Ecological Considerations in Setting MFLs and Lake Regulation Targets for the Ocklawaha Chain of Lakes Rolland Fulton, Ph.D. Environmental Scientist VI Bureau of Environmental Sciences St. Johns River Water Management District
Goals and Constraints Our primary ecological goal in setting targets for water levels in Lake Apopka and the Harris Chain is healthy lake and floodplain wetland plant communities for fish and wildlife. Our primary economic constraint is flood control. Other important considerations are water quality, recreation, navigation, and water storage and supply.
Two Sets of Lake Level Targets Similar but Different Minimum flows and levels (MFL) Targets must be protected (St. Johns River Water Management District rule) Targets are minimums (more water is OK) Purpose is to prevent over-use of water Goal is to prevent significant harm Lake level regulation Targets used for modeling of regulation schedules Targets require both high and low levels (more water is not always OK) Purpose is fluctuation in lake levels for healthy littoral wetland communities Goal is to create optimal conditions
Ecological Value of Lake Level Fluctuation (1) Increased areas of wetland and submersed vegetation habitat Sufficient flooding of floodplain habitat No encroachment by upland plants Protection of organic soils Seed germination in exposed sediments Consolidation of exposed sediments Expansion to deeper areas during lowwater periods
Ecological Value of Lake Level Fluctuation (2) Benefits to fish and wildlife Increases in wetland and submersed aquatic vegetation habitat Production of invertebrates and forage fish Nesting, spawning and feeding habitat Consolidation of near-shore sediments
Ecological Value of Lake Level Fluctuation (3) Water quality improvements Increases in wetlands habitat filter runoff Reduction in resuspension of soft sediments, resulting from consolidation and stabilization by vegetation
Lake Littoral Wetland Upland s Communities Swamp Elevation Marsh SAV Submersed Aquatic Vegetation Distance
Ecological Values of Lake Littoral Wetlands Aquatic plant production and diversity Habitat for fish, birds, mammals, reptiles and prey species Fish spawning and juvenile nursery areas Protection of shorelines from erosion Water quality protection
Data from 20 field wetland transects are used to set target levels Up to 2,485 ft long Vegetation Soils Survey (horizontal and vertical)
Example of wetland transect data
Lake Level Targets are Defined Statistically 1. Magnitude (how high or low) 2. Duration (how long) 3. Return Interval (how often)
Elevation Target High Water Level Uplands Swamp High Target Sufficient flooding to protect hardwood swamp and prevent encroachment by upland species Water should be above the average elevation of hardwood swamp for at least 30 60 days at least once every 2 years (on average) Marsh SAV (Submersed Aquatic Vegetation) Distance
Elevation Target Average Water Level Uplands Swamp Average Target Protect wetlands and organic soils from over-draining Water should not drop lower than 0.3 ft below the average elevation of deep organic soils or of upper wetland habitat for more than 180 days No more than once every 1.7 year or 2 years (on average) Marsh SAV (Submersed Aquatic Vegetation) Distance
Elevation Target Low Water Level Uplands Swamp Low Target Expose marsh habitat for seed germination and sediment consolidation Water should drop to roughly the lower elevation of shallow marsh but not for more than 120 days No more than once every 5 years, but at least once every 10 years (on average) Marsh SAV (Submersed Aquatic Vegetation) Distance
Determining Whether Lake Level Targets are Met Actual or modeled lake levels over a long period are compared with targets. Were levels exceeded (magnitude)? For how long (duration)? How frequently did this occur (how often)?
Nine Water Resource Value Assessments for Each Lake Fish and wildlife Detrital material transfer Aesthetic and scenic Filtration and absorption of nutrients Sediment loads Water quality Recreation Navigation Maintenance of freshwater supply
Assessing Water Quality Impacts of MFLs Water level fluctuations beneficial to long-term water quality However, water quality does tend to deteriorate during low-water periods
Issues With Assessing Water Quality Impacts of MFLs in the Ocklawaha Chain of Lakes Basin restoration has resulted in lower nutrient loading and improved water quality. These changes make assessment of relationships between water levels and water quality more difficult.
Issues With Assessing Water Quality Impacts of MFLs in the Ocklawaha Chain of Lakes Hurricanes of 2004 had large impacts on water quality. Large pulse of external loading to the lakes in watershed runoff High winds and waves disturbed sediments and shallow vegetated habitats Varying periods of unusually poor water quality following hurricanes
Assessing Water Quality Impacts of MFLs Used water level and water quality data beginning in 2001, and excluded periods impacted by 2004 hurricanes. For each of basin lakes, evaluated relationships between water level and water quality (TP, TN, chlorophyll, TSS, Secchi transparency). Also examined water elevations at which fish kills were reported.
Assessing Water Quality Impacts of MFLs Define threshold water elevations of concern at which water quality parameter doubles (or Secchi depth transparency is halved), or at which fish kills frequently have been reported. Use hydrological modeling to determine whether frequency of occurrence of these water elevations of concern change under MFL conditions.
Hypothetical Example of Negligible Effect of MFLs on Metric Water Quality Threshold elevation Duration (days) Number of years threshold elevation not exceeded Existing MFL Difference Chlorophyll 62.3 7 6.3 6.5 0.2 Chlorophyll 62.3 30 3.9 4.0 0.1 Chlorophyll 62.3 120 0.8 0.8 0
Metric Hypothetical Example of Substantial Effect of MFLs on Water Quality Threshold elevation Duration (days) Number of years threshold elevation not exceeded Existing MFL Difference Chlorophyll 62.3 7 6.3 20.5 14.2 Chlorophyll 62.3 30 3.9 16.0 12.1 Chlorophyll 62.3 120 0.8 10.8 10