Major Professor: Christopher Meindl, Ph.D.
James Ivey, Ph.D.
Thomas Whitmore, Ph.D.
John Osegovic, Ph.D.
University of South Florida St. Petersburg
June 25, 2020
Crescent Lake is a stormwater retention water body situated in St. Petersburg, Pinellas County, Florida. The Florida Department of Environmental Protection recognizes Crescent Lake as impaired because the lake sustains high total phosphorus and chlorophyll-a concentrations. Prior work done by Jacobs Engineering Inc., an environmental consultant agency hired by St. Petersburg, has suggested that summer thermal stratification forms and that water quality declines in fall months with concurrent proliferation of phytoplankton. The cause of increased phytoplankton growth in the fall is not fully understood. The fall increase in phytoplankton might occur because of a mixing event that dissipates thermal stratification and introduces nutrients from bottom waters (hypolimnion) to surface waters (epilimnion). Reduced oxygen levels, which also have been observed in the fall, might be the result of anoxic hypolimnetic waters that reach the epilimnion. The objective of this study was to determine whether mixing causes phytoplankton growth and reduces dissolved oxygen in surface waters. Nutrient, chlorophyll-a and dissolved oxygen concentrations, as well as temperature and other limnological variables were measured at three depths in Crescent Lake from July to December 2019. A heat budget was calculated for comparison with other warm temperate lakes of Florida. This study showed that thermal stratification was present from July through mid-November with a disparity in concentration of several variables from the epilimnion to the hypolimnion. Mixing homogenized temperature, density and total dissolved solids in the water column for approximately a two-week period in mid-November. Mixing lowered dissolved oxygen concentrations in the epilimnion during this period. Nutrient concentrations increased in the epilimnion but decreased in the hypolimnion relative to the period of stratification. Chlorophyll-a in surface waters showed a steady increase from mid-September until early December and peaked just after the period of mixis. It appears that an increase in nutrients in the epilimnion during mixis might contribute to phytoplankton growth in Crescent Lake. The calculated heat budget of 4450 cal cm-2 is slightly lower than previously reported values of warm-temperate Florida lakes because Crescent Lake has a small volume and low light penetration.
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Stanley, Benjamin, "Determination of Thermal Stratification, Mixing Pattern, Water Quality and Heat Budget of an Urban Florida Lake" (2020). USF St. Petersburg campus Master's Theses (Graduate).