Platte Lake CROW WING & MORRISON COUNTIES

Transcription

1 Platte Lake CROW WING & MORRISON COUNTIES Summary Platte Lake is located in Crow Wing and Morrison Counties of north-central Minnesota, six miles northeast of the town of Harding at the north end of the Mississippi River-Sartell Watershed. Platte Lake has four inlets and one outlet, which classifies it as a drainage lake. The Platte River flows through the lake and provides a broad connection to Sullivan Lake. The Platte River then eventually joins the Mississippi River. Water quality data have been collected for Platte Lake since These data show that Platte Lake is eutrophic (page 8). Eutrophic lakes are usually shallow and have "green" water throughout the summer, with some possible larger algae blooms in late summer. In 2002, 2004 and 2006, Platte Lake phosphorus concentrations exceeded the Impaired Waters Standard (35 ug/l) for the Northern Lakes and Forest Ecoregion. Platte Lake is currently not considered impaired for eutrophication, but it was considered on the border-line in the 2008 assessment cycle. Platte Lake will be evaluated again in the 2010 assessment cycle, and currently transparency in Platte Lake is showing a statistically significant declining trend in water quality. Care should be taken to avoid any further decline in water quality in Platte Lake. The Platte Lake Property Owners Association has been involved in many activities including water quality monitoring and the Healthy Lakes Initiative. Vitals MN Lake ID: County: Crow Wing and Morrison Ecoregion: Northern Lakes and Forest Major Drainage Basin: Upper Mississippi River Latitude/Longitude: / Water Body Type: Public Waters Monitored Sites (Primary): 203 Monitored Sites (Secondary): 101, 102, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 3901, 3902 Invasive species present: Curly-leaf pondweed Physical Characteristics Surface area (acres): 1,661 Littoral area (acres): 1,611 % Littoral area: 97% Max depth (ft): 23 (m): 7.0 Mean depth (ft): 8 (m): Lakeshed size (acres): 11,467 Lakeshed : lake area ratio 6.9:1 Inlets 4 Platte River Outlets 1 Platte River Accesses 1 Data Availability Transparency data Chemical data Numerous yearly Secchi readings from through the MPCA CLMP program. Total Phosphorus and Chlorophyll a data have been collected in 1997, 2002, and Inlet/Outlet data Tributary data have been collected in 1997 and Recommendations For recommendations refer to page 13. Platte Lake Report generation: RMB Environmental Laboratories 1 of 13

2 Figure 1. Map of Platte Lake illustrating bathymetry, lake sample site locations, stream inlets and outlets and aerial land use. The pink shaded areas in the lake illustrate the littoral zone, where the sunlight can usually reach the lake bottom, allowing aquatic plants to grow. Lake Site Depth (ft) Monitoring Programs MPCA: CLMP: CLMP: (3902)* Primary Site 21 CWP: 1997, 2004; CLMP+: 2002; Outdoor Corps: (102) 15 CLMP: <5 shore CLMP: CLMP: CLMP: <5 shore CLMP: 2008 The following sites are not included in this table: sites (data only from , CLMP). KEY: MPCA Minnesota Pollution Control Agency; CWP Clean Water Partnership Grant, CLMP MPCA Citizens Lake Monitoring Program Platte Lake Report generation: RMB Environmental Laboratories 2 of 13

3 The information below describes available chemical data for Platte Lake through The data set is limited, and all parameters, with the exception of total phosphorus, chlorophyll a and Secchi depth, are means for just data. Minnesota is divided into seven ecoregions based on land use, vegetation, precipitation and geology. The MPCA has developed a way to determine the "average range" of water quality expected for lakes in each ecoregion. For more information on ecoregions and expected water quality ranges, see page 11. Parameter Mean Ecoregion Range 1 Impaired Waters Standard 2 Interpretation Total phosphorus (ug/l) > 35 Results are higher than the expected range for the ecoregion, Chlorophyll a (ug/l) > 12 which is most likely due to the Chlorophyll a max (ug/l) 24 <15 shallow nature of the lake. For more information about Impaired Secchi depth (ft) < 4.5 Waters Assessment, see page 12. Dissolved oxygen see page 8 Temperature data indicated that the lake was well mixed all summer long with no distinct thermal stratification. Total Kieldahl Nitrogen Indicates insufficient nitrogen to support summer nitrogen-induced (mg/l) algae blooms. Alkalinity (mg/l) Indicates a low sensitivity to acid rain and a good buffering capacity. Color (Pt-Co Units) Indicates fairly clear water with some tannins (brown stain from wetlands) or sediment. ph Characteristic of a hard water lake. Lake water with ph less than 6.5 can affect fish spawning and the solubility of metals in the water. Chloride (mg/l) Slightly above the ecoregion average but still considered low level. Total Suspended Solids 3.3 <1-2 Slightly above the ecoregion average but still considered low (mg/l) level. Conductivity (umhos/cm) Within the ecoregion average range. Total Nitrogen :Total Phosphorus 23:1 25:1 35:1 Indicates the lake is phosphorus limited, which means that algae growth is limited by the amount of phosphorus in the lake. Data Source: Minnesota Pollution Control Agency Assessment 1 The ecoregion range is the 25 th -75 th percentile of summer means from ecoregion reference lakes 2 For further information regarding the Impaired Waters Assessment program, refer to 3 Chlorophyll a measurements have been corrected for pheophytin Units: 1 mg/l (ppm) = 1,000 ug/l (ppb) Platte Lake Report generation: RMB Environmental Laboratories 3 of 13

4 Water Quality Characteristics - Historical Means Years monitored: Parameters Site *203 Total Phosphorus Mean (ug/l): Total Phosphorus Min: Total Phosphorus Max: Number of Observations: 30 4 Chlorophyll a Mean (ug/l): Chlorophyll-a Min: Chlorophyll-a Max: Number of Observations: 31 4 Secchi Depth Mean (ft): Secchi Depth Min: Secchi Depth Max: Number of Observations: *Primary site Site 201 Site 202 Site 207 Site 101 Figure 2. Platte Lake total phosphorus, chlorophyll a and transparency historical ranges. The arrow represents the range and the black dot represents the historical mean (Primary Site 203). Figure adapted after Moore and Thornton, [Ed.] Lake and Reservoir Restoration Guidance Manual. (Doc. No. EPA 440/ ) Platte Lake Report generation: RMB Environmental Laboratories 4 of 13

5 Transparency (Secchi Depth) Transparency is how easily light can pass through a substance. In lakes, it is how deep sunlight penetrates through the water. Plants and algae need sunlight to grow, so they are only able to grow in areas of lakes where the sun penetrates. Water transparency depends on the amount of particles in the water. An increase in particulates results in a decrease in transparency. The transparency varies year-to-year due to changes in weather, precipitation, lake use, flooding, temperature, lake levels, etc. The annual means for Platte Lake range from ft (Figure 3). Annual means for sites 201 and 202 are almost identical. Site 207 generally had the lowest transparency. For trend analysis, see page Transparency: Annual Means 7 Secchi Depth (ft) Site 201 Site 202 Site 203 Site Figure 3. Annual mean transparency for sites 201, 202, 203 and 207. Platte Lake transparency ranges from 2.5 to 15.5 feet throughout the summer. Figure 4 shows the seasonal transparency dynamics. Platte Lake transparency is highest in early June, declines steadily throughout the summer, and then rebounds somewhat in late September. Some lakes vary throughout the summer while some lakes stay constant. The transparency dynamics have to do with algae population dynamics and lake turnover. Platte Lake Report generation: RMB Environmental Laboratories 5 of 13

6 12 Seasonal Transparency Dynamics Secchi Depth (ft) May 24-May 7-Jun 21-Jun 5-Jul 19-Jul 2-Aug 16-Aug 30-Aug 13-Sep 27-Sep Mean Poly. (Mean) Figure 4. Seasonal transparency dynamics and year-to-year comparison (site 201). The yellow line represents the average best fit line. User Perceptions When volunteers collect Secchi depth readings, they record their perceptions of the water based on the physical appearance and the recreational suitability. These perceptions can be compared to water quality parameters to see how the lake "user" would experience the lake at that time. Looking at transparency data, as the Secchi depth decreases, the perception of the lake's physical appearance rating decreases. Platte Lake was rated as having "definite algae" 64% of the time between (Figure 5). 4% 2% Physical Appearance Rating 2% Crystal clear water 30% 30% Not quite crystal clear a little algae visible 64% Definite algae green, yellow, or brown color apparent 64% 4% High algae levels with limited clarity and/or mild odor apparent 0% Severely high algae levels Figure 5. Physical appearance rating, as rated by the volunteer monitor ( ). Platte Lake Report generation: RMB Environmental Laboratories 6 of 13

7 As the Secchi depth decreases, the perception of recreational suitability of the lake decreases. Platte Lake was rated as having "swimming and aesthetic enjoyment of the lake slightly impaired because of algae levels" 46% of the time from (Figure 6). 5% 8% Recreational Suitability Rating 8% Beautiful, could not be better 41% Very minor aesthetic problems; excellent for swimming, boating 46% 41% 46% Swimming and aesthetic enjoyment of the lake slightly impaired because of algae levels 5% Desire to swim and level of enjoyment of the lake substantially reduced because of algae levels 0% Swimming and aesthetic enjoyment of the lake nearly impossible because of algae levels Figure 6. Recreational suitability rating, as rated by the volunteer monitor ( ). Total Phosphorus Platte Lake is phosphorus limited, which means that algae and aquatic plant growth is dependent upon available phosphorus. Total phosphorus was evaluated in Platte Lake in 1997, 2002, and (Figure 7). The total phosphorus concentrations ranged from 6-49 ug/l, not including one 81 ug/l data point in The Impaired Waters Standard for the Northern Lakes and Forest Ecoregion is 35 ug/l. In 2002, 2004 and 2006, phosphorus concentrations exceeded the Figure 7. Historical total phosphorus concentrations (ug/l) for site 203 in Platte Lake (data sets from 1997, 2002, ). standard. Platte Lake is currently not considered impaired for eutrophication, but care should be taken to avoid any further decline in water quality. Phosphorus should continue to be monitored to track any future changes in water quality. Platte Lake Report generation: RMB Environmental Laboratories 7 of 13

8 Chlorophyll a Chlorophyll a is the pigment that makes plants and algae green. Chlorophyll a is tested in lakes to determine the algae concentration or how "green" the water is. Chlorophyll a concentrations greater than 10 ug/l are perceived as a mild algae bloom, while concentrations greater than 20 ug/l are perceived as a nuisance. Figure 8. Historical chlorophyll a concentrations (ug/l) for Platte Lake (data sets from 1997, 2002, and ). Chlorophyll a was evaluated in Platte Lake in 1997, 2002, (Figure 8). Chlorophyll a concentrations exceeded 10 ug/l each year except 1997, which indicates mild algae blooms. In 2005, chlorophyll a concentrations exceeded 20 ug/l, which indicates a nuisance algae bloom. Dissolved Oxygen Depth (m) Dissolved Oxygen (mg/l) Dissolved oxygen (DO) is the amount of oxygen dissolved in lake water. Oxygen is necessary for all living organisms to survive, except for some bacteria. Living organisms breathe oxygen that is dissolved in the water. Dissolved oxygen levels of <5 mg/l are typically avoided by game fish. Platte Lake is a relatively shallow lake, with a maximum depth of 23 feet and a mean depth of 8 feet. Dissolved oxygen profiles from 2003 indicate that Platte Lake mixes throughout the summer (Figure 9). The sunlight can reach the bottom of 97% of the lake, allowing aquatic plants to grow. These plants produce oxygen as a byproduct of photosynthesis, which keeps the water column fully oxygenated. Figure 9. Dissolved oxygen profile for Platte Lake on September 3, Platte Lake Report generation: RMB Environmental Laboratories 8 of 13

9 Trophic State Index Phosphorus (nutrients), chlorophyll a (algae concentration) and Secchi depth (transparency) are related. As phosphorus increases, there is more food available for algae, resulting in increased algal concentrations. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases. The results from these three measurements cover different units and ranges and thus cannot be directly compared to each other or averaged. In order to standardize these three measurements to make them directly comparable, we convert them to a trophic state index (TSI). Trophic State Index 100 Platte Lake TSI Total Phosphorus 53 TSI Chlorophyll-a 51 TSI Secchi 53 TSI Mean 52 Trophic State: Eutrophic Numbers represent the mean TSI for each parameter. The mean TSI for Platte Lake falls in the eutrophic range (Figure 10). There is good agreement between the TSI for phosphorus, chlorophyll a and transparency, indicating that these variables are strongly related. Platte Lake Eutrophic lakes (TSI 50-70) are characterized by "green" water most of the summer. "Eu" means true and the root "trophy" means nutrients; therefore, eutrophic literally means true nutrients or truly nutrient (phosphorus) rich. Eutrophic lakes are usually shallow, and are found where the soils are fertile. Eutrophic lakes usually have abundant aquatic plants and algae. Hypereutrophic Eutrophic Mesotrophic Oligotrophic Figure 10. Trophic state index chart with corresponding trophic status. TSI Attributes Fisheries & Recreation <30 Oligotrophy: Clear water, oxygen throughout Trout fisheries dominate. the year at the bottom of the lake, very deep cold water Bottom of shallower lakes may become anoxic (no oxygen). Trout fisheries in deep lakes only. Walleye, Tullibee present Mesotrophy: Water moderately clear most of the summer. May be "greener" in late summer. No oxygen at the bottom of the lake results in loss of trout. Walleye may predominate Eutrophy: Algae and aquatic plant problems possible. "Green" water most of the year. Warm-water fisheries only. Bass may dominate Blue-green algae dominate, algal scums and aquatic plant problems. Dense algae and aquatic plants. Low water clarity may discourage swimming and boating Hypereutrophy: Dense algae and aquatic Water is not suitable for recreation. plants. >80 Algal scums, few aquatic plants. Rough fish (carp) dominate; summer fish kills possible. Source: Carlson, R.E A trophic state index for lakes. Limnology and Oceanography. 22: Platte Lake Report generation: RMB Environmental Laboratories 9 of 13

10 Trend Analysis For detecting trends, a minimum of 8-10 years of data with 4 or more readings per season are recommended. Minimum confidence accepted by the MPCA is 90%. This means that there is a 90% chance that the data are showing a true trend and a 10% chance that the trend is a random result of the data. Only short-term trends can be determined with just a few years of data, because there can be different wet years and dry years, water levels, weather, etc., that affect the water quality naturally. There is not enough historical data to perform trend analysis for total phosphorus or chlorophyll a on Platte Lake. Sites 201, 202, 203 and 207 have enough transparency data to perform a shortterm and long-term trend analysis. The data was analyzed using the Mann Kendall Trend Analysis. Lake Site Parameter Date Range Trend Probability 201 Transparency Declining 99.9% 201 Transparency Declining 99.9% 202 Transparency Declining 99.9% 202 Transparency Declining 99.9% 203 Transparency Declining 99.9% 203 Transparency Declining 99.9% 207 Transparency Declining 99.9% Secchi Depth (ft) Transparency Trends for Platte Lake Site 201 Site 202 Site 203 Site 207 Linear (Site 201) Linear (Site 202) Linear (Site 203) Linear (Site 207) 0 6/15/95 6/15/96 6/15/97 6/15/98 6/15/99 6/15/00 6/15/01 6/15/02 6/15/03 6/15/04 6/15/05 6/15/06 6/15/07 6/15/08 Figure 11. Transparency trends (ft) for sites 201, 202, 203 and 207 from Sites 201, 202, 203 and 207 show a statistically significant declining trend in transparency from (Figure 11). The transparency has declined an average of approximately 1-2 feet in Platte Lake since Transparency monitoring should continue at all four sites so that this trend can be tracked in future years. Platte Lake Report generation: RMB Environmental Laboratories 10 of 13

11 Ecoregion Comparisons Minnesota is divided into seven ecoregions based on land use, vegetation, precipitation and geology. The MPCA has developed a way to determine the "average range" of water quality expected for lakes in each ecoregion. From , the MPCA evaluated the lake water quality for reference lakes. These reference lakes are not considered pristine, but are considered to have little human impact and therefore are representative of the typical lakes within the ecoregion. The "average range" refers to the 25 th - 75 th percentile range for data within each ecoregion. For the purpose of this graphical representation, the means of the reference lake data sets were used. Platte Lake is in the Northern Lakes and Forests Ecoregion. The total phosphorus and chlorophyll a for Platte Lake are higher than the expected ecoregion ranges (Figures 12a-b). The Secchi depth for Platte Lake is lower than the expected ecoregion average (Figure 12c). Platte Lake is relatively shallow compared to the other large lakes in Crow Wing County, which contributes to its differing water quality and dynamics Total Phosphorus (ug/l, ppb) Chlorophyll-a (ug/l, ppb) Secchi depth (ft) increased algae crystal clear 0 0 NLF Platte Lake Ecoregion NLF Ecoregion Platte Lake a b c 25 NLF Ecoregion Platte Lake Figures 12a-c. Platte Lake ranges compared to Northern Lakes and Forest Ecoregion ranges. The Platte Lake total phosphorus and chlorophyll a ranges are from 30 and 31 data points, respectively, collected in May-September of 1997, 2002, and The Platte Lake Secchi depth range is from 372 data points collected in May-September from Platte Lake Report generation: RMB Environmental Laboratories 11 of 13

12 Inlet/Outlet Data Assessment Inlet and outlet data was collected for Platte Lake in 1997 and There are three main inlets and one outlet to Platte Lake (Figure 13). In order to compare the inlets and outlets, two field measurements needed to be recorded: 1) the water velocity flowing at each site and 2) the concentration of phosphorus in that water. These two measurements are used to determine the phosphorus loading, or in other words, the lbs of phosphorus entering the lake from that source. Comparing loading, taking into account the water volume of each inlet, is more accurate than just comparing concentrations. A water source with high flow and low phosphorus could be putting as many pounds of phosphorus into the lake as a source with low flow and high phosphorus. Figure 13. Platte Lake inlet and outlet monitoring sites. Flow and total phosphorus data exist for Platte Lake; however, actual stream measurements such as cross sections and water depth are not available. So actual loading calculations cannot be completed, but general conclusions can be drawn by looking at the flow and total phosphorus data. Flow data show that, of the three inlets, S (Platte River) carries the largest volume of water into Platte Lake (Figure 14). Total phosphorus data show that S (Platte River inlet) and S (Platte River outlet) have the lowest phosphorus concentrations (Figure 15). In summary, the two minor inlets (S and S ) have higher phosphorus concentrations but lower flow than the major inlet (Platte River S ) and the outlet (Platte River S ). This result is typical in lakes. In order to get a better comparison of loading from the three inlets, the data should be updated and complete with water volume measurements. Figure 14. Platte Lake tributary flow. Figure 15. Platte Lake tributary total phosphorus. Platte Lake Report generation: RMB Environmental Laboratories 12 of 13

13 Assessment/Findings Recommendations Transparency Transparency monitoring at sites 201, 202, 203 and 207 should be continued annually. It is important to continue transparency monitoring weekly, or at least bimonthly, every year to enable year-to-year comparisons and trend analyses. Impaired Waters Assessment 303(d) List There are two main types of Impaired Waters Assessment for lakes: eutrophication (excess phosphorus) for aquatic recreation and mercury in fish tissue for aquatic consumption. Platte Lake was listed as impaired for mercury in fish tissue in the 2006 Impaired Waters List; however, it is part of the statewide mercury TMDL, so it was removed from the 2008 Impaired Waters List. Platte Lake is not listed as impaired for eutrophication. Aquatic Recreational Use Assessment 305(b) In the 2008 MPCA Aquatic Use Assessment (305(b)), Platte Lake was classified as having insufficient data for this assessment. Inlet/Outlet Assessment Platte Lake already has some tributary data (page 12). This data could be updated and supplemented with complete water volume measurements at the inlets and outlets to determine loading from each site. See page 12 for more explanation. Organizational contacts and reference sites Platte Lake Property Owners Association Crow Wing Soil and Water Conservation District DNR Fisheries Office Regional Minnesota Pollution Control Agency Office Regional Board of Soil and Water Resources Office N Platte Lake Rd Hillman, MN Clearwater Road, Baxter, MN 56425, (218) Minnesota Drive, Brainerd, MN 56401, (218) College Road, Suite 105, Baxter, MN 56425, (218) Minnesota Drive, Brainerd, MN 56401, (218) Platte Lake Report generation: RMB Environmental Laboratories 13 of 13

14 Platte Lake Lakeshed Assessment The lakeshed vitals table identifies where to focus organizational and management efforts for each lake. Criteria were developed using limnological concepts to determine the effect to lake water quality. Lakeshed Vitals Rating Major Basin Upper Mississippi River descriptive Major Watershed Mississippi River-Sartell descriptive Minor Watershed descriptive Lakeshed Platte Lake ( ) descriptive Ecoregion Northern Lakes and Forests descriptive Lake Area 1,662 acres descriptive Miles of Shoreline descriptive Miles of Stream 7.12 descriptive Miles of Road 31.5 descriptive Lake Max Depth 23 ft. (7 m) descriptive Lake Mean Depth 8 ft. (2.4 m) - Water Residence Time 2.5 years + Municipalities None + Sewage Management Individual waste treatment systems (septic systems and holding tanks inspections only for property - sales and building permit requests) Public Drainage Ditches None + Lake Management Plan Healthy Lakes & Rivers Partnership program, Lake Vegetation Survey/Plan Survey Completed Forestry Practices Two areas of proposed clear-cut with reserves in lakeshed - Development Classification General Development - Shoreline Development Index Total Lakeshed to Lake Area Ratio (total lakeshed includes lake area) 6.9:1 x Public Lake Accesses 1 x Inlets 4 Platte River, 3 unnamed x Outlets 1 Platte River x Feedlots 4 - Agriculture Zoning 20 acres within 200 ft. of lake; 1,539 acres > 200 ft. from lake - Public Land : Private Land 0.05:1 - Wetland Coverage 28.5% + Lake Transparency Trend Declining trend (99.9% probability) - Exotic Species Curly-leaf pondweed - Rating Key: + beneficial to the lake - possibly detrimental to the lake x warrants attention RMB Environmental Laboratories, Inc. 1 of Platte Lake Lakeshed Assessment

15 Lakeshed Understanding a lakeshed requires the understanding of basic hydrology. A watershed is the area of land that drains into a surface water body such as a stream, river, or lake and contributes to the recharge of groundwater. There are three categories of watersheds: 1) basins, 2) major watersheds, and 3) minor watersheds. Platte Lake is found within the Upper Mississippi River Basin, which includes the Mississippi River-Sartell Major Watershed as one of its sixteen major watersheds (Figure 1). The basin covers 20,000 square miles, while the Mississippi River-Sartell Watershed covers 1,019 square miles (approximately 652,439 acres). Platte Lake falls within minor watershed 15055, one of the 66 minor watersheds that comprise the Mississippi River-Sartell Major Watershed (Figure 2). Within this watershed hierarchy, lakesheds also exist. A lakeshed is defined simply as the land area that drains to a lake. While some lakes may have only one or two minor watersheds draining into them, others may be connected to a large number of minor watersheds, reflecting a larger drainage area via stream or river networks. Platte Lake falls within the Platte Lake ( ) lakeshed, covering 11,467 acres (includes lake area) (Figure 3). Even though Platte Lake receives water from minor watershed 15056, for the purpose of this assessment it is decided that only the immediate lakeshed be inventoried and assessed. Platte Lake Lakeshed Water Quality Protection Strategy Each lakeshed has a different makeup of public and private lands. Looking in more detail at the makeup of these lands can give insight on where to focus protection efforts. The protected lands (easements, wetlands, public land) are the future water quality infrastructure for the lake. Developed land and agriculture have the highest phosphorus runoff coefficients, so this land should be minimized for water quality protection. The majority of land within Platte Lake s lakeshed is made up of private forested uplands. This land can be the focus of development and protection efforts in the lakeshed. Developed Agriculture Private (79%) 17% Public (4%) Forested Uplands Other Wetlands Open Water County State Federal Land Use (%) 3.5% 20% 25% 6.5% 24% 17% 0% 4% 0% Runoff Coefficient Lbs of phosphorus/acre/ year Description Focused on Shoreland Cropland Focus of development and protection efforts Open, pasture, grassland, shrubland Protected Potential Phase 3 Discussion Items Shoreline restoration Restore wetlands; CRP Forest stewardship planning, 3 rd party certification, SFIA, local woodland cooperatives Protected by Wetland Conservation Act County Tax Forfeit Lands State Forest National Forest RMB Environmental Laboratories, Inc. 2 of Platte Lake Lakeshed Assessment

16 Figure 1. Upper Mississippi Basin and the Mississippi River-Sartell Watershed. Figure 2. Minor Watersheds & contribute water to Platte Lake. Figure 3. The Platte Lake ( ) lakeshed (Aerial imagery M). RMB Environmental Laboratories, Inc. 3 of Platte Lake Lakeshed Assessment

17 Land Cover / Land Use The activities that occur on the land within the lakeshed can greatly impact a lake. Land use planning helps ensure the use of land resources in an organized fashion so that the needs of the present and future generations can be best addressed. The basic purpose of land use planning is to ensure that each area of land will be used in a manner that provides maximum social benefits without degradation of the land resource. Changes in land use, and ultimately land cover, impact the hydrology of a lakeshed. Land cover is also directly related to the lands ability to absorb and store water rather than cause it to flow overland (gathering nutrients and sediment as it moves) towards the lowest point, Figure 5. The Platte Lake ( ) lakeshed land cover ( typically the lake. Impervious intensity describes the lands inability to absorb water; the higher the % impervious intensity the more area that water cannot penetrate in to the soils. Monitoring the changes in land use can assist in future planning procedures to address the needs of future generations. Phosphorus export, which is the main cause of lake eutrophication, depends on the type of land cover occurring in the lakeshed. Figure 5 depicts Platte Lake s lakeshed land cover. The University of Minnesota has online records of land cover statistics from years 1990 and 2000 ( Table 1 describes Platte Lake's lakeshed land cover statistics and percent change from 1990 to Due to the many factors that influence demographics, one cannot determine with certainty the projected statistics over the next 10, 20, 30+ years, but one can see the transition within the lakeshed from agriculture, grass/shrub/wetland, and water acreages to forest and urban acreages. The largest change in percentage, as well as acreage, is the increase in forest cover (23.6%), with increases of 23.6% and 818 acres, respectively. In addition, the impervious intensity has increased, which has implications for storm water runoff into the lake. The increase in impervious intensity is consistent with the increase in urban acreage. RMB Environmental Laboratories, Inc. 4 of Platte Lake Lakeshed Assessment

18 Table 1. Platte Lake's lakeshed land cover statistics and % change from 1990 to 2000 ( % Change Land Cover Acres Percent Acres Percent 1990 to 2000 Agriculture 2, , % Decrease Forest 3, , % Increase Grass/Shrub/Wetland 2, , % Decrease Water 1, , % Decrease Urban % Increase Impervious Intensity % 0 11, , % Decrease % Decrease % Decrease % Increase % Increase % Increase % Increase Total Area 11,467 11,467 Total Impervious Area (Percent Impervious Area Excludes Water Area) % Increase Demographics Platte Lake is classified as a general development lake. General development lakes usually have more than 225 acres of water per mile of shoreline and 25 dwellings per mile of shoreline, and are more than 15 feet deep. The Minnesota Department of Administration Geographic and Demographic Analysis Division extrapolated future population in 5-year increments out to These projections are shown in Figure 6 below. Compared to Crow Wing County as a whole, Platte Lake Township has a slightly lower extrapolated growth projection, while Roosevelt Township has a higher extrapolated growth projection. Percentage of 2006 Population 60% 50% 40% 30% 20% 10% Population Growth Projection Platte Lake Township; 2006 population: 347 Roosevelt Township; 2006 population: 571 Crow Wing County; 2006 population: 61,038 0% Extrapolation Figure 6. Population growth projection for Crow Wing County and the townships around Platte Lake. Figure excludes two Morrison County townships (source: RMB Environmental Laboratories, Inc. 5 of Platte Lake Lakeshed Assessment

19 Status of the Fishery (DNR, as of 07/14/2003) Platte Lake has a reputation for providing a good winter bluegill fishery and "hammerhandle" northern pike. Maximum depth of the lake is only about 23 feet and extensive vegetation beds, both emergent and submergent, can be found around the lake. Much of the north shore is characterized by floating bog and shrub/sedge swamps. The lake characteristics such as shallowness of the basin, presence of abundant vegetation, and infrequent low oxygen periods makes the lake better suited to support a fish community dominated by bullheads, crappies, and northern pike. Platte Lake shares a wide connection to Sullivan Lake under County Road 51. Boat traffic between the lakes is dictated by water levels and clearance under the road bridge. While northern pike abundance did show a decline from the last survey, numbers continue to be high. And despite a slight improvement in length and weight from the last survey, the pike are still small by anglers standards averaging 19 inches and 1.6 pounds. Bluegills showed an increase in abundance with an average size of 5.5 inches. A few larger bluegills up to 8 inches were observed but would be probably be a rare catch. Two other panfish species in Platte Lake are black crappies and pumpkinseeds. Crappie abundance appears normal for the lake type while pumpkinseed numbers were high. Average size of the crappies was about 8 inches while the pumpkinseeds averaged only about 5 inches in length. Crappies up to 11 inches have been documented in past surveys. The black crappie catches were much higher in the 1997 and 1993 surveys suggesting that the population may be cyclic in abundance. Finding a walleye in Platte Lake could be a challenge, but may be worth the effort as the average size seen in the survey was an impressive 22 inches. Walleye numbers did show an improvement from the last survey but continue to be low for the lake type. Despite almost annual walleye stocking in previous years until 1997, net catches have always been low. The current walleye population is sustained through natural reproduction in the lake, immigration from Sullivan Lake, and an occasional spring fry stocking. Largemouth bass are another gamefish species which could provide a memorable fishing trip. While the average size appears to be around 12 inches, trophy size bass up to 20 inches have been observed in previous surveys and reported by anglers. All three species of bullheads were sampled in the survey with the black bullhead being the most common. Despite the abundance of black bullhead, their numbers remain within the normal range when compared to similar lake types. Brown bullhead numbers were also within their normal range while yellow bullheads were high. Average size of the bullheads was between 9 and 10 inches although some larger bullheads up to 12 were observed. Other species of the fish community include yellow perch and bowfin or dogfish. Yellow perch numbers were down from the last survey and are low for the lake type which may be the result of high pike numbers. An abundant perch population is desirable to maintain good growth in the gamefish populations and they have also been found to help maintain a well balanced bluegill population with fast growing, quality individuals. See the link below for specific information on gillnet surveys, stocking information, and fish consumption guidelines. RMB Environmental Laboratories, Inc. 6 of Platte Lake Lakeshed Assessment