 11. Lake Tarpon, Brooker Creek and South Creek watershed boundaries. Atkins Lake Tarpon Water Quality Management Plan Final May

32 Surface water quantity Within the Lake Tarpon watershed, there were three surface water level monitoring stations. Two of the sites collected daily field measurements at a combination of SWFWMD and USGS sites (22112 and , respectively). Site reports data at 15-minute real-time frequency; data are also reported as daily values. This site is the only active site in the Lake Tarpon watershed. There were 68 surface water level gage monitoring stations in the Brooker Creek watershed; 65 are located in the Brooker East portion and three are located in the Brooker Creek West portion of the watershed. There are 12 real-time stations which correspond in location with subsequent field measurement stations. The majority of stations are maintained by SWFWMD, with USGS and Lakewatch data also available. There are several stations which still remain active in There are seven surface water flow monitoring stations in the Brooker Creek watershed. One of the stations ( ) has corresponding real-time data collection available. There were four surface water level monitoring stations in the South Creek watershed, all located at or near the S-551 structure on the Lake Tarpon Outfall Canal. There are four real-time stations which correspond in location with subsequent field measurement stations. The stations are maintained by either SWFWMD or USGS. There are several stations still active in There is one surface water flow monitoring station in the South Creek watershed. The station ( ) has real-time data collection available and remains active Groundwater data The H&H model (ICPRv4) that was used to develop the hydrologic loads for the WQMP has the capability to predict the extent of inundation associated with a design-level event and changes in inundation extents under managed lake level operations or other improved conditions. The advantage of using the updated (version 4) model is the inclusion of a surficial groundwater component and its ability to provide continuous and long term simulations that allow predictions of flows and stages over time, and under different conditions in lake inputs or management strategies over time. As such, a comprehensive understanding of the available groundwater quality and quantity data is important to the development of the WQMP. An overview of the groundwater ambient data available for the development of the WQMP is presented below Groundwater quality data There were a total of 16 groundwater quality sampling locations sampled by the USGS National Water Information System (NWIS) within the project study area. Groundwater quality sampling locations were identified within the Lake Tarpon and Brooker Creek watersheds. No groundwater quality sampling locations were identified in the South Creek watershed. While various parameters were sampled, conductivity and chloride concentrations were the dominant parameters quantified specifically within the Lake Tarpon watershed Groundwater quantity data There were 11 groundwater level monitoring stations identified in the Lake Tarpon watershed. Ten of the stations are maintained by SWFWMD wherein field measurements are recorded daily. Two of the sampling locations (22768 and ) were monitored in real-time, resulting in data available at 15 minute intervals. Groundwater level data are available as both daily field measurements and real-time at station Several of the groundwater level monitoring stations remain active with data available within the current year (2016). There were 67 groundwater level monitoring stations identified in the Brooker Creek watershed, 22 of which are within the western portion and 45 in the eastern portion of the watershed. The majority Atkins Lake Tarpon Water Quality Management Plan Final May

33 of sites are maintained by SWFWMD or Tampa Bay Water, and field measurements are recorded daily. Seven of the eight stations which provide 15 minute real-time data correspond with field measurement locations. One station, , is maintained by USGS. Several of the groundwater level monitoring stations remain active and data are available within the current year (2016). Included in the 67 groundwater level monitoring stations identified in the Brooker Creek watershed are those from the Eldridge well field area among other deep monitoring wells, measuring aquifer levels. This data was utilized indirectly in the modeling through the use of the time varying Floridian Aquifer piezometric surface as a groundwater boundary. There were two groundwater level monitoring stations identified in the South Creek watershed. Both sites are maintained by SWFWMD and field measurements are recorded daily. Sampling appears to have been discontinued in 2009 at both locations Aquatic Vegetation In the DBMP (Atkins 1998) the importance of hydrilla control efforts was noted. In particular, it seemed that the chemical treatment of ca. 500 acres of hydrilla in the early 1990s might have resulted in a change in the lake s ecology from that of a macrophyte-dominated system to a phytoplankton-dominated system (Deitche and Hicks 2006). However, this transition is not evident based upon more recent vegetation surveys and phytoplankton abundance. The DBMP Update (2006) summarizes SAV abundance within Lake Tarpon over several years provided by Pinellas County Environmental Management. A spreadsheet documenting the monthly acres of floating aquatic plants and hydrilla treated in Lake Tarpon from July 1993 to June 2014 was provided by SWFWMD. The FFWCC provided aquatic vegetation survey data specific to Lake Tarpon from 1982 to The FFWCC performs annual public water body assessments which include a cursory evaluation of plant communities. The surveys provide notes based upon the visual assessment of how conditions have changed since the previous survey. It was recommended by FFWCC that the surveys be used as informative supplemental material and not included as a quantitative dataset by which an assessment of community abundance or change be evaluated. More recently, the FFWCC has performed hydroacoustic surveys of Lake Tarpon in 2014, 2015 and 2016 to provide a lake-wide snap shot of SAV coverage within the lake. Additionally, the bathymetry and lake volume were calculated for Lake Tarpon in both 2006 and 2015, as provided by the University of South Florida (USF), which provides an estimated hydrilla coverage for that time period in the Lake Internal Cycling Processes Phosphorus inputs into lakes include external sources from the watershed and internal sources from the lake sediments. Phosphorus flux from sediments is sometimes sufficient to maintain anthropogenic eutrophication of a lake even when external phosphorus loads have been reduced or eliminated (Larsen et al. 1979, Ryding 1981) as seen in Figure 12. Neither the DBMP nor the 2001 SWIM Plan include sediment removal or sediment inactivation as a cost-effective management option for the lake, because the relatively low organic content of those sediments makes nutrient fluxes out of the sediments and into the water column a likely minor component of the overall lake nutrient budget. The County collected sediment samples on September 23, 2015 as part of an effort to quantify the potential release of nutrients into the water column from lake sediments. Preliminarily sites were selected which were thought to provide a comprehensive assessment of the variable sediment interfaces within the lake (i.e., sandy, mixed sand and muck vs muck). Piston cores were retrieved at each site and homogenized prior to sample collection. The County collected cores from approximately 20 sites of which only two locations had evidence of appreciable muck. The grain size analysis results performed by FDEP Atkins Lake Tarpon Water Quality Management Plan Final May

34 confirmed the field observations of limited organic material on the lake bottom. Based on these findings it was determined by the County that there was little value in moving forward with sediment flux experiments in the lake due to the overall lack of muck present. Figure 12. Influence of reduced external phosphorus reductions on phosphorus in water column (after Reddy et al. 1999). Atkins Lake Tarpon Water Quality Management Plan Final May

35 3. Modeling Component A hydrodynamic model and pollutant loading spreadsheet were developed to provide the mechanisms for evaluating the relationships between watershed pollutant loadings and lake water quality. These relationships, in turn, were used to examine proposed lake level management scenarios with respect to flooding in the watershed. Discussions with researchers from the University of Florida suggested a careful application of an empirical model approach due to the absence of adequate local data needed for an EPA Water Quality Assessment Program (WASP) or other mechanistic water quality model. Consequently, the Atkins/ESA team recommended a model approach that relies on both empirically- derived water quality targets and a hydrodynamic model for water volume and pollutant loading estimates. A more comprehensive discussion for each of these model approaches is detailed in Task 1.3- Modeling Plan Final (Appendix C) Hydrologic/Hydraulic (H & H) Model As part of the Lake Tarpon WQMP, various scenarios were evaluated in an attempt to identify alternatives to alleviate the ecological stress currently being experienced in Lake Tarpon. In order to quantify the inflows to the lake and evaluate the flooding impacts that could result from modifications to the lake operations, a validated existing conditions H&H computer model was required. The ICPRv4 was selected for the analysis of the H&H conditions within the Lake Tarpon watershed. The ICPRv4 model is an integrated groundwater and surface water interaction model, capable of simulating overland flow through either a traditional node link schema (1-D) and/or an overland flow mesh (2-D). The hydrologic component of the model performs runoff calculations concurrently with the hydraulic calculations and relies upon land use and soils data to spatially define the hydrologic characteristics of the watershed. Discharges are routed through the system to compute water surface elevations at nodes and discharges at links throughout the hydraulic network. An important feature of the ICPRv4 modeling software was its ability to perform long term simulations in addition to single storm events. This feature was critical for the Lake Tarpon watershed. Continuous simulation modeling was used to simulate monthly inflows into Lake Tarpon from ground and surface water. Results from this simulation allowed the conditions within the lake to be summarized, including total inflow; outflow; change in stage; change in storage; lake residence time; and impacts of management scenarios. The inflows generated from the ICPRv4 model were then converted into pollutant loads by applying applicable time varying concentrations to each inflow stream. Task 2.1- Lake Tarpon Watershed and Water Quality Evaluation Report provides a comprehensive description of the model evaluation, development and simulations (Appendix A). Despite efforts to improve the lake conditions through implementation of several projects recommended in the DBMP (see Section 1.1) water quality in Lake Tarpon continues to be a concern. As part of the Lake Tarpon WQMP, an in lake model was created based upon empirically derived equations to ultimately identify alternatives to alleviate the ecological stress currently being experienced in Lake Tarpon. As a primary input into the lake model, pollutant loads from inflow sources were quantified. Using a monthly time step to aggregate values, water quantity loadings from the ICPRv4 continuous simulation model were converted into pollutant loads. Load conversions were based upon measured or estimated pollutant loading rates of each input stream for TN and TP which are the pollutants of interest for this study. Atkins Lake Tarpon Water Quality Management Plan Final May

36 3.2. Pollutant Loading Model Components Developing estimates of pollutant loads requires estimating both the runoff volume to the lake and the corresponding concentration of the pollutants under consideration. For the Lake Tarpon model, these loads were calculated for the following parameters, with specific details associated with each parameter described in subsequent subsections: Atmospheric Deposition - loads associated with direct wet rainfall falling on the lake surface Basin Inflow event mean concentrations (EMC) based loading from drainage basin runoff incorporating applicable load reductions from BMPs Tributary Inflows Measured Brooker Creek concentrations applied to model generated creek flows, which were calibrated in the H&H model via comparison with gaged inflows Groundwater Seepage volume applied to seasonal ground water concentrations measured around Lake Tarpon Rainfall is the primary driver of the hydraulic model simulation and is applied over the watershed in 15-minute intervals based upon the NexRAD grid cells. For areas outside of the lake boundary, rainfall is either stored in the basin, converted to runoff, or seeps into groundwater. Each of these components contains a separate pollutant loading concentration and is transported to the lake as described in one of the subsections below Atmospheric Deposition Direct rainfall is the portion of rainfall that falls directly on the surface of the lake and carries with it a pollutant load associated with atmospheric deposition. Regionally, atmospheric deposition data has been collected at Chesnut Park, from the Bay Region Atmospheric Chemistry Experiment (BRACE), and through the National Atmospheric Deposition Program. The closest station, Chesnut Park, only had data from June 1997 to June This data was not used directly as it only represented a limited time period, compared to the model simulation period. The data from BRACE represented a detailed account of nutrient loads to Tampa Bay, including both dry and wet deposition focusing on the nitrogen component. This data set was not used as it was not able to directly correlate to a loading rate for the wet atmospheric deposition for nitrate and ammonia. Instead, monthly data from the National Atmospheric Deposition Program site FL05 at the Chassahowitzka National Wildlife Refuge was used as it provided a longer term data set. Atmospheric deposition data from Chesnut Park indicated an annual loading for TN and TP of 9.99 tons per year and 0.2 tons per year respectively as extrapolated from the two year sampling program. These values were compared to the model simulated results and indicated that total nitrogen and phosphorus percent contribution of the watershed were within 6% for TN and 1% for TP, noting that the total nitrogen estimated loadings also have a 6% unknown source. The atmospheric deposition for TN was further calculated as the sum of measured wet atmospheric deposition nitrate (NO 3 ) and ammonium (NH 4 ) falling directly on Lake Tarpon. Total Phosphorus (TP) data was not available at the Chassahowitzka site. Instead a ratio was developed between TN and TP based upon the bulk atmospheric deposition samples collected at Chesnut Park. The ratio of 1:0.020 was applied to the monthly TN data collected above to determine the monthly TP concentrations. Monthly atmospheric deposition values used for this study are presented in Table 11 within Appendix A. It is of note that the monthly atmospheric data series only began in August The data gap for values back to January 1995 was filled in with the first complete year of data (1997). Two months of erroneous data were noted in May and June Atkins Lake Tarpon Water Quality Management Plan Final May

37 of 1998, which had values significantly higher than any other nutrient value collected. For this analysis, the erroneous values were removed and estimated from values for adjacent months Basin Inflows (Surface Water Loads) Basin inflows from the ICPRv4 continuous simulation model consist of 364 individual drainage basins. Each basin was modeled with initial abstraction to reflect whether or not it had water quality treatment. If a basin was identified as having water quality treatment, an initial abstraction of one inch over the basin was applied to simulate the development s required water quality treatment volume. Gross Pollutant Loads Gross pollutant loads are defined as the amount of pollutant that is generated from a basin. This load is calculated as the product of the estimated runoff volumes times the EMC for each selected pollutant: Gross loading (lb/mo) = (Runoff volume (ac-ft/mo) x EMC (mg/l) x conversion factor (2.718)) The event mean concentration (EMC) is the expected mean concentration of a pollutant in the runoff that is discharged from a particular land use during a typical (average) storm event. EMC values used in the Lake Tarpon study were obtained from literature values from Harper and Baker (2007) and Harper (2011). These are typical values used for and accepted by the FDEP for estimating pollutant loads for various National Pollutant Discharge Elimination System (NPDES) and TMDL permits throughout both Central and South Florida. Values shown here are similar to those used in the recently completed (for the TBEP) Old Tampa Bay Integrated Model s watershed loading model. For example, the EMC value used in the Old Tampa Bay Model for the urban land use category is 1.9 mg TN/L, which matches up with the EMC value listed below for the most common urban land use category in the Lake Tarpon watershed, that of medium density residential (at 1.87 mg TN/L). Table 1 summaries the surface water EMC values utilized in the Lake Tarpon pollutant loading model. Atkins Lake Tarpon Water Quality Management Plan Final May

38 Table 1. Surface Water EMC Values EMC LANDUSE CATEGORY TN (mg/l) TP (mg/l) SOURCE Residential Low Density < 2 Dwelling Units Florida Runoff EMC Database Final May 2012 (FDEP) Residential Med Density 2 >5 Dwelling Unit Florida Runoff EMC Database Final May 2012 (FDEP) Residential High Density Florida Runoff EMC Database Final May 2012 (FDEP) Commercial and Services Florida Runoff EMC Database Final May 2012 (FDEP) Industrial Florida Runoff EMC Database Final May 2012 (FDEP) Institutional Florida Runoff EMC Database Final May 2012 (FDEP) Recreational Florida Runoff EMC Database Final May 2012 (FDEP) Golf Courses Assumed Agriculture Pastures Open Land Florida Runoff EMC Database Final May 2012 (FDEP) Pine Flatwoods Florida Runoff EMC Database Final May 2012 (FDEP) Longleaf Pine Xeric Oak Florida Runoff EMC Database Final May 2012 (FDEP) Hardwood Conifer Mixed Florida Runoff EMC Database Final May 2012 (FDEP) Lakes 0 0 All EMC assumed zero Reservoirs 0 0 All EMC assumed zero Stream and Lake Swamps (Bottomland) Florida Runoff EMC Database Final May 2012 (FDEP) Wetland Coniferous Forests Florida Runoff EMC Database Final May 2012 (FDEP) Cypress Florida Runoff EMC Database Final May 2012 (FDEP) Wetland Forested Mixed Florida Runoff EMC Database Final May 2012 (FDEP) Freshwater Marshes Assumes Average of Wet Prairie and Marl Prairie Wet Prairies Assumes Average of Wet Prairie and Marl Prairie Emergent Aquatic Vegetation Assumes Average of Wet Prairie and Marl Prairie Intermittent Ponds Assumes Average of Wet Prairie and Marl Prairie Transportation Florida Runoff EMC Database Final May 2012 (FDEP) Communications Florida Runoff EMC Database Final May 2012 (FDEP) Utilities Florida Runoff EMC Database Final May 2012 (FDEP) Net Pollutant Loads For the surface water component of the pollutant loading model, the actual amount of pollutant runoff discharged to Lake Tarpon is referred to as net pollutant load. Net pollutant loads build upon the gross pollutant load calculation, by first determining the efficiency of the existing stormwater runoff treatment facility BMP. The net load is calculated as the difference between the gross pollutant load and the amount of pollutant removed from the treatment facility: Net loading (lb/mo) = Gross loading x (1 BMP removal efficiency) Best Management Practices (BMPs) BMP removal efficiency is typically based upon literature values of field tests of various BMPs. In these tests a significant portion of the removal efficiency of the load from dry or percolation based units is actually due to a removal of water, whereby the BMP promotes percolation rather than settling or other removal methods. In contrast, wet detention pond removal efficiencies are driven by permanent pool volume and residence time. As to not overstate the removal efficiency of watershed loads, for basins with identified treatment, no additional load reduction was applied other than that associated with water removal through initial abstraction already factored into ICPRv4 s hydrology engine. The exception to this method is the one CIP project implemented in the watershed: Lake Tarpon Sub-basin 6, Florida Department of Transportation (FDOT) pond modification, FY 2005 FY Removal efficiency for total suspended solids (TSS), TP and TN are 90%, 85% and 50% respectively, based upon predicted load reductions specified in the project development report summary. Atkins Lake Tarpon Water Quality Management Plan Final May

39 The additional pollutant removal from the applied Alum Treatment is applied to this basin s outfall stream. Adjusting these percent reductions for the basin from predicted to measured efficiencies resulted in minimal changes to the surface loadings to Lake Tarpon of 2.6, 1.2 and 1.4% for TSS, TP and TN respectively. It is of note that the vast majority of the rainfall events in a given year are less than one inch, whereby first flush methodology captures a significant portion of typical events occurring in a particular year Tributary Inflows The major tributary inflow into Lake Tarpon comes from Brooker Creek. The model simulation includes an accounting for flows from Brooker Creek. A control structure was located at the model boundary and reflected inflow into the Brooker Creek system from Island Ford Lake consistent with the operational data provided by SWFWMD. This flow was converted into a monthly nutrient load by multiplying a monthly concentration by the total flow using the formula below. Tributary loading (lb/mo) = Tributary volume (ac-ft/mo) x Conc. (mg/l) x conversion factor (2.718) Water quality data was generated as the monthly average of samples collected from either the County Station as part of Pinellas County Environmental Management s Surface Water Ambient Water Monitoring Program (SWARM), or at USGS station These stations are seen spatially in Figure 13. If no samples were collected in a given month, then the previously recorded value was held constant representing the best available information. For context, the monthly time series values generated back to 1984 for TN and TP are shown in Figure 14. The monthly averages used over the 1995 to 2012 model simulated time period, based upon the nutrient samples, are presented in Table 13 within Appendix A. An elevated value recorded in the summer of 1997, which had nutrient values double that of the majority of the other samples for both nitrogen and phosphorus, occurred directly after a storm event. Atkins Lake Tarpon Water Quality Management Plan Final May

40 Figure 13. Brooker Creek Water Quality Sampling Locations Atkins Lake Tarpon Water Quality Management Plan Final May

41 Figure 14. Brooker Creek Total Nitrogen and Total Phosphorus Concentrations Groundwater Inflows ICPRv4 includes an inflow summation that includes: surficial groundwater inflow, surficial groundwater outflow, and leakance inflow. In Lake Tarpon, over the majority of the model simulation, groundwater was flowing into Lake Tarpon at a much more significant rate than it was flowing out. Similarly, the leakance parameter did not play a significant role in controlling lake levels and is also minimal compared to the groundwater inflow component. Thereby, the seepage value reported by ICPRv4 into Lake Tarpon will be taken as the surficial groundwater inflow component. In 2004, the County undertook the Lake Tarpon Groundwater-Nutrient Study (Leggette, Brashears and Graham, Inc., 2004). This study, cooperatively funded by both the County and SWFWMD, captured groundwater samples all around the lake to assess groundwater nutrient loading to Lake Tarpon. The samples had a wide range of nutrient values measured between 12.8 and 0.1 mg/l for TN and 1.53 and for TP. Given the distribution of the data, a geometric mean was used to obtain the values applied to the groundwater data to obtain the ground water loading. Table 2 summarizes the geometric mean of the wet and dry season typical values for Lake Tarpon. It is of note that this data would be inclusive of septic tank seepage as well as fertilizer spraying that seeps into the groundwater. Table 2. Groundwater Typical Values Season TN mg/l TP mg/l Wet Dry Atkins Lake Tarpon Water Quality Management Plan Final May

42 4. Historical and recent conditions in the lake and watershed This section summarizes the historical and current conditions of the lake and contributing watershed. An evaluation of the historical and current conditions of the lake and watershed was performed by reviewing and synthesizing existing information and conducting a site visit. Lake eutrophication in general is a natural process whereby nutrient enrichment and biological productivity increase. Lake eutrophication can be exacerbated by anthropogenic land uses (Gill et al. 2005) or other anthropogenic activities. The accelerated eutrophication due to human activities is termed cultural eutrophication. Increased nutrients associated with eutrophication can increase algal growth (algal blooms) and macrophyte abundance (Smith et al. 1999), in turn increasing turbidity, particulate organic matter, and dissolved organic matter in lakes. Historical water quality patterns in Lake Tarpon and the implications of relevant state and federal regulations for water quality were characterized, leading to a more detailed investigation into the interaction of the external pollutant loads and ambient water quality. Many factors may influence in-lake water quality including: long-term hydrologic alterations, pre development, stormwater runoff, historic and current point source discharges, septic tank discharge, accumulated sediment, extent of submerged aquatic vegetation (SAV) and emergent aquatic vegetation (EAV), and hydrologic connections to forested wetlands. The potential effects of these factors on water quality in Lake Tarpon were investigated in this effort Lake and watershed description, including alterations to hydrology Lake Tarpon is the largest lake in Pinellas County with a surface area of over four square miles. The lake has a north to south orientation, with a maximum water depth of approximately 20 feet (Figure 15). The lake s historically excellent water quality and healthy fish and wildlife populations made it an important recreational resource for many years. Lake Tarpon and its watershed have been substantially altered from natural conditions. The lake historically had no outfall other than a tidally influenced sinkhole on the northwest shore of the lake. The tidal influence resulted in wide fluctuations of the lake level and occasional pulses of saline water and estuarine organisms. In response to several flood events the USACE constructed an outfall canal and control structure at the south end of the lake in The outfall canal crossed a natural basin divide, discharging water to Old Tampa Bay near Safety Harbor. Two years later, SWFWMD built an earthen dike around the sinkhole leaving the outfall canal as the only point of discharge, and eliminating the tidal and salt-water influence. Concurrent with these structural modifications was urban development of the lake s shoreline and adjacent watershed. Construction of the outfall canal and the sinkhole dike has largely reduced the number of flooding events around the lake; however, combined with rapid watershed urbanization they may have contributed to the observed ecological degradation. Atkins Lake Tarpon Water Quality Management Plan Final May

43 Figure 15. Lake Tarpon bathymetry. Atkins Lake Tarpon Water Quality Management Plan Final May

44 4.2. Regulatory implications Since 1999, the Total Maximum Daily Load (TMDL) program in Florida has been focused on identifying water quality impairments and developing and adopting TMDLs to restore water quality. More recently, the TMDL program is transitioning to project implementation and water quality restoration, with the cost of restoration being passed to local governments and other Municipal Separate Storm Sewer System (MS4) permit holders. Previously, MS4 permits focused on documenting the extent and status of stormwater infrastructure, as well as documentation of monitoring programs and various management activities. Recent permits have included requirements to begin the process of implementing pollutant load reduction quantities spelled out in various TMDLs. This marks a transition in MS4 permitting in Florida, from its original function as a planning tool to guide stormwater management towards that of a regulatory tool to enforce TMDL implementation. These changes have made it increasingly important for local governments to verify that water quality impairments identified by FDEP are accurate, that the TMDLs developed and adopted by FDEP are scientifically defensible, and that water quality restoration projects can be reasonably expected to restore water quality. Failure to do so may mean that a community will be legally obligated to implement projects that are not necessary and may not actually restore water quality. In addition, the implementation of problematic TMDLs can divert the limited resources of local governments from addressing actual water quality problems, and thus delay the restoration of waterbodies with verified impaired water quality. FDEP identifies four distinct waterbodies (waterbody identification [WBID]) in the Lake Tarpon watershed and one for the Brooker Creek system (Table 3). These WBID are shown on Figure 16. Section 303(d) of the Clean Water Act (CWA) requires that each state develop an impaired waters list to identify rivers, lakes, coastal waters, and estuaries that do not meet water quality standards. FDEP currently identifies the following impairment designations for each of the WBIDs within the Lake Tarpon and Brooker Creek systems: Lake Tarpon: DO and Nutrients (Historic TSI) Lake Tarpon Outlet: No impairments Lake Tarpon Canal (1541A): DO and Nutrients Lake Tarpon Canal (1541B): DO Brooker Creek: Coliforms In 1998, FDEP developed the first list of impaired WBIDs (303(d) list) based primarily on the states (b) Water Quality Assessment Report which used a watershed approach to evaluate the water quality throughout the state. At that time, two of the four Lake Tarpon WBIDS were identified as impaired with the projected year of TMDL development of 2008 (Table 3): WBID 1541A (Lake Tarpon Canal [downstream from the outfall structure) for depressed DO, elevated nutrients and fecal coliform bacteria WBID 1541B (Lake Tarpon Canal [upstream from the outfall structure]) for depressed DO The (d) list does not include a determination of impairment for Lake Tarpon (1486A) or the Lake Tarpon Outlet (1486). Brooker Creek (1474) was also placed on the 303(d) list in 1998 Atkins Lake Tarpon Water Quality Management Plan Final May

45 for impairments due to Dissolved Oxygen, Coliforms, Nutrients with a projected year for TMDL development of Subsequently, in 2002, Brooker Creek was determined to meet water quality standards for both fecal and total coliform bacteria and FDEP recommended de-listing Brooker Creek for bacterial contamination. The nutrient impairment was also removed in 2002 based on evidence that the water quality met the existing state standards. However, the DO impairment within Brooker Creek was deemed accurate although FDEP was unable to link low levels of DO to any pollutant. In the (d) list, FDEP concluded that low DO levels in Brooker Creek were at least partly due to the number of swamps and other wetlands in the watershed. Figure 16. Water Body Identification (WBID) Numbers. Atkins Lake Tarpon Water Quality Management Plan Final May

46 Table 3. Impairment designations within Lake Tarpon and Brooker Creek WBIDs Year of water quality review Lake Tarpon Lake Tarpon Outlet Lake Tarpon Canal (marine) Lake Tarpon Canal (fresh) Brooker Creek 1486A A 1541B no impairments no impairments 2002 DO Nutrients (Historic TSI) FDEP Impairment Status DO Nutrients (Historic TSI) DO Nutrients (TSI) Delist: DO (data used for impairment classification taken from 1486) Delist: nutrients (TSI)- assessed as a lake not a stream no impairments DO Nutrients Coliforms DO - - DO Nutrients DO DO Coliforms Nutrients Delist: Coliforms Nutrients Coliforms DO Nutrients DO Coliforms The (d) list of water quality identifies Lake Tarpon as impaired for both DO and nutrients based on historic TSI values (Table 3). For DO, the impairment was expected to be addressed in a TMDL that was projected to be completed in The DO impairment noted for Lake Tarpon was based on FDEP s previous DO standard of 5 mg/l. The failure to meet DO standards was attributed to elevated levels of Biological Oxygen Demand (BOD). The median BOD level in Lake Tarpon was listed as 3.0 mg/l. A BOD level of 3.0 mg/l would fall in the top 30 percent of values for Florida streams (FDEP, 2006); however, it is not clear whether or not a BOD level of 3.0 mg/l is excessive for Florida s lakes. Additionally, revised DO standards apply to surface waters within the upper two meters of the water column. As such, Lake Tarpon surface water DO would not be classified as impaired. For nutrients, FDEP determined in 2002 that Lake Tarpon was limited by both nitrogen and phosphorus, and that nutrient reductions would be developed by SWFWMD through the development of a Pollutant Load Reduction Goal (PLRG). The impairment determination for Lake Tarpon shown in the 303(d) list states that For the historical listing ( ), annual average TSI values in the verified period exceeded the minimum historical annual average value of TSI units by more than 50 percent in and A 50 percent increase in the minimum annual average historical period TSI value would equal a value of or higher (i.e., 150 percent of 43.42). However, the TSI values listed by FDEP in its (d) list are 61.45, 60.38, 55.46, and 57.9, for the years 2000, 2001, 2003, 2004, and 2005, respectively. While two of the five years do exceed the prior TSI threshold value of 60, and all of the values shown are more than 10 TSI units higher than the lowest annual average value of 43.42, none of the annual average values shown are higher than the 50 percent increase referenced in the 303(d) list. Following further analysis by FDEP, Lake Tarpon was identified as a phosphorus-limited system, based on an average TN:TP ratio of 29.5 (FDEP, 2014). The determination that Lake Tarpon is a phosphorus-limited system differs from both the (d) list, where the lake was determined to be co-limited by both nitrogen and phosphorus and also from the Lake Tarpon SWIM Plan (2001) Atkins Lake Tarpon Water Quality Management Plan Final May

47 where evidence was presented (Figure 17) that supports a determination of nitrogen limitation of phytoplankton abundance. Figure 17. Plots of mean annual chlorophyll-a concentrations vs mean annual TN concentrations for the period of (from 1998 Study). Additionally, the Lake Tarpon Outlet (1486A) was identified for depressed DO and elevated nutrients based on TSI. However, in 2009, the Lake Tarpon Outlet was delisted for DO because it was determined that the data used for the impairment classification in 2002 was from Lake Tarpon (1486) and not the Lake Tarpon Outlet (1486A). In 2013, the Lake Tarpon Outlet was delisted for elevated nutrients because the water quality within the WBID was compared to the lake criteria not stream criteria as would be appropriate based on the waterbody type classification. Atkins Lake Tarpon Water Quality Management Plan Final May

48 4.3. Previous Pollutant Load Estimates In the DBMP, completed in 1998, a water budget was the first step in the development of a lakewide nutrient budget. The water budget was based on an assumed storage volume of 19,155 acre-feet, which was based on an average depth of 7.3 feet and a surface area of 4.1 square miles. The volume of the lake was assumed to be constant and sediment accumulation rates were judged to be minor, since the lake s water level is managed within a relatively narrow range and very low levels of organic sediment were observed in most of the open waters of the lake, as discussed in Section 2.4. Major conclusions for the lake water budget are found in Table 4. Based on terms of the water budget, the nutrient budget for the lake was developed for both TN and TP. Stormwater loads of TN and TP were estimated by multiplying appropriate nutrient concentration data (EMC values) with the previously derived inflow and outflow volume estimates from the lake water budget. An important component of the lake s nutrient balance was developing an estimate for the amount of nutrients (both TN and TP) that settle out of the water column. Using estimates supplied by KEA (1992), net sedimentation rates were included in the nutrient balance estimates for the lake. As a result, the nutrient balance for the lake was simplified (Atkins, 1998) as: INFLOW LOADINGS = OUTFLOW LOADINGS + NET SEDIMENTATION Table 4. Input and output components of the Lake Tarpon water and nutrient budget (from 1998 Study). Source (Atkins, 1998) Water budget (percent) Nutrient budget (percent) Inflow Outflow TN TP Inputs Stormwater (un-gaged) Direct Precipitation Brooker Creek (gaged) Groundwater Seepage from surficial aquifer Other (Floridan aquifer, septic tanks) < Outputs Outfall Structure Evaporation Sedimentation and uptake Permanent removal (e.g. fish harvesting) - - <2 4 For TN, the major conclusions of the nutrient budget in the DBMP (Atkins 1998) are provided in Table 4. Comparing the water budget to the nutrient budget, atmospheric deposition of TN is proportionally less important, which reflects lower TN concentrations in rainfall than in stormwater runoff. Gaged and un-gaged portions of the watershed are proportionally similar, in terms of TN loads and hydrologic loads. Septic tanks, however, were a minor portion of the hydrologic budget, but a more important source of TN loads (Atkins 1998). However, the TN load estimates for septic tank systems listed in Upchurch (1998) are 15 percent of the load estimates in the DBMP. Consequently, the 2001 SWIM Plan states that based on groundwater monitoring conducted by Upchurch (1998) the impact of septic tank leachate on the lake is not conclusive. Atkins Lake Tarpon Water Quality Management Plan Final May

49 The nutrient budget for the lake estimated 62 percent of the TN load left via outflows from the Lake Tarpon Outfall Canal, with 37 percent of TN loads remaining behind via the processes of sedimentation and uptake by macrophytes. These results are contradictory to the more recent nutrient loading estimates modeled as part of this effort. A more detailed comparison between the previous (DBMP 1998) and current modeling effort is provided in Section 4.4. Less than 2 percent of the TN load was estimated to be removed from the lake via fish harvesting. Comparing the water budget to the nutrient budget, atmospheric deposition of TP is proportionally less important, which reflects lower TP concentrations in rainfall than in stormwater runoff (Table 4). Runoff from the un-gaged portions of the watershed are proportionally more important for TP loads than the water budget would suggest, while TP loads from Brooker Creek are proportionally less important than the water budget. As in TN, the TP loads from septic tanks (Atkins, 1998) were estimated to be more important than their contribution to the water budget. However, the TP load estimates for septic tank systems listed in Upchurch (1998) are 6 percent of the load estimates in the DBMP. As was the case for TN, the TP loads attributed to septic tanks are considered to be inconclusive in the 2001 SWIM Plan. The nutrient budget for the lake estimated that 24 percent of the TP load to the lake left via outflows from the Lake Tarpon Outfall Canal, versus the 62 percent estimate for TN loads (Table 4). This would suggest that most of the TP loads to the lake remain behind in the lake. In fact, the DBMP (Atkins, 1998) concluded that 72 percent of the TP load to the lake stayed within the lake via the dual processes of sedimentation and uptake by macrophytes. A more detailed comparison between the previous (DBMP 1998) and current modeling effort is provided in Section 4.4. Approximately 4 percent of the TP load was estimated to be accounted for via fish harvesting. The gaged Brooker Creek basin was estimated to account for 19 percent of the hydrologic load to the lake, and 19 percent of the TN load (Table 4). However, Broker Creek was estimated to load only about 10 percent of the TP loads to the lake. The importance of any stormwater projects in Brooker Creek would vary, depending upon the relative importance of TN vs. TP to the lake s water quality. For the un-gaged portions of the watershed, the DBMP estimated that these areas contributed 42, 49 and 69 percent of the hydrologic, TN and TP loads, respectively (Table 4). As in Brooker Creek, the importance of any stormwater projects in the un-gaged portions of the Lake Tarpon watershed would vary, depending upon the relative importance of TN vs. TP to the lake s water quality. Of the six identified sources of nutrient inflows to Lake Tarpon, only direct runoff and septic tanks were considered to be sufficiently manageable sources that external loads could be reduced in any manner. As stated in the 2001 SWIM Plan, the importance of septic tanks to overall nutrient loads is considered to be inconclusive. The DBMP nutrient budget for Lake Tarpon (Atkins, 1998) concluded that 37 and 72 percent of the TN and TP loads to the lake, respectively, remain within the lake through the processes of sedimentation and uptake by macrophytes. These processes represent internal nutrient fluxes that could potentially influence water quality to a greater extent than any single external load reduction strategy. However, a more recent investigation (completed in September 2015) into the potential sediment nutrient flux in Lake Tarpon indicated that the lake bottom is predominantly characterized by sandy soils which are not conducive for substantial nutrient fluxes. The discrepancy with the earlier modeling effort is likely due to an underestimation in groundwater contributions via the mechanistic model used to quantify loads. Atkins Lake Tarpon Water Quality Management Plan Final May

50 Based on these results, the DBMP recommended that management of internal nutrient fluxes be an important part of lake management. The DBMP thus included recommendations related to modifications to the lake level operation schedule that would decrease residence times and increase flushing rates and dilution in the lake. As well, strategies for nutrient removal through mechanical harvesting of nuisance species (especially hydrilla) were recommended. These proposed management actions were also cited for their ability to help maintain the abundance of non-nuisance SAV, as well as their importance in terms of improving the quality of fish habitat. The DBMP recommended that the County collect appropriate hydrologic and water quality data, and use those data to calculate water and nutrient budgets for Lake Tarpon on an annual basis. The DBMP Update (Pinellas County, 2006) presents revised nutrient budgets using new data developed following the adoption of the DBMP, which were then compared to the DBMP nutrient budgets. The following summarizes the findings and conclusions from this work. Direct runoff was calculated using the loads determined from four years of priority tributary sampling. Nutrient loads from tributary discharges calculated using concentration data from the tributary study were lower than the modeled loads presented in the DBMP. Annual loading of TN was tons/year, while the annual TP loading was 0.55 tons/year (Baltus & Squires, 1999). A more detailed comparison of model output is contained within the portion of this report that summarizes results from the ICPR model. With regard to the atmospheric deposition component, the Tampa Bay Estuary Program (TBEP) reported a bay-wide loading of 838 tons TN/year (Poor et al., 2001). Extrapolation of the area of Lake Tarpon relative to total bay-wide loadings resulted in a value of 8.38 tons TN/year using TBEP data. County staff collected bulk atmospheric deposition samples at Chestnut Park from June 1997 to June 1999 (Baltus & Squires, 1999). The average of the Baltus & Squires (1999) and TBEP (2001) data is 9.55, which is close to the 9.9 tons/year value presented in the DBMP. In the DBMP budgets there are 4.29 tons/year of unaccounted nutrients. The County speculated that possible sources for this loading may come from the Boot Ranch development, un-gaged portions of Brooker Creek, nitrogen fixation by blue-green algae, or Lake St. George. The basis for the amount of "unaccounted" nutrient loads is not fully known, but a more detailed comparison of the output of the current ICPR model, versus earlier efforts, is contained within the portion of this report that summarizes results from the ICPR model External Pollutant Loads The following section presents the results of the ICPR v4 H&H modeling effort to quantify water and nutrient budgets for Lake Tarpon (for more detail related to the modeling calibration results review Appendix A). The water quantity results were converted into contributions into Lake Tarpon based upon the representative concentration of each inflow stream for TN and TP. The results presented as annual totals are seen in Figures 18 and 19 for TN and TP respectively. In both graphs the annual rainfall total is also shown. In general, rainfall peaks and valleys tend to trend with the rise and fall of modeled nutrient inflows into Lake Tarpon. Over the simulation period, for TN, atmospheric deposition represented 19% of the TN loading and Basin Runoff, Groundwater inflows, and Brooker Creek inflows represented 10%, 41%, and 30% respectively. The load distribution of TP included 5% atmospheric deposition and Basin Runoff, Groundwater inflows, and Brooker Creek inflows represented 25%, 39%, and 30% respectively. Atkins Lake Tarpon Water Quality Management Plan Final May

51 The ICPRv4 model computed nutrient results were compared to the results of annual nutrient loads assembled between 1997 and 2004 to match the time period of the previously calculated nutrient budgets for Lake Tarpon presented in the 2006 Lake Tarpon Drainage Basin Management Plan Update (2006 Study). The percentage of the modeled inflows compared to the previous study (2006 Study) are shown in Figures 20 and 21 for TN and TP, respectively. In these figures, the 2006 Study included annual nutrient budget the County assembled between 1997 and 2004 for total nitrogen, and between 1997 and 2002 for total phosphorus, combining data collected by the County and the previous DBMP. The biggest difference in the distributions is the load split between ground and surface water. This difference is likely due to the methodology the previous work used to calculate the basin runoff. Without a model to track the surface water percolating into the ground and reemerging into the lake or creek as baseflow or seepage, the previous work would have had to assign a greater contribution to surface water to maintain the water balance. The previous estimates as presented in the DBMP contradict the modeling effort presented above. The discrepancy between modeling efforts is likely, at least partly, due to the use of a mechanistic water quality model in prior efforts, as well as an overestimate of surface water nutrients loads which could be more pulsed than the more even load of nutrients via groundwater found in this effort. Groundwater load estimates from this study are substantially higher, for nitrogen, than what Upchurch (1998) estimated. While some of this is likely due to differences in modeling approaches, it is worth noting that the amount of nitrogen load exported from Lake Tarpon (ca. 22 tons / yr; Table 7) is five to ten times higher than groundwater load estimates from Upchurch (1998). Since the nutrient load leaving the lake is a measured value (from flows over the outfall structure multiplied by TN concentrations of the lake) it is likely that the TN export estimates from the lake are fairly accurate. As such, this would indicate that TN loads from groundwater developed by Upchurch (1998) are likely underestimates, as those loads would only be able to account for a small fraction of the nitrogen that leaves the lake, which in turn has to have first been loaded into the lake. Atkins Lake Tarpon Water Quality Management Plan Final May

52 Figure 18. Annual Total Nitrogen Load to Lake Tarpon (2016 Study). Figure 19. Annual Total Phosphorus Load to Lake Tarpon (2016 Study). Atkins Lake Tarpon Water Quality Management Plan Final May

53 Utilizing ICPRv4 enables tracking of even the smallest amount of rainfall, through the surficial groundwater table and its emergence into the lake. The combination of ground and surface water components between this work and the previous nutrient budgets does show similar total loadings. For TN the combination of groundwater and basin runoff represents 51% of the total inflow, while the previous study had TN at 47%. A similar result can be shown for TP, with a modeled contribution of 64% compared to the previous study s 71%. Modeled (2016 Study) 2006 Study Figure 20. Percent Contribution of Total Nitrogen to Lake Tarpon by Source During 1997 to 2004 (2016 Study compared to 2006 Study). Modeled (2016 Study) 2006 Study Figure 21. Percent Contribution of Total Phosphorus to Lake Tarpon by Source During 1997 to 2002 (2016 Study compared to 2006 Study). Comparing the quantity of the loads between this modeling effort and the 2006 study showed that over comparable time periods, the resulting total inflows of nutrients were comparable. This data comparison is presented in Table 5. Atkins Lake Tarpon Water Quality Management Plan Final May

54 Table 5. Brooker Creek Annual Average Load Comparison ( ). TN (tons/year) TP (tons/year) 2016 Study Study Comparing the total loadings over the years the annual nutrient budgets were calculated, including 1997, 1998, 2000, 2001, 2002, 2003, and 2004, shows the average annual TN load from known sources is 70.1 tons per year. This number compares well to the model calculated 65.4 tons per year over the same time period. TP was calculated in the previous study at 5.37 tons per year, which compares well to this model s calculation of 5.3 tons per year. This data is also shown in Table 6. Table 6. Annual Average Nutrient Load Comparison (2006 and 2016 Studies). TN (tons/year) TP (tons/year) Year 2016 Study 2006 Study* 2016 Study 2006 Study* Annual Average *Yearly breakdown of loads from previous study not available. Only annual average data available. Tables 7 and 8 summarize data on TN loads by loading source, by year for both TN and TP. The amount of TN and TP leaving Lake Tarpon is also included; those estimates are based on monthly lake-wide TN and TP concentrations multiplied by monthly discharge rates out of the lake and into the freshwater portion of the Lake Tarpon Canal. The years used in the comparison in Table 6 include only one year that entirely overlaps between the two studies (1997). Nutrient loads from the 2006 study are for a generic year, rather than any given year. For the 2016 study (this effort) values are available for individual years. The years 1997 to 2004 were chosen (for comparison to the 2006 Study) based on the fact that they included an El Niῆo event (1997 to 1998); a series of wet (2002), dry (2000) and normal years; as well as the very busy 2004 hurricane season. This allows the reader to see the variability in years, as opposed to the generic year output from the 1998 study. On average, while yearly average TN and TP loads are similar between the 2006 and 2016 studies, the year to year variation can be considerable. While the data in Tables 7 and 8 are shown as annual averages, they are based on monthly values summed over each year. For greater detail, monthly loading rate estimates are presented for TN and TP in Appendix D. The years chosen include those where the authors felt the most accurate Atkins Lake Tarpon Water Quality Management Plan Final May

55 information was available for both inflows and outflows, since the numbers include the amount of TN and TP exported from the lake. On average, the amount of incoming nitrogen (from all sources) that is exported from Lake Tarpon to Tampa Bay annually is about 22.2 tons. For phosphorus, the amount of incoming loads that are exported averages 0.73 tons. For nitrogen, the loads leaving the lake are smaller than those entering it, which indicates that there is little to no nitrogen fixation or other internal nutrient sources in Lake Tarpon. Additionally, the calibrated model shows that there is very little interaction between Lake Tarpon and the underlying Floridian Aquifer. Also, the proportion of phosphorus not exported but left behind in the lake is greater than for nitrogen, which is consistent with the lake being a more substantial sink for phosphorus than is the case for nitrogen. Table 7. (2016 Study). Annual Average TN Loads by Source, Compared to TN Loads Leaving Lake Tarpon Year Atmospheric deposition Basin runoff Nitrogen Loads Groundwater Brooker Creek Total Load to Lake Exported from Lake (tons) (tons) (tons) (tons) (tons) (tons) Means Atkins Lake Tarpon Water Quality Management Plan Final May

56 Table 8. Annual Average TP Loads by Source, Compared to TP Loads Leaving Lake Tarpon. Year Atmospheric deposition Basin runoff Phosphorus Loads Groundwater Brooker Creek Total Load to Lake Exported from Lake (tons) (tons) (tons) (tons) (tons) (tons) Means A summary of the 2016 Study water and nutrient budget components for Lake Tarpon are presented in Table 9. Table 9. Input and output components of the Lake Tarpon water and nutrient budget (from 2016 Study) Current Study Water budget (percent) Nutrient budget (percent) Inflow Outflow TN TP Inputs Stormwater (un-gaged) Direct Precipitation Brooker Creek (gaged) Groundwater Seepage from surficial aquifer Outputs Outfall Structure Evaporation Sedimentation and uptake Atkins Lake Tarpon Water Quality Management Plan Final May

57 5. Identify relationships that may affect lake conditions This section addresses links between external (watershed) and/or internal processes which could explain the water quality changes that have been documented in Lake Tarpon over the last 30 years. Relationships that may affect lake conditions are presented below. Land use, point source history, septic tank/sanitary sewer coverage, lake level history and water quality data were analyzed. Results have been evaluated to determine if there is clear evidence of an ongoing and negative impact of nutrient supply or other factors on nutrient-related water quality parameters. If multiple and corroborating lines of evidence of such relationships are found, these would indicate the possibility of ongoing nutrient-related impacts to water quality. Surface water quality data were compiled from various sources to develop a comprehensive database for Lake Tarpon including the following data sources: the County, SWFWMD, the FDEP Impaired Waters dataset (IWR Run 50) and the WaterAtlas ( The majority of the surface water data compiled was acquired through the County monitoring effort. The County monitoring program began in 1991 with five fixed sampling locations. The data collected included, but is not limited to, chlorophyll-a and nutrients. In 2003, a randomized sampling design was implemented wherein the lake was divided into forty hexagons. Four sampling sites are randomly selected each sampling period from four of the established hexagons. Eight sampling periods were identified (four dry season and four wet season periods). A total of 32 random sampling sites are collected each year. Prior to 1991, a robust data collection effort by the USGS is available within the Lake Tarpon watershed; however, chlorophyll-a concentrations are not included, even though chlorophyll-a concentrations are reported by the EPA in SWFWMD collected water quality data from Lake Tarpon over the period 1988 to 1991, which included both chlorophyll-a and nutrient concentrations. To maximize the comparability of information, period of record water quality data as provided by the County, were used as the primary data source. A Quality Assurance/ Quality Control (QA/QC) evaluation of the comprehensive dataset (including all data providers) identified data associated with special studies, which did not reflect ambient conditions for the lake as a whole. As such, only data collected and analysed by the County were used to evaluate the ambient water quality conditions within Lake Tarpon. The County surface water quality monitoring effort sampling techniques and analysis have remained consistent allowing for a comparable dataset over time. Monthly and annual summary statistics were generated for the purposes of statistical analysis. It should be noted that the proposed management actions provided as part of this effort are based on a comparison of water quality data, lake level data, water age and pollutant loading model output on a monthly time step from 1995 to Although more recent water quality are available, attributing "causation" to any values would be compromised by including data on only a portion of the potential forcing factors being examined, if (for example) there was data on rainfall but not hydrilla treatment, or if water quality data were available, but not data on the amount of water leaving (or entering) the lake. As such, water quality data beyond 2012 cannot be similarly interpreted as data from the years 1995 to Prior to performing statistical analysis, the data have to be characterized in such a manner that the appropriate technique can be utilized. Parametric statistical analysis requires the data being analyzed to satisfy three assumptions: that they come from independent sampling events, that there is a normal (i.e., bell-shaped curve) distribution of the data, and that the data have equal variances for variables tested against each other. Prior to selecting the appropriate statistical tool, Atkins Lake Tarpon Water Quality Management Plan Final May

58 a test to determine the normality (Shapiro-Wilk) and variance for each dataset was performed. Overall, the majority of datasets did not satisfy the last two assumptions; therefore, non-parametric analysis was performed. Non-parametric statistical analysis (Spearman) are performed on the ranks of individual observations, which is a data transformation in which discrete values are replaced for further analysis by their ranks within the dataset. A p-value statistic of less than or equal to 0.05 was used to identify statistically significant relationships. In lieu of a coefficient of determination (r 2 ) which indicates how well data fit a parametric statistical model, non-parametric statistical analysis provide a correlation coefficient (ρ) which demonstrates the association between the variables of interest as well as directionality (values range between -1 to +1). For example, a correlation coefficient of indicates a strong, inverse correlation, while a correlation coefficient of would indicate a weak, direct correlation. It should be noted that the symbols for probability due to chance (p) and correlation coefficient (ρ) appear similar, but they are different in their meaning. Multiple water quality analysis techniques were used to ascertain specific influences on water quality. These techniques included the following analysis: General lake and watershed characterization Impairment designation using Numeric Nutrient Concentration (NNC) criteria Determination of the limiting nutrient of concern (NOC) Correlation between NOC and annual rainfall Correlation between NOC and lake water elevation Correlation between NOC and residence time of water in the lake 5.1. Ambient water quality Until 2013, FDEP used the TSI for the determination of nutrient imbalances in lakes and estuaries in the State of Florida. TSI was utilized by FDEP to determine nutrient impairment for lakes and estuaries until the adoption of the state-wide NNC criteria in TSI is calculated based on the calculated nutrient limitation (e.g. nitrogen, phosphorus or co-limited). In order to violate the TSI guidance criteria, a single year s exceedance during a specified period of record is necessary. In regards to data sufficiency, one sample is required from each quarter of the calendar year. Specific to lakes, TSI targets were allocated based upon color classification. High color lakes (Lake Tarpon long-term geometric mean color=52 PCU) are those with levels of color in excess of 40 platinum-cobalt units (PCU). High color lakes had an assigned TSI threshold of 60, which equates to do not exceed values for chlorophyll-a of 20 µg/l, a TP concentration of 0.07 mg/l and a TN concentration of 1.2 mg/l. For NNC, adopted by FDEP in 2013, lakes must first be classified as a low color ( 40 PCU) or high color (> 40 PCU) lake. High color lakes (as appears to be the case for Lake Tarpon) are then assigned NNC criteria based upon whether or not chlorophyll-a concentrations exceed guidance criteria of 20 µg/l. If the annual geometric mean chlorophyll-a concentration exceeds 20 µg/l for a high color lake, the TP and TN criteria revert to stricter standards, not to exceed 0.05 and 1.27 mg/l, respectively. If, however, the annual geometric mean chlorophyll-a concentration is below 20 µg/l, then guidance Atkins Lake Tarpon Water Quality Management Plan Final May

59 criteria increase so that annual geometric means of nutrients cannot exceed values of 0.16 and 2.23 mg/l for TP and TN, respectively. The guidance language within FAC (2b) (1a) is somewhat confusing, as it suggests that minimum criteria are applicable, whereas FDEP's use of NNC for determining impairment status has only used the highest allowable nutrient criteria guidance for lakes that are not impaired for chlorophyll-a. These criteria thus allow for a higher concentration of TN or TP if some feature (e.g., high tannins, nutrient uptake by SAV, etc.) moderates the transformation of nutrients into phytoplankton biomass. With NNC, impairment occurs when there is more than one exceedance in any three consecutive year periods, as opposed to the prior TSI method, wherein a single year s exceedance would trigger a finding of impairment. A minimum of four temporally independent sampling events are required in the calendar year to calculate the annual geometric mean, with at least one sampling event occurring during the periods from May to September and October to April. For Lake Tarpon, a preliminary determination of impairment status with NNC criteria suggests that the lake could potentially be impaired for chlorophyll-a, but not nutrients (Figures 22 and 23). The water quality data shown in the 2014 FDEP 303(d) list and other examined data clearly show that annual average chlorophyll-a concentrations exceed 20 µg/l on a regular basis, especially after While the basis for the post-1993 increase in chlorophyll-a is not fully understood, it could be that the previously extensive hydrilla coverage that peaked in 1992, and was then reduced via herbicide applications. If the extensive hydrilla coverage, mostly represented an SAV community outside the footprint of native SAV beds, then the sustained increase in chlorophyll-a after 1992 could be due to a reduced presence of an alternative destination for nutrients. There is insufficient data to prove or disprove this suggestion. However, if hydrilla coverage prior to 1993 consisted of an SAV community that was, for the most part, spatially distinct (i.e., in deeper waters) from most of the native SAV, then the large scale hydrilla coverage that was lost in the early 1990s might represent an SAV coverage that has not yet been (and may not every be) replaced by native SAV. The total SAV coverage for Lake Tarpon, estimated at 21 percent, is roughly half the coverage (40 percent) required for lakes in Hillsborough County to have, on average, chlorophyll-a concentrations lower than 20 µg/l (Robison and Eillers 2014). Atkins Lake Tarpon Water Quality Management Plan Final May

60 Figure 22. Lake Tarpon annual geometric chlorophyll-a mean compared to FDEP chlorophyll-a NNC for colored lakes. Analysis used only the County data. Atkins Lake Tarpon Water Quality Management Plan Final May

61 Figure 23. Lake Tarpon annual geometric TN and TP mean compared to FDEP NNC for colored lakes. Analysis used only the County data. NNC guidance states that high color lakes with chlorophyll-a concentrations higher than 20 µg/l are impaired for nutrients when annual geometric means exceed 1.27 and 0.05 mg/l for TN and TP, respectively. In the (d) list, the median values for the verified period are listed as 1.11 and 0.04 mg/l for TN and TP, respectively, values that are below the nutrient thresholds for impairment with NNC. These findings were confirmed by an independent assessment of the Lake Tarpon water quality data applying the NNC which indicated that Lake Tarpon is impaired for the biological indicator of chlorophyll-a, but not for nitrogen or phosphorus (Figures 22 and 23; Table 10). Atkins Lake Tarpon Water Quality Management Plan Final May

62 Table 10. Results of NNC evaluation for Lake Tarpon (WBID 1486A), WBID Waterbody Name Year Chlorophyll-a, corrected (µg/l) Geometric Mean Total Nitrogen (mg/l) Total Phosphorus (mg/l) 1486A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON A LAKE TARPON Note: Shaded cells indicate an exceedance of colored lake NNC criteria 5.2. Identification of nutrient of concern The determination of the limiting or co-limiting nutrients of concern can assist in the determination of projects for successful lake management. The ratio of TN and TP is a typical metric used to provide a cursory determination of the nutrient of concern responsible for phytoplankton production. For Lake Tarpon, the average TN:TP ratio for the period of is 31 which is just above the threshold of 30 that the FDEP uses to identify phosphorus limitation for algal growth (Figure 24; FDEP 2007). While identifying phosphorus as a nutrient of concern, additional evaluation of in-lake and watershed dynamics are necessary to explain changes in water quality in Lake Tarpon. A correlation between annual TN or TP and chlorophyll-a was performed to assist in identifying the sensitivity of phytoplankton production to the availability of nutrients. A strong significant correlation between TN and chlorophyll-a was identified (Figures 25; p= ; ρ=0.63). In contrast, there was not a statistically significant correlation between TP and chlorophyll-a (Figure 26; p>0.05). The results of the TN and TP correlations analysis indicate that an increase in nitrogen availability, but not phosphorus, is correlated with an increase in phytoplankton production, regardless of the TN:TP ratio. Atkins Lake Tarpon Water Quality Management Plan Final May

63 Phosphorus-limited above line Nitrogen-limited below line Figure 24. Total Nitrogen:Total Phosphorus ratio over time in Lake Tarpon. Figure 25. Correlation between annual geometric TN and Chlorophyll-a in Lake Tarpon ambient water quality over the period of (p= ). Graphic provides a visual representation of the degree of correlation. Atkins Lake Tarpon Water Quality Management Plan Final May

64 Figure 26. Correlation between annual geometric TP and Chlorophyll-a in Lake Tarpon ambient water quality over the period of (p=>0.05). Graphic provides a visual representation of the degree of correlation External nutrient load relevance to water quality Quantification of external nutrient loads is most important (from a resource management aspect) if a corresponding link to ambient lake water quality is identified. As appropriate, annual nutrient loads from each component were compared to Lake Tarpon nutrient and chlorophyll-a concentrations. Comparison of annual time steps were utilized to identify potential external or internal drivers impacting Lake Tarpon water quality as it is consistent with the management paradigm related to lake level and SAV management. If fluctuations in nutrient loading are evident based on changes in land use, and regional and/or seasonal rainfall or infrastructure degradation substantially impact the nutrient availability within Lake Tarpon, a commensurate response in phytoplankton production should be evident. Appendix E provides summary tables of the nonparametric correlations completed to evaluate the pollutant of concern and/or potential causative factors impacting water quality in Lake Tarpon. Time steps other than an annual basis were also examined, with results summarized in Appendix F. On a monthly time step, for example, there is a positive correlation between TN loads and chlorophyll-a, as opposed to the inverse relationship found when TN loads are compared to average chlorophyll-a values on an annual time step. The basis for the positive correlation appears to be due to the influence of water temperature. Water temperature alone explains more of the month to month variation in chlorophyll-a concentrations than TN loads, as the months with the highest water temperature have the highest chlorophyll-a concentrations, and months with the lowest water temperatures have the lowest chlorophyll-a concentrations. Consequently, individual months with the lowest TN loads do not have the lowest chlorophyll-a concentrations, and months with the lowest chlorophyll-a concentrations do not have the lowest TN loads (Appendix F). As both NNC criteria and the State of Florida s TMDL program (as well as the Tampa Bay Nitrogen Management Consortium s Reasonable Assurance Plan) are based on annual time steps, the Atkins Lake Tarpon Water Quality Management Plan Final May

65 remainder of this chapter focuses on the relationships between forcing factors and water quality on an annual time step Relationships with ambient water quality In the most recent lake TMDL documents produced by FDEP (FDEP 2014 a,b,c,d), total annual rainfall and average annual chlorophyll-a concentrations were used to investigate whether stormwater run-off had a direct impact on water quality. For scenarios in which a positive correlation between rainfall and chlorophyll-a concentrations were observed, stormwater run-off and groundwater discharge should be investigated as a prominent nutrient source. For Lake Tarpon, no significant Spearman correlation between phytoplankton production and annual rainfall was identified (p=0.12). Similarly, no correlations were found between annual TN or TP atmospheric deposition loads and ambient in-lake nutrient or chlorophyll-a concentrations (Tables 11 and 12). Those significant correlations that were identified indicate inverse correlations suggesting a reduction in ambient nutrients or phytoplankton production with increased external nutrient loadings, regardless of source (Tables 11 and 12) Evidence of historical land use changes A reduction in citrus-related agriculture is widespread throughout Florida, but different patterns exist in the County, compared to the state as a whole. Due to a combination of land development (commercial or residential), freezes, citrus canker and citrus-greening, the acreage of farmland devoted to citrus has declined since the 1960 s in the County (data from FDACS; Figure 27). Statewide, citrus increased dramatically from the 1920s to the 1960s, followed by declines in the 1980s perhaps precipitated by hard freezes in 1983 and 1985, followed by slight increases during the 1990s, and then a more recent reduction perhaps associated with citrus greening. Table 11. Spearman Correlation Analysis of Rainfall or Total Nitrogen Annual Loads with Lake Tarpon Ambient Total Nitrogen or Chlorophyll-a concentrations. Lake Tarpon Total Nitrogen Chlorophylla Statistics Rainfall (in) Atmospheric Deposition Total Nitrogen Annual Loadings (tons) Basin runoff Groundwater Brooker Creek Total Inflow ρ p-value N ρ p-value N *bold denotes statistically significant correlations Atkins Lake Tarpon Water Quality Management Plan Final May

66 Table 12. Spearman Correlation Analysis of Rainfall or Total Phosphorus Annual Loads with Lake Tarpon Ambient Total Phosphorus or Chlorophyll-a concentrations. Lake Tarpon Total Phosphorus Chlorophylla Statistics Rainfall (in) Atmospheric Deposition Total Phosphorus Annual Loadings (tons) Basin runoff Groundwater Brooker Creek Total Inflow ρ p-value N ρ p-value N *bold denotes statistically significant correlations In contrast to the more dynamic state-wide pattern, citrus in the County shows evidence of simply a decline over time, dating back to the 1950s. Much of the County was historically managed for citrus (Figure 27). However, more recent data reflects extensive urbanization within the County, with only 0.3% (625 acres) classified as agricultural (Pinellas County Planning Department 2008). An evaluation of the soils within the watershed contributing to Lake Tarpon (including Brooker Creek and South Creek) indicated that the well-drained Type A soils typically favoured by citrusgrowers were common only on the west side of Lake Tarpon. This finding was corroborated by aerial imagery (1951) for the southwestern portion of the Lake Tarpon watershed which shows evidence of agriculture adjacent to the lake (Figure 28). Previous studies in Polk, Seminole and Orange Counties have linked recent improvements in water quality within various lakes and springs to declining citrus production within the receiving water body watershed or (for spring-fed systems) groundwater recharge zones (Atkins 2009; ESA and Atkins, 2015; ESA and Atkins, 2016). Although there is no specific evidence indicating that nitrogen loads to Lake Tarpon were higher in the early-1950s, it is likely that that was the case, as citrus groves were a much more dominant feature of the general Pinellas County landscape (Figure 27). Atkins Lake Tarpon Water Quality Management Plan Final May

67 Figure 27. Trend in acres of land classified as Orange Bearing in Florida and as Orchards in the County (data from FDACS). Atkins Lake Tarpon Water Quality Management Plan Final May

68 Figure 28. Historical aerial photography from 1951 with scattered characteristic features of citrus related agriculture adjacent to the west side of Lake Tarpon Influence of lake elevation and residence time on water quality A significant positive correlation between annual rainfall and water elevation was identified (p=0.0306; ρ=0.509). Based on the high infiltration soils within the Lake Tarpon watershed, water elevation is likely to respond quickly during rainfall events. Evaluations of Lake Tarpon water elevations compared to chlorophyll-a and nutrient concentrations were performed and are presented in Appendix E. A significant inverse correlation was identified between annual water elevation and phytoplankton abundance, indicating that chlorophyll-a concentrations declined as water elevation rose (p<0.044; Figure 29). Additionally, a direct correlation between the coefficient of variation (the standard deviation divided by the mean) in annual water elevation and chlorophylla was found (Figure 30; p=0.004; ρ=0.637) indicating that larger fluctuations in water elevation within a given year resulted in increased chlorophyll-a concentrations. No correlations were found between TN and TP and water elevation (p=0.407 and p=0.597, respectively). However, other factors also vary as a function of the amount of rainfall in a given year, including that of the residence time of the water within the lake which was evaluated here as the relationship between water age for a given year and the annual average chlorophyll-a value for that same year. When examined, it was found that annual average chlorophyll-a values were inversely correlated with the average water age. Thus years with high rainfall have both higher lake levels Atkins Lake Tarpon Water Quality Management Plan Final May

69 and (on average) younger water; therefore the residence time of water in Lake Tarpon is shorter in years of above-average rainfall. Those years of above average rainfall thus have a number of features that influence water quality: the lake elevation is higher, the residence time of the water in the lake is shorter, and the amount of variation in lake elevations is less. These factors all influence water quality, so that lake water quality is best in years of above-average rainfall, even though years of above-average rainfall are also years with above-average stormwater loads and above-average external TN loads. Figure 29. Significant inverse correlation between annual average water elevation and chlorophyll-a concentrations (p=0.044). Atkins Lake Tarpon Water Quality Management Plan Final May

70 Figure 30. Significant direct correlation between the annual coefficient of variation in water elevation and chlorophyll-a concentrations (p=0.004) Submerged Aquatic Vegetation Sequestering of nutrients from the water column by SAV communities is well documented (Kadlec and Wallace 2009, Knight et al. 2003, Blindow et al. 2002, Canfield et al. 1984, Havens 2003, Shireman et al. 1985) and, as such, SAV is an important consideration for improving water quality in lakes. Prior work has found that the amount of lake volume occupied by SAV can strongly influence phytoplankton biomass (e.g., Canfield et al. 1984, Moreno 2010). Quantifying the relationship between SAV and nutrients in central Florida lakes is impossible in the absence of SAV abundance data. Robison and Eilers (2014) correlated SAV data from 98 Hillsborough County lakes with corresponding water quality which indicated an inverse correlation between chlorophyll-a and percent area covered (PAC). In other words, greater SAV coverage resulted in lower chlorophyll-a concentrations within the lake. Managing SAV lakes using PAC as a metric was proposed whereby a PAC 45 percent was considered optimal (Robison and Eilers 2014) Treatment processes for nuisance SAV (including hydrilla) There are multiple control methods used to manage hydrilla; chemical, mechanical, biological and physical (IFAS 2016). Mechanical control requires the physical removal of the plant material using machinery to cut the aquatic plant below the water surface. The cut plant material is then removed from the waterbody via a harvester or track hoes and draglines. Ultimately, the plant material is removed from the site for final disposal. Alternately, shredders are another form of mechanical control which cut, chop, slurry or shred the vegetation at which time the organic material is permitted to decompose at the waterbody bottom or harvested from the surface. In regards to biological control, grass carp (an herbivorous fish) are successful at consuming aquatic vegetation but are not selective in their consumption of plant material, potentially resulting in significant reduction of the native SAV population. Based on a recent SAV survey performed in Lake Tarpon (USF 2015), there is an extensive and healthy SAV community within the lake which could be compromised with the addition of grass carp. The physical control for nuisance aquatics has been Atkins Lake Tarpon Water Quality Management Plan Final May

71 performed by multiple techniques including manual removal, barriers (fences, booms, and cables), cutting/shearing, water level fluctuations, sediment removal, light attenuations, nutrient manipulation, and/or aeration. For water level fluctuations to be successful, water elevations must be drawn down sufficiently to expose the lake bottom and prevent tuber development within the hydrosoil (Shireman 1980). Lake level modifications to this extreme are not feasible in Lake Tarpon. Chemical controls via herbicide applications have been successful at temporarily controlling hydrilla infestations; however, the plant organic material is generally left in place to degrade and decompose within the waterbody resulting in a release of nutrients back into the water column. Herbicide application has been the management tool preferred for the control of hydrilla within Lake Tarpon. The large-scale treatments have been correlated with elevated phytoplankton production in the lake, ostensibly related to the influx of nutrients associated with decaying hydrilla plant material. As such, physical or mechanical control is recommended ensuring that the plant material be removed from the system Correlation with ambient water quality SAV communities are categorized into two categories: native and non-native. Unfortunately, the results of water quality improvements and non-native invasive species control sometimes conflict. However, there is information regarding the impact of SAV management on water quality in other lakes in Florida, as well as anecdotal information from various lakes in central Florida. In Lake Tarpon, large-scale hydrilla treatments is positively correlated with Chlorophyll-a concentration for the years 1994 to 2002 (Figure 31). The data set used in Figure 31 is restricted to a sub-set of the total years for which both parameters (chlorophyll-a and acres of hydrilla treated) are available, as they represent the time period during which the authors of this report have the greatest confidence. For example, Figure 32 shows approximately 500 acres of hydrilla were mapped in 1992, prior to 350 acres of hydrilla treatment quantified in After 2000, the data set displayed in Figure 32 shows an excess of 300 acres of hydrilla were treated in 2006, yet the highest amount of hydrilla acreage mapped between 2002 and 2006 was not even half the amount that was estimated to have been treated in Therefore, the data shown in Figure 31 represents a time period during which the amount of hydrilla and the amount of hydrilla treated with herbicides matches up better (although not perfectly) when trying to determine if a relationship between chemical treatment of hydrilla and water quality might exist. A linear regression equation was used (rather than correlation analysis) to allow for the derivation of an empirical relationship between the two variables. The findings displayed in Figure 31 support the prior finding that while herbicide treatments to control non-native invasive SAV are an important component of lake management, large-scale removal of SAV also removes a dissolved nutrient reduction mechanism and can result in a decline in water quality. Atkins Lake Tarpon Water Quality Management Plan Final May

72 Annual average chlorophyll a (ug/l) y = x R² = ; p < Acreage treated Figure 31. Annual Average Chlorophyll-a Concentrations VS Acreage of hydrilla Treated in the Same Year (1994 to 2002). Since 2009, minimal hydrilla coverage has been identified within Lake Tarpon (Figure 32) and no hydrilla treatments have been performed. In the absence of hydrilla, recent mapping efforts completed by USF (2015) indicate extensive native SAV along the shoreline of Lake Tarpon. Prior to 2009, there were intermittent periods of expanded hydrilla coverage corresponding with increased hydrilla treatment. The highest recorded extent of hydrilla coverage was reported in 1992 which corresponded with the lowest annual chlorophyll-a concentrations (Figure 32). In contrast, the highest reported annual chlorophyll-a concentrations, in 2000, correspond with the most extensive Hydrilla treatment effort. A more thorough visual inspection of water quality trends in Lake Tarpon indicates that a discernible degradation in water quality is observed within a 2 to 3 year period of substantial hydrilla treatment. The connection between water quality and large-scale hydrilla management efforts is complex, and other factors also influence water quality (such as lake elevations and variability in lake elevations). However, the majority (not all) of the highest chlorophyll-a values have occurred within a few years after the largest hydrilla treatment efforts, and the link between such treatments are consistent with the conclusions presented within the 2001 SWIM Plan (SWFWMD 2001). As such, the management of hydrilla coverage and associated treatment practices should be considered when making decisions related to water quality improvements. Additionally, the development of a site-specific alternative criteria (SSAC) for chlorophyll-a is deemed useful to help determine the appropriateness of the default standard of 20 µg/l as concentrations have exceeded 20 µg/l during three of the last six years in the absence of hydrilla treatments. Atkins Lake Tarpon Water Quality Management Plan Final May

73 Figure 32. Period of Record data depicting hydrilla treatment and coverage associated with corresponding annual total nitrogen, total phosphorus, chlorophyll-a, rainfall and water elevation. Hydrilla is capable of growing under very low light conditions when compared to other aquatic vegetation. While aquarium studies have reported hydrilla thriving with light levels as low as 7 to 20 µe/m 2 s (0.35 to 1 %; Van et al. 1976, Bowes et al. 1979, Steward 1991), lake measurements report percent light transmittance on average of 1.76% (Canfield et al. 1985) or 19% (Caffrey 2006). In order to determine the minimum light requirement for Hydrilla in Lake Tarpon, the following calculations were completed. The minimum light available in Lake Tarpon was calculated using Beers law. Initially, the light attenuation coefficient (k) was calculated using monthly average Secchi depth values over the period of January 1993 to December 2013 using Equation 1. Values ranged from 0.79 to 3.77 with a median of 1.89 (Table 13). The percent subsurface irradiance (Equation 2) was calculated using one foot water depth increments for multiple k scenarios (Figure 33). A recent SAV and bathymetry survey completed by USF (2015) indicates that Lake Tarpon extends to 24 feet deep in some areas. The mapped SAV deep edge in Lake Tarpon was 6.25 ft of water depth (not 6.25 feet NGVD29) based on data collected in September 2015 (USF 2015) which is in agreement with the percent subsurface irradiance calculated based on the median light attenuation coefficient ( 2.7%; Figure 34). The Lake Tarpon water elevation, taken upstream of the Lake Tarpon Canal Outfall Structure, ranged from 3.12 to 3.28 ft NGVD 29 over the period in which the bathymetry and SAV mapping effort were performed (Figure 35). In 2014, the Lake Tarpon water elevation ranged as high as ft NGVD 29. Under a scenario in which the Lake Tarpon water elevation is reduced from 3 to 2 ft NGVD 29, an additional 117 acres would be available for Hydrilla or SAV growth. A reduction in water elevation from 3 to 1 ft NGVD 29 could make an Atkins Lake Tarpon Water Quality Management Plan Final May

74 additional 320 acres susceptible to Hydrilla infestation due to increase in light irradiance under lower water elevations. Equation 1: Equation 2: k= 1.65/SD where SD= secchi depth in meters. Iz=Io * e -kz Where: Iz = light at depth z in meters, Io = light at water surface, k= light attenuation coefficient, and z is the depth in meters. Table 13. Summary statistics generated from Lake Tarpon light attenuation coefficient Summary Statistic K Minimum % Percentile % Percentile 1.50 Median 1.89 Average 1.90 Maximum 3.77 Figure 33. Incremental subsurface irradiance based on water depth under multiple light attenuation scenarios for Lake Tarpon. Portion of lake (acres) represented by each one-foot water depth increment. Atkins Lake Tarpon Water Quality Management Plan Final May

75 Figure 34. Incremental subsurface irradiance using median light attenuation coefficient. The existing Lake Tarpon SAV deep edge is referenced (USF 2015). Figure 35. Lake Tarpon Water Elevation (ft NGVD 29) at the time of the SAV and bathymetry mapping effort performed by USF (2015). Atkins Lake Tarpon Water Quality Management Plan Final May

76 6. Proposed actions to protect and/or improve lake condition As shown in Section 5.1, water quality within Lake Tarpon exceeds NNC criteria for the biological response variable of chlorophyll-a, but not for the nutrients TN or TP. As such, phytoplankton biomass may be considered problematic from a regulatory standpoint, but nutrient concentrations are not, at least using NNC guidance as listed in the State of Florida s Surface Water Quality Standards (FAC ). These findings could be interpreted in two opposite manners that chlorophyll-a guidance criteria are overly restrictive for Lake Tarpon, or that nutrient guidance is not restrictive enough. Additional studies, include paleolimnological, should be performed to make a scientifically informed conclusion. Section 5.2 summarizes findings that support the earlier contention (SWFWMD 2001) that phytoplankton growth in Lake Tarpon is most strongly limited by the availability of nitrogen, rather than phosphorus. However, the analyses conducted in this report (summarized in Section 5.3) showed that variation in chlorophyll-a concentrations in the lake were not explained by TN loads from ungaged stormwater runoff, gaged TN loads from Brooker Creek, groundwater loads of TN, atmospheric loads of TN, or the combination of TN loads from all sources combined. Therefore, the traditional lake management paradigm wherein external loads of nutrients are viewed as the primary factor controlling water quality, does not appear to be valid for Lake Tarpon. The factor(s) that showed a statistically significant impact on chlorophyll-a in Lake Tarpon were lake levels, the annual variation in lake levels, and residence time (as quantified in terms of water age ). As shown in Section 5.4, chlorophyll-a concentrations and lake levels were inversely correlated high lake levels were associated with low chlorophyll-a concentrations and low lake levels were associated with high chlorophyll-a concentrations. In contrast, the annual fluctuation in lake levels was positively correlated with chlorophyll-a, such that years with greater variation in lake levels were more often than not years with higher concentrations in chlorophyll-a, and years with low levels of variation in lake levels also had, on average, lower chlorophyll-a concentrations. Lake levels and residence time are correlated with each other, and both reflect the influence of rainfall. In years with above-average rainfall, lake levels are higher, the variation in lake levels is reduced (since lake levels do not drop that low) and the residence time of water is reduced. Thus, the beneficial influences of years with high rainfall most likely manifest themselves through the combination of higher lake levels, shorter residence times, and reduced variation in lake levels. What is important to note is that those same years (with above average rainfall) are also years with above-average nutrient loads, but better than average water quality. As such, a lake management approach that solely focuses on external nutrient loads would most likely not be successful. While there was not a statistically significant (at p = 0.05) relationship between the acreage of hydrilla chemically treated with herbicides and chlorophyll-a, it was statistically significant during those years when large-scale hydrilla treatment projects were undertaken, and during which the data on hydrilla abundance and hydrilla treatment acreages are in their best agreement. It is also consistent with a wider body of literature (Terrell et al. 2000, Wang et al. 2013) that suggests that widespread herbicide applications can help to flip a lake from being macrophyte dominated to being dominated by phytoplankton. As such, and consistent with findings in the 2001 SWIM Plan, it would appear that controlling the abundance of hydrilla or expanding native SAV may be an important lake management concern. Atkins Lake Tarpon Water Quality Management Plan Final May

77 Consequently, management actions to protect the water quality and ecological health of Lake Tarpon are based on the need to manage for the following conditions: Maintain, as much as possible, as high of a lake level as can be done without compromising flood protection in the watershed Minimize, as much as possible, the variation in water levels in the lake while also taking into account that such conditions, while favourable for water quality, may be problematic for the growth of cattails along parts of the shoreline Hydrilla should be controlled in favor of the expansion of native SAV species. Large -scale chemical treatments should be avoided, in favor of mechanical and physical removal. Protect, preserve and expand native SAV plant communities which provide important water quality and habitat benefits Manage external loads at or below current levels to remain protective Refine current estimates of groundwater loads, and monitor groundwater quality, as groundwater appears to be a more dominant source of nutrients than stormwater runoff 6.1. Approaches to managing stormwater and groundwater Historically, external nutrient load reductions have been the default focus for lake management. More recently, awareness to the role internal processes play in lake water quality has been incorporated into lake management through the collaboration between regulatory agencies and researchers. For example, both FDEP and the USEPA previously explicitly stated, through various TMDLs, that local, regional, state and federal resources should be directed at reducing the external loads of nutrients to impaired waterbodies through various regulatory and non-regulatory programs. While this approach is broadly consistent with prior successful water resource management efforts, external nutrient loads are sometimes not the primary stressor to water quality in Florida lakes (e.g., Terrell et al. 2000). As shown in Section 5.3, there is no statistically significant correlation between external nutrient loads (on an annual time step) and concentrations of either TN or chlorophyll-a. This could indicate that existing stormwater permitting and regulatory programs have reduced the impact of stormwater loads, by reducing pollutants in stormwater runoff. However, there are relationships between external loads and water quality on a monthly time step, which indicate that nutrient loading should not be dismissed as irrelevant to the lake s water quality. The loading model conducted as part of this study found that groundwater was a more important source of nutrient loading than stormwater runoff. This finding is based on much more limited data than is available for stormwater runoff, which could have resulted in either under-reporting or overreporting the influence of groundwater. At a minimum, the quality of groundwater should be considered at least as important to water quality as stormwater runoff. Therefore, the collection and/or monitoring of the quality of groundwater should be a focus of future studies. As a test of the validity of the assumption that external nutrient loads are the primary factor influencing water quality, it is valuable to see if implementing TMDL-required loads has resulted in the desired benefit to water quality. One such test of that assumption includes Lakes Lulu, May and Shipp all located in the City of Winter Haven. All three lakes have fully met their regulationrequired reductions in stormwater loads of TP, yet a follow up study found that none of the lakes showed signs of improving water clarity or decreased concentrations of chlorophyll-a (Atkins Atkins Lake Tarpon Water Quality Management Plan Final May

78 2009). As disappointing as the results were for these lakes, the lack of response of water quality to reduced stormwater loads is consistent with the findings of Terrell et al. (2000) who examined water quality data from 127 Florida lakes over a 30-year period. Terrell et al. (2000) found overall trends of decreasing concentrations of TP, but no trend in TN over time. Despite the downward trend in TP and the lack of a trend in TN, chlorophyll-a concentrations clearly increased over time across the state. The authors concluded that altered hydrology and impacts from management efforts to control hydrilla through herbicide applications were more important influences on water quality than nutrient concentrations and/or nutrient loads. These findings should not be construed as suggesting that stormwater should be dismissed as potential stressors, or that stormwater projects are not worth considering, especially as much of the watershed was developed prior to the implementation of regulations on stormwater treatment; these areas might be candidates for stormwater retrofit, especially for those areas where runoff is routed directly to the lake. Of potential interest, a recent study in the Tampa Bay watershed found that wet detention ponds might be more effective at reducing the impact of road runoff than they are given credit for, in a regulatory sense (Tomasko et al. 2013). In that study, the authors found that wet detention ponds reduced TN concentrations by approximately 30 to 40 percent, comparing inflows to outflows of three different wet detention ponds, and consistent with efficiencies assumed by both regulatory programs and stormwater modeling efforts (such as for this project). In their study, the authors found that concentrations of dissolved inorganic nitrogen, the most biologically-available and problematic form of nitrogen, were reduced by more than 80 percent, on average (Tomasko et al. 2013) Approaches to managing lake levels Chlorophyll-a concentrations in Lake Tarpon are inversely correlated with lake levels, on both monthly and annual time steps (summarized in Section 5.4). That is, lowest chlorophyll-a concentrations mostly occur with higher lake levels and the highest chlorophyll-a concentrations occur during times of lower lake levels. These results are similar to findings in other parts of Florida where it was found that when there was a relationship between lake levels and chlorophylla, the majority of those relationships were inverse (where higher chlorophyll-a values occurred at low, not high lake levels; Atkins 2008). However, although the trend is accurately described as an inverse one, year to year variation can result in higher chlorophyll-a concentrations occurring in years of high water, and lower chlorophyll-a concentrations occurring in years of lower water levels. While the concept of managing lake levels in Lake Tarpon to increase the annual range of water levels was suggested as having potential value in terms of managing shoreline vegetation (e.g., KEA 1992, SWFWMD 2001), the results shown in Section 5.4 show that chlorophyll-a concentrations were positively correlated with the degree of variation in lake levels, at least on an annual time step. That is, the greater the amount of variation in water levels in a given year, the higher the chlorophyll-a concentration in Lake Tarpon, on average. While previous studies concluded that modifying lake levels would benefit emergent vegetation along the shoreline, many of the problem areas with monocultures of Typha latifolia, have been effectively managed through other means, such as the bulrush restoration project on the east side of Lake Tarpon, north of Chesnut Park. For fisheries, Allen et al. (2003) concluded that Lake Tarpon supports important recreational fisheries in Florida and ranks among the top lakes in the state for largemouth bass Micropterus Atkins Lake Tarpon Water Quality Management Plan Final May

79 salmoides. The authors of that study paid particular attention to the benefits (if any) to fisheries that could occur as a result of implementing proposed lake level modifications, as outlined in the Lake Tarpon Drainage Basin Study (Coastal Environmental 1996). Allen et al. (2003) concluded that low water level during spring and summer of 2000 was related to a weak largemouth bass year class and that the proposed minimum in the revised operating schedule may cause relatively weak largemouth bass year classes at Lake Tarpon. Overall, it was found the fisheries in Lake Tarpon were healthy, and that previously suggested actions to modify lake levels were not needed to either protect or restore the health of the lake s fisheries Approaches to managing submerged aquatic vegetation (SAV) As had been previously determined for Lake Tarpon (e.g., SWFWMD 2001) the amount of SAV in the lake appears to influence water quality. Section shows evidence of an overall pattern wherein highest levels of chlorophyll-a are associated with years with the greatest amount of chemical treatment of nuisance SAV. This correlation is highly variable, in part because water quality is most strongly influenced by chemical treatment of hydrilla, and hydrilla is no longer abundant in Lake Tarpon (see Figure 32). As previously documented, the amount of lake volume occupied by SAV can strongly influence phytoplankton biomass in Florida lakes (e.g., Canfield et al. 1984, Moreno 2010). Locally, Robison and Eilers (2014) correlated SAV data from 98 Hillsborough County lakes with water quality, and found an inverse relationship between chlorophyll-a concentrations and percent of the lake bottom with SAV coverage. In their study, Robison and Eilers (2013) determined that lakes in Hillsborough County typically had chlorophyll-a concentrations less than 20 µg / liter when more than 45 percent of the lake bottom was covered by SAV. In contrast, the SAV community in Lake Tarpon occupies about 21 percent of the lake bottom. The ability of Lake Tarpon to achieve a SAV coverage of 45 percent is restricted by the combination of water clarity and bathymetry, and it could be that Lake Tarpon could never achieve an SAV coverage of 45 percent. However, it is worth acknowledging that without such a high SAV coverage, it may be impossible for Lake Tarpon to achieve chlorophyll-a values lower than 20 µg / liter. Hydrilla is an invasive, exotic aquatic weed in Florida, which seems to have become established in the state in the 1960 s. The species has been shown to be able to grow at rates up to an inch a day, which allows it to out-compete most native species of SAV. Hydrilla also has several competitive advantages over native SAV related to modes of reproduction and its physiological ecology that result in it quickly becoming a nuisance if conditions allow for expansive growth (e.g., Canfield et al., 1985, Langeland 1996, Bowes et al. 1977, Van et al. 1976, Rybicki and Carter 2002, Caffrey 2006). Given the advantages listed here, this nuisance SAV species can quickly give rise to extensive monospecific stands which are difficult to manage. Should the public determine that hydrilla is seriously impeding boating access to portions of the lake, regulatory agencies can come under significant pressure to do something about the infestation. More often than not, this has resulted in large-scale chemical treatment of hydrilla. In response to chemical treatment, the typical water quality response is that of increased concentrations of chlorophyll-a. As shown in Section 5.5.2, eight of the ten highest annual average chlorophyll-a concentrations occurred within two years after the three years with the largest values for chemical treatment of hydrilla. As a result of nutrient release into the water column (after chemical treatment of hydrilla), the lake often experiences elevated levels of chlorophyll-a and reduced water clarity; these conditions often give rise to public desire to then do something about the water quality issues in the lake. The post-1993 sustained increase in chlorophyll-a could be explained if the previously extensive hydrilla coverage, which peaked in 1992, was mostly restricted to areas interior to the native SAV Atkins Lake Tarpon Water Quality Management Plan Final May

80 in the lake, and if the extensive hydrilla treatment efforts in the early 1990s focused mostly on areas interior (closer to the middle of the lake) compared to the current, mostly native, SAV beds. Unfortunately, there is insufficient data to prove or disprove this concept. But, the large scale hydrilla coverage that was lost in the early 1990s might represent an SAV coverage that has not yet been (and may not every be) replaced by native SAV. The total SAV coverage for Lake Tarpon, estimated at 21 percent, is roughly half the coverage (40 percent) required for lakes in Hillsborough County to have, on average, chlorophyll-a concentrations lower than 20 µg/l (Robison and Eillers 2014). If native SAV coverage could be expanded, perhaps by planting native species in areas with sufficient light but where a lack of propagules limits their spread, then such actions might be able to improve water quality, as would be predicted based on the relationships identified by Robison and Eillers (2014). A more holistic approach to lake management thus would involve proactive approaches to managing the abundance of hydrilla, to keep it from becoming problematic in the first place. As such, it is recommended that a technical advisory committee (TAC) be developed including at a minimum representatives from the County, SWFWMD and FFWCC to develop approaches to vegetation management. Fortunately, hydrilla is now a minor component of the SAV community in Lake Tarpon, comprising less than three percent of the SAV community lake-wide (Appendix G). As shown in Section 5.5.2, it appears that SAV of all types grow down to approximately six feet of water depth, as no SAV was found at depths greater than seven feet. Based on average water clarity measurements, it appears that SAV in Lake Tarpon grows down to a depth with just under two percent of the immediately sub-surface irradiance (Section 5.5.2). However, should the lake level be lowered via some activity, then a larger amount of lake bottom could become established with SAV, as more of the lake bottom would fall within the zone where two percent of sub-surface irradiance reaches the bottom. Due to growth aspects discussed above, the species most likely to first establish itself in newly available areas should lake levels be lowered would be hydrilla. Therefore, maintenance of a high (and non-varying) lake level would not only benefit water quality via the direct relationships discussed in Section 6.2, it would also help to minimize the potential impacts to water quality that could occur if hydrilla were to become established in deeper portions of the lake that could become newly available habitat, and if that hydrilla acreage was then treated via application of herbicides General overview of project types recommended At least on an annual basis, the results shown here do not indicate that large-scale stormwater retrofit projects are needed to restore water quality in Lake Tarpon. While stormwater management remains an important part of preserving and protecting water quality, large regional alum-treatment projects such as have been used in other systems do not appear to be necessary. However, this should not be misinterpreted to mean that stormwater runoff is not important to the lake, but that existing regulatory guidance on stormwater treatment appears to be sufficient to keep stormwater from requiring additional attention. In part, this could be due to the fact that much of Lake Tarpon s watershed, particularly on the western shoreline, is comprised of Type A soils, which can infiltrate rainfall quite rapidly, thus reducing direct runoff into the lake. As such, it might be worth considering using stormwater infiltration systems (SIAs) or raingardens as a stormwater treatment system for those areas where runoff is not effectively dealt with and as a proactive protective measure. In areas with soil types amenable to their use, raingardens are not only more effective at reducing nutrient loads - compared to typical wet detention ponds (e.g., Harper and Baker 2007) but they also can recharge the surficial aquifer in areas where high infiltration soils are covered with impervious features such as rooftops, roadways and parking lots. However, stormwater infiltration into the surficial aquifer must be done in a careful manner, so that a nutrient load from surface Atkins Lake Tarpon Water Quality Management Plan Final May

81 water runoff is not simply shifted into a problem with the surficial aquifer. Many factors control the fate of nutrients in groundwater, depending upon the nutrient of concern, as well as soil characteristics. For example, the nitrogen-limited waters of Lake Tarpon suggest that groundwater transport could be problematic, as nitrate can travel long distances in some soil types. However, should soils be conducive to the linked processes of nitrification and de-nitrification, then hardly any nitrogen loaded into groundwater would make it to the lake itself. Prior to widespread adoption of a raingarden type of approach to stormwater treatment, it would benefit the County to first determine if soil characteristics are such that nitrogen transport in groundwater would be problematic. For systems such as Lake Tarpon, where lake levels have been maintained at higher and less variable levels than was the case in years past, it appears that water quality, the SAV community and the lake s fisheries are in a healthy state. As groundwater was found to be a more important source of nutrient loading than stormwater runoff, the characterization and monitoring of groundwater quality should be considered. As in any model, output is a function of the quality of data. The characterization of water quality in groundwater is much more limited it terms of data than is the case for stormwater runoff. It is therefore recommended that the County consider additional efforts to characterize and/or monitor the quality of groundwater in the Lake Tarpon basin, and also to consider the value of directly measuring groundwater seepage, rather than solely relying upon the combination of water budget calculations and groundwater quality samples taken in the watershed, at some distance away from the lake s shoreline. Maintaining a higher lake level should create conditions that have been associated with the recent water quality in Lake Tarpon, as chlorophyll-a concentrations are inversely correlated with lake levels. However, Lake Tarpon does not meet NNC criteria for chlorophyll-a, even though it does meet NNC criteria for both TN and TP (Section 5.1). These results could be interpreted as the chlorophyll-a criteria is overly restrictive, or that the nutrient criteria are not restrictive enough. Additionally, the status and trends of SAV in Lake Tarpon should be mapped and monitored on a regular basis, given the importance of SAV on water quality. In 2014, FFWWCC and SWFWMD independently implemented hydroacoustic mapping of waterbodies within their jurisdiction. The frequency of such mapping efforts for Lake Tarpon will be largely dependent upon the available budget, staff and necessity. As the health of Tampa Bay is measured in part by the abundance of seagrass, so should the health of Lake Tarpon be measured, in part, by quantifying the species composition, areal coverage, and percentage of the water column occupied by both native and nuisance species of SAV. As in Tampa Bay, a bi-annual (every other year) monitoring frequency might be useful. Atkins Lake Tarpon Water Quality Management Plan Final May

82 7. Project Descriptions The projects described below are based on a hold the line approach to nutrient management. While there is no evidence that large-scale stormwater retrofit projects are warranted for the lake, future development is expected and the current healthy conditions of the lake could be compromised. The County s stormwater manual ( lays out a number of ways to reduce the impacts of future development and stormwater runoff. This document should be a guiding principle on how growth should be accommodated in the Lake Tarpon watershed. While large-scale stormwater projects are not recommended, there are still a number of actions that could be taken to further improve and protect the lake; help to monitor the lake to ensure water quality is maintained; or determine appropriate criteria to determine future water quality impairments. These projects are described in more depth in the following sections Stormwater Infiltration Areas / Raingardens Type A soils, while not common in the Brooker Creek watershed, are the most common soil type on the west side of Lake Tarpon (Figure 36). Figure 36. Spatial distribution of Type A soils in the Lake Tarpon watershed. Atkins Lake Tarpon Water Quality Management Plan Final May

83 Type A soils are characterized by having minimum infiltration rates of 0.30 to 0.45 inches per hour (Harper and Baker 2007) which indicates the greatest ability for stormwater to be retained of any soil type. Percolation rates of Type A soils can be as high as 6 inches per hour (USDA 2006) although the assumption of rates that high has not been tested in the Lake Tarpon watershed. The Winter Haven Chain of Lakes Water Quality Management Plan (Atkins 2010) determined that rather than using traditional wet detention ponds for stormwater treatment, Stormwater Infiltration Areas (SIAs or raingardens ) would be the preferred approach for dealing with untreated stormwater runoff from developed landscapes on Type A soils. As in the Lake Tarpon watershed, much of the development in Winter Haven took place on the well-drained Type A soils, but traditional approaches to lake management have involved the use of wet detention ponds next to the lake edge, rather than SIAs up on the higher elevations of the watershed (Atkins 2010). In the City of Winter Haven, the preferred stormwater treatment system outlined in the Water Quality Management Plan (Atkins 2010) was the use of SIAs or raingardens. SIAs are basically dry retention basins that are designed to reduce the impacts of stormwater runoff by directing runoff into the surficial aquifer in areas with the appropriate soil types. SIAs are also referred to as raingardens or dry retention systems and they can include features that promote infiltration into the groundwater via open areas of the correct soil types such as bio-infiltration basins (e.g., Figure 37) or through the use of underground slotted pipes designed to infiltrate stormwater runoff. Figure 37. Photos of two constructed SIAs / raingarden projects in City of Winter Haven. SIAs differ from traditional stormwater treatment projects in that they direct surface water to the surficial aquifer and they have the benefit of not only having higher nutrient removal efficiencies, compared to wet detention ponds (i.e., in excess of 80% removal of TN and TP; Harper and Baker 2007) but also increasing aquifer recharge due to greater surface water infiltration. In addition, SIAs can be constructed with minimal impacts to existing infrastructure (see photos above) and they can be designed to fit within existing infrastructure in urbanized landscapes. Additionally, Atkins Lake Tarpon Water Quality Management Plan Final May

84 SIAs can be constructed in such a manner that they do not reduce flood protection, by occupying land adjacent to existing inlets to storm sewer systems (Figure 38). When considered as being equivalent to dry retention systems, SIAs would be given greater credit for nutrient removal (as per Harper and Baker 2007) than the more traditional wet detention pond. As such, SIAs are not viewed by regulatory agencies as a way to contaminate groundwater, but as a method to more effectively remove TN and TP than is the case with more traditional stormwater treatment systems. Figure 38. Photo of constructed SIA / raingarden project in City of Winter Haven incorporating existing inlet to storm sewer system (at slightly raised elevation). Prior to implementation, it should be noted that many factors control the fate of nutrients in groundwater, and that the nitrogen-limited waters of Lake Tarpon should be protected at least as well with a raingarden-type system as with traditional stormwater detention ponds. While it is true that nitrate can travel long distances in some soil types, other soil types are ones that are conducive to the linked processes of nitrification and de-nitrification, which would result in hardly any nitrogen making it to the lake itself. Prior to widespread adoption of a raingarden type of approach to stormwater treatment, it would benefit the County to first determine if soil characteristics are such that nitrogen transport in groundwater would be problematic. By integrating SIAs into areas with existing flood protection infrastructure, such systems can be constructed in such a manner that they do not compromise flood protection at the expense of enhancing infiltration of stormwater into the surficial aquifer. The Type A soils in the Lake Tarpon watershed are mostly developed and much of the developed portions already have SIA-like treatment of stormwater. For example, much of the road runoff from US 19 is treated via grassy swales, which already act to reduce runoff via percolation. However, there are some areas where untreated stormwater runoff discharges directly into the Atkins Lake Tarpon Water Quality Management Plan Final May

85 lake; these locations are a small subset of the areas of developed land shown in Figure 39, which displays the land use within the areas of Type A soils in the Lake Tarpon watershed. Site visits to the residential areas identified on Figure 39 (D. Tomasko, personal observation) indicated that the area of the Tarponaire Mobile Resort, located just south of the Knights Sink / Tarpon Sink complex represents a location where untreated stormwater runoff (which directly flows into the lake) could potentially be diverted into an SIA or community raingarden. SIAs have been designed, permitted and constructed at a cost of less than $10,000 each. Although the amount of stormwater runoff that can be expected to infiltrate into the groundwater varies from site to site, the average ratio between the size of the watershed and the size of the SIA for proposed projects in the City of Winter Haven (Atkins 2010) was approximately 7 to 1. Therefore, an SIA designed to infiltrate the entirety of the untreated stormwater runoff from the Tarponaire Resort (which is approximately 13 acres in size) would need to be 1.9 acres in size. The Pinellas County Stormwater Manual should be used for size and design criteria for the raingardens or other SIAs in this area. The manual recommends designing individual rain gardens to accommodate runoff from drainage areas up to 3 acres each. These raingardens could be developed as a part of the community, as has been done in the City of Winter Haven (Figure 40). While there are sitespecific differences between costs associated with projects in the City of Winter Haven and the Lake Tarpon Watershed, the typical rain garden project in the City of Winter Haven (e.g., 28) has come in at a cost of approximately $7,000; although the cost of land and construction can vary tremendously. Atkins Lake Tarpon Water Quality Management Plan Final May

86 Figure 39. Land use type within areas with Type A soils in the Lake Tarpon watershed. Atkins Lake Tarpon Water Quality Management Plan Final May

87 Figure 40. Photos of constructed raingarden project in City of Winter Haven constructed as a community amenity, along with associated educational signage. Atkins Lake Tarpon Water Quality Management Plan Final May

88 7.2. Groundwater characterization and protection The reduction of nutrient loads via groundwater infiltration could be compromised if nutrient content of stormwater runoff was sufficiently high to exceed the assimilative capacity for removal of nitrogen through microbial uptake and/or denitrification. While it is possible (as per discussions with researchers working in the field) for groundwater contamination to occur in urban settings, this finding has been restricted to areas where high nutrient contents of stormwater runoff were due to the combination of large-scale reuse application (as in golf courses) in combination with wastewater effluent from treatment plants without (for example) biological nitrogen removal. Due to the importance of groundwater loads to the overall nutrient budget for Lake Tarpon, the characterization and monitoring of groundwater quality should be pursued. The County should consider additional efforts to characterize and/or monitor the quality of groundwater in the Lake Tarpon basin. In addition, the County should consider directly measuring groundwater seepage, which has been done in at least four lakes in the Winter Haven Chain of Lakes system (e.g., Atkins 2009). Such efforts can result in refinement of pollutant loading models, in some case by refining the water budget, and in other locations by refining the nutrient content loaded through groundwater seepage, rather than modifying the water budge estimates themselves. Based on the project conducted in the City of Winter Haven, a detailed groundwater seepage study for Lake Tarpon, with three to six sites, conducted over the course of an entire year, could be completed for perhaps $125,000 to $250,000, depending on the frequency of sampling, and the desire to also collect water level data at locations along the shoreline and uphill from paired seepage meters, which would be submerged in the lake itself Managing lake levels A major finding of this study is that current management of lake levels, as opposed to previously proposed modifications of lake levels (i.e., SWIM 2001, Atkins 1998) appear to be sufficient to protect the water quality of Lake Tarpon (see Section 5.4). As such, no recommendation is made at this time to adjust the structure or to alter the structure to release water at different times of the year. Instead, the recommendation is for the agencies involved with controlling the outfall structure to continue with the status quo procedures. To investigate the impacts of maintaining the lake water elevation at 3.2 feet NGVD29, an additional modeling effort was completed which showed that the current lake level management procedures do not cause or contribute to any existing flooding problems in the Lake Tarpon watershed (up to a lake elevation of 5.25 feet NGVD29). A more comprehensive documentation of the Tarpon Woods modeling effort can be found in Appendix H Managing aquatic vegetation The health of Lake Tarpon is intimately tied to the health of both lakeside and submerged aquatic vegetation (SAV). The findings of the report conducted by USF researchers (Appendix G) is that the SAV community in particular is healthy, with the previously problematic nuisance species hydrilla comprising less than three percent of the total SAV coverage. It is possible that the low frequency of occurrence of hydrilla is due in part to the maintenance of higher lake levels. More specifically, the lowering of lake levels (whether due to drought or management actions) could allow for both native and nuisance exotic species of SAV to grow into areas of the lake that were previously deeper than what could have been occupied otherwise. Due to several growth aspects, hydrilla could out compete other SAV species and be able to grow into newly available areas of the lake bottom, which could then grow to nuisance levels (even if water levels would then be raised again) with the possibility of subsequent pressure for lake managers to do something about such Atkins Lake Tarpon Water Quality Management Plan Final May

89 an infestation. Results summarized in Section 5.5 show the value of maintaining the status quo approach to SAV management, which is dependent upon the management of lake levels, as outlined in Section 7.3. It would be beneficial for the County to repeat the SAV mapping and interpretation report conducted as part of this report (Appendix G). The mapping and interpretive report were conducted at a total project cost of $30,000. Assuming a desire to map SAV in Lake Tarpon at a frequency similar to the mapping of seagrasses in Tampa Bay, which is done for a similar reason detecting trends (if any) in a biological indicator of system health SAV mapping would be conducted every two years. Beginning in 2014, the FFWCC initiated an annual hydroacoustic mapping effort using the subscription based BioBase software to provide bathymetry, soil composition, and vegetation biovolume data for selected lakes throughout the state, including Lake Tarpon. The hydroacoustic survey data is paired with point intercept data to supplement the mapping effort. This effort is estimated to cost $2,000 plus the cost of an annual BioBase subscription. In addition to the need to manage SAV, emergent aquatic vegetation (EAV) along the shoreline must also be appropriately managed. As an example of the types of results that should be replicated, the County s recently completed cattail management efforts on the eastern shoreline appear to have been successful in replacing a monospecific cattail fringe with a more diverse community of emergent species. The techniques applied by the County to manage cattails seem to be working, and should be repeated in other locations, as needed. In cases where monocultures of nuisance EAV are present, an approach consisting of the following steps is recommended: 1) physical removal of the above-ground biomass of the nuisance EAV, with biomass taken off-site rather than being left to decay within the lake or along the shoreline, 2) after an appropriate length of time (to be determined by monitoring), careful application of herbicides to reduce regrowth of nuisance EAV species, and 3) after die-off of regrowth of nuisance EAV by herbicide application, replanting of native species at suitable combinations of elevation and species, typically at 3-foot centers. Although costs can vary, prior experience in other lakes suggest that such work can be done at a rate of approximately $10,000 per acre Development of site-specific water quality criteria for Lake Tarpon The determination that Lake Tarpon is impaired is problematic and warrants further assessment. For Lake Tarpon, paleolimnological studies could help derive scientifically sound and locally relevant water quality targets for chlorophyll-a, as the lake meets NNC criteria for TN and TP but not for chlorophyll-a. An appropriately developed paleolimnological study could develop site specific and scientifically sound criteria for TN, TP and chlorophyll-a for Lake Tarpon, rather than having management actions viewed as being successful or not based largely on whether the lake met the more generic and state-wide standards set out in FAC For example, regardless of whether lakes are classified using TSI or the more recently-adopted NNC criteria, the highest allowable chlorophyll-a concentration for a high color lake such as Lake Tarpon is 20 µg chlorophyll-a / liter. In developing their criteria, FDEP has stated (FDEP presentation dated June 17, 2009) that guidance on NNC is based on results from Salas and Martino (1991). In that paper, the authors derived nutrient targets that match up well with FDEP guidance on nutrient concentrations for oligotrophic, mesotrophic and eutrophic lakes. However, none of the lakes examined by Salas and Martino (1991) had chlorophyll-a concentrations as high as what FDEP considered are likely to occur in lakes with nutrient concentrations as high as those shown for even eutrophic lakes. Additionally, Bachmann et al. (2012) listed water quality in lakes FDEP has considered representative of benchmark conditions. These lakes are meant to represent water bodies with low levels of human disturbance, enabling full support of the most sensitive designated use Atkins Lake Tarpon Water Quality Management Plan Final May

90 Upon examination of the data listed in Bachmann et al. (2012), 11 of the 30 benchmark lakes chosen by FDEP had mean TN values higher than the median value of annual means for Lake Tarpon during the years 1995 to Paleolimnological studies have provided evidence of the historical water quality conditions within the Central Florida waterbodies of Lake Hollingsworth and Lake Parker (Brenner et al. 1999) as well as Lakes Conine, Haines, Hartridge, Howard and May, all in Central Florida (Whitmore and Brenner 1995). In all seven of these lakes, paleolimnological studies indicated that default water quality targets used by FDEP to determine impairment thresholds were overly strict, in some cases by a significant amount. For example, paleolimnological examinations of Lakes Conine, Hartridge, Howard and May resulted in a determination that the target chlorophyll-a levels used in TMDL development (FDEP 2007) were increased from their default value of 5 µg Chlorophyll-a/L to 20 µg Chlorophyll-a/L, as supported by results from Whitmore and Brenner (1995). For Lake Hollingsworth, Brenner et al. (1999) show evidence that the impaired condition of Lake Hollingsworth in the 1990s was actually substantially improved over conditions that existed in the 1960s. If the historical conditions of Lake Tarpon are such that a chlorophyll-a concentration of 20 µg/l is overly conservative, this would result in the lake continuously failing established regulatory lake criteria well into the future, even if it had water quality conditions typical of pre-development conditions. While development of these alternative chlorophyll-a and nutrient targets would likely be done by researchers with prior experience in such techniques (e.g., Tom Whitmore with USF-St. Petersburg), the ability of FDEP to allow for target values of chlorophyll-a, TN, and TP that are higher than those contained within NNC (as outlined in FAC ) is key to how successful such efforts would be for guiding the County in managing the water quality of Lake Tarpon. The proposed and potential techniques for developing an alternative water quality target for Lake Tarpon are based on the following strategy: Determine the rate of sediment accretion in Lake Tarpon using techniques such as changes in lead isotopes or cesium-157 abundance over time (e.g., Whitmore and Brenner 1995, Brenner et al. 1999) Based on sediment dating techniques, identify a sediment depth that can be considered characteristic of pre-development conditions in the lake At or below that sediment depth, sufficient samples should be gathered so that a statistical distribution of data can be developed for derived values of chlorophyll-a, TP and TN (if possible) Based on a cumulative frequency plot (or similar techniques) water quality criteria could be established using, for example, the upper 80th percentile distribution to represent predevelopment conditions As outlined above, prior techniques to develop site specific water quality targets via reconstructing historical water quality conditions have been conducted on a variety of lakes in Central Florida. However, the results from such work have not always been determined to be adequate for the development of site-specific water quality targets for lakes. For paleolimnological work to become useful in the context of TMDLs and resource management activities, data need to be collected, analyzed and interpreted within the context of the development of a Type III SSAC, as outlined in Rule , F.A.C. Atkins Lake Tarpon Water Quality Management Plan Final May

91 Pinellas County should consider working with FDEP to convert results from paleolimnological work into a locally-derived SSAC for nutrients and chlorophyll-a for Lake Tarpon. As in any SSAC, the proposed alternative criteria must meet the following tests: 1) they must fully protect the designated use of the water body, 2) they must support or be consistent with the narrative nutrient criterion in subparagraph (47) (b), 3) they must be based on a sound, scientific rationale, and 4) if relevant, they must protect downstream waters. Based on previous experience working with the City of Lakeland, the cost of developing SSAC for Lake Tarpon would potentially come to something between $50,000 to $85,000, depending upon the costs of collecting paleolimnological samples, the need for sufficient replication, and the need for coordinating with FDEP and other agencies on the study design, interpretation of results, and the ease with which results could be integrated into SSAC guidance documents and then acted upon. Atkins Lake Tarpon Water Quality Management Plan Final May

92 8. References Cited Allen, M.S., Tate, W., Tugend, K.I., Rogers, M., & Dockendorf, K.J Effects of Water Level Fluctuations on the Fisheries of Lake Tarpon. Gainesville: The University of Florida Department of Fisheries and Aquatic Sciences. Atkins The Lake Tarpon Drainage Basin Management Plan. Prepared for Pinellas County. Tampa, Florida. Atkins Winter Haven Chain of Lakes ground water seepage study. Final draft. Prepared for the Florida Department of Environmental Protection, Bureau of Watershed Management, Tallahassee, FL. Atkins Winter Haven Chain of Lakes Water Quality Management Plan. Final draft for the City of Winter Haven, Winter Haven, FL. Atkins Prioritizing Future Actions Related to Impaired Lakes and the FDEP TMDL Program. Final report submitted to Polk County Parks and Natural Resources Division. Baltus, L. A. & Squires, A.P Atmospheric Nutrient Deposition in Two Watersheds of Pinellas County, FL (draft). Clearwater, Florida: Pinellas County Department of Environmental Management, Water Resources Management Section. Blindow, I., A. Hargeby, and G. Andersson Seasonal changes of mechanisms maintaining clear water in a shallow lake with abundant Chara vegetation. Aquatic Botany 72: Bowes, G., T. K. Van, L. A. Garrard and W. T. Haller Adaptation to low light levels by hydrilla. Journal of Aquatic Plant Management 15: Caffrey, A Factors affecting the maximum depth of colonization by submersed macrophytes in Florida lakes. M.S. Thesis paper. University of Florida Graduate School. P. 48. Canfield, D.E., Jr, J.V. Shireman, D.E. Colle, W.T. Haller, C.E. Watkins, II., and M.J. Maceina Predication of Chlorophyll a concentrations in Florida lakes: Importance of Aquatic Macrophytes. Canadian Journal of Fisheries and Aquatic Science 41: Canfield, D., K. Langeland, S. Linda, and W. Haller Relations between water transparency and maximum depth of macrophyte colonization in lakes. Journal of Aquatic Plant Management 23: Champeau, T.R Lake Tarpon Fisheries Investigations. Florida Game and Freshwater Fish Commission. Lakeland, FL. Champeau, T.R Lake Tarpon fisheries investigations. Florida Game and Fresh Water Fish Commission. Lakeland, FL. Coastal Environmental, Inc Lake Tarpon Drainage Basin Management Plan Task , Development of Monitoring Programs. St. Petersburg, Florida. Atkins Lake Tarpon Water Quality Management Plan Final May

93 Deitche, S.M. and D. Hicks Lake Tarpon Drainage Basin Management Plan Update. Prepared by Pinellas County Department of Environmental Management. Clearwater, Florida. ESA and ATKINS Lake Tennessee Water Quality Management Plan. Final Report submitted to Polk County Natural Resources. ESA and ATKINS City of Lakeland Water Quality Management Plan. Draft Report submitted to City of Lakeland. FDEP TMDL Report: Nutrient TMDL for the Winter Haven Southern Chain of lakes (WBIDs 1521, 1521D, 1521E, 1521F, 1521G, 1521H, 1521J, 1521K). Division of Water Resource Management, Bureau of Watershed Management, Tallahassee, Florida. FDEP 2014a. TMDL Report: Nutrient TMDL for Lake Bonny WBID 1497E. Prepared by FDEP Bureau of Watershed Management. Tallahassee, Florida. Pp. 48. FDEP 2014b. TMDL Report: Nutrient TMDL for Deer Lake WBID 1521P. Prepared by FDEP Bureau of Watershed Management. Tallahassee, Florida. Pp. 43. FDEP 2014c. TMDL Report: Nutrient TMDL for Lake Hollingsworth WBID 1549X. Prepared by FDEP Bureau of Watershed Management. Tallahassee, Florida. Pp. 47. FDEP 2014d. TMDL Report: Nutrient TMDL for Lake Lena WBID Prepared by FDEP Bureau of Watershed Management. Tallahassee, Florida. Pp. 47. Gill, A.C., A.K. McPherson, and R.S. Moreland Water quality and simulated effects of urban land use change in J.B. Converse Lake watershed, Mobile County, Alabama, : U.S. Geological Survey Scientific Investigations Report pp. Gremillion, P.T., C.E. Mericas and H.S. Greening Lake Okeechobee aquatic weed harvesting demonstration project. Final report prepared for the South Florida Water Management District, West Palm Beach, Florida. Harper, H. and D. Baker Evaluation of Current Stormwater Design Criteria within the State of Florida. Final Report to FDEP. 327 pp. Harper, H New Updates to the Florida Runoff Concentration (emc) Database. Oral Presentation at Florida Stormwater Association. December 2011, Tampa, Florida. Havens, K.E Submerged aquatic vegetation correlations with depth and light attenuating materials in a shallow subtropical lake. Hydrobiologia 493: Huber, W.C., P.L. Brezonik, J.P. Heany, R.E. Dickinson, S.D. Preston, D.S. Dwornik, and M.A. DeMaio A classification of Florida lakes. University of Florida Water Resources Research Center. Publication No. 72. Gainesville, Florida. Kadlec, R.H. and S.D. Wallace Treatment Wetlands. CRC Press. Boca Raton KEA Final Comprehensive Report: Lake Tarpon Diagnostic/Feasibility Studies. Final report submitted to the Pinellas County Department of Environmental Management. Atkins Lake Tarpon Water Quality Management Plan Final May

94 Kercher, S. And J. Zelder Flood tolerance in wetland angiosperms: a comparison of invasive and noninvasive species. Aquatic Botany 80: Knight, R.L., B. Gu, R.A. Clarke, and J.M. Newman Long-term phosphorus removal in Florida aquatic systems dominated by submerged aquatic vegetation. Ecological Engineering 20(1):45-63 Langeland, K Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), The Perfect Aquatic Weed. Castanea 61: Larsen, D.P., J. Van Sickle, K.W. Malueg, and P.D. Smith The effect of wastewater phosphorus removal on Shagwa Lake, Minnesota: Phosphorus supplies, lake phosphorus and chlorophyll a. Water Res. 13: Leggett, Brashears, and Graham, Inc and SDI Environmental, Inc Lake Tarpon Ground- Water Nutrient Study. Project Number Prepared for Pinellas County Department of Environmental Management and SWFWMD. Moreno, M Analysis of the relationship between submerged aquatic vegetation (SAV) and water trophic status of lakes clustered in Northwestern Hillsborough County, Florida. Water Air Soil Pollution. Pp. 8. National Agricultural Statistics Service (NASS) Pinellas County Lake Tarpon Drainage Basin Management Plan Update. Prepared by Pinellas County Department of Environmental Management. Clearwater, Florida. Poor, N Contribution of Industrial Atmospheric Ammonia Emissions to Nitrogen Loading in the Tampa Bay Estuary, Tampa, FL USA. Submitted to the FDEP Contract AQ-156. Reddy, K.R., G.A. O Connor, and C.L. Schelske (Editors) Phosphorus biogeochemistry of subtropical ecosystems. CRC Press LLV. 687 pages. Robison, D. and D. Eilers An assessment of the relationship between submerged aquatic vegetation and water clarity in Florida Lakes. Presentation at Florida Lake Management Society s 25th Annual Technical Symposium. June 16-19th, Stuart, Florida. Rybicki, N. and V. Carter Light and temperature effects on the growth of wild celery and hydrilla. Journal of Aquatic Plant Management. 40: Ryding, S.O Reversibility of man-induced eutrophication. Experiences of a lake recovery study in Sweden. Int. Rev. Hydrobiol. 66: Shireman, J., W. Haller, D. Canfield, and V. Vandiver. The impact of aquatic plants and their management techniques on the aquatic resources of the United States: An overview. Submitted to Environmental Research Laboratory Environmental Protection Agency. P Smith, V.H., G.D. Tilman, J.C. Nekola Eutrophication: impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems. Environmental Pollution 100: SWFWMD Lake Tarpon Surface Water Improvement and Management (SWIM) Plan. Atkins Lake Tarpon Water Quality Management Plan Final May

95 Terrell, J.B., Watson, D.L., Hoyer, M.V., Allen, M.S. and D.E. Canfield Temporal water chemistry trends ( ) for a sample (127) of Florida Waterbodies. Lake and Reservoir Management. 16: Tomasko, D.A., Keenan, E.H., Paynter, S. and M. Arasteh Assessing the environmental impact: Are stormwater ponds more effective than presumed? Florida Water Resources Journal. October 2013, pp Upchurch, S. B., Lake Tarpon Ground-Water Nutrient Study. Final report submitted to the Southwest Florida Water Management District. Prepared by ERM-South. USDA Soil Survey of Pinellas County, Florida. Final Report. 199 pp. USF Lake Tarpon Submerged Aquatic Vegetation Survey Deliverables, Notes and Observations. Final Deliverable to Pinellas County, Van, T., W. Haller, and G. Bowes Comparison of the photosynthetic characteristics of three submersed aquatic plants. Plant Physiology 58: Want, B., Fayun, L. and Z. Fan Nutrient release during the decomposition of submerged macrophyte (Hydrilla verticillate). Journal of Flood, Agriculture & Environment. 11: Atkins Lake Tarpon Water Quality Management Plan Final May

96 Appendices

97 Appendix A. Lake Tarpon Watershed and Water Quality Evaluation Report Atkins Lake Tarpon Water Quality Management Plan Final May 2017

98 Appendix B. Surface Water Resource Assessment Atkins Lake Tarpon Water Quality Management Plan Final May 2017

99 Appendix C. Modeling Plan Atkins Lake Tarpon Water Quality Management Plan Final May 2017

100 Appendix D. Monthly Pollutant Loads and Water Quality Results Atkins Lake Tarpon Water Quality Management Plan Final May 2017

101 Appendix E. Non-parametric analysis summary tables Atkins Lake Tarpon Water Quality Management Plan Final September 2016

102 Appendix F. Monthly Water Quality Data Analysis Memo Atkins Lake Tarpon Water Quality Management Plan Final May 2017

103 Appendix G. SAV Assessment Atkins Lake Tarpon Water Quality Management Plan Final May 2017

104 Appendix H. Lake Tarpon Modeling Scenarios Atkins Lake Tarpon Water Quality Management Plan Final May 2017

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