SEASONAL FISH KILL CONTROLS WITHIN A SMALL LAKE IN BLOOMINGTON, ILLINOIS

Shallow lakes are prone to a depletion of dissolved oxygen in the summer and winter months, which can lead to fish kills. Oxygen depletion can be caused by diurnal fluctuations in oxygen generation. During the day, algae generate oxygen and consume it at night. A disruption in this cycle caused by phytoplankton die-off, lake destratification, or high concentrations of nitrate or phosphate can alter the dissolved oxygen content of the lake and lead to fish kills. The Lakeside Country Club in Bloomington, Illinois is home to a small (0.025 km²), shallow lake, with a maximum depth of 4.7 m, that experiences seasonal fish kills. With no inputs other than direct runoff from the golf course, fertilizer runoff from the course serves as the sole point source for nutrients in the lake. With many uncertainties of the urbanized lake and the land surrounding the lake, it was necessary to collect initial survey data. A bathymetry survey along with vertical profiles of temperature, dissolved oxygen (DO), pH, and specific conductance at 0.5 m interval depths were completed in May of 2019. In addition, two buoys with temperature and light intensity loggers were deployed in the central part of the lake to record measurements every 15 minutes at depth intervals of 0.3 meters. At four locations, water samples were collected from the surface and at the bottom for analyses of major anions (Cl^-, NO₃-N, PO₄^3-, SO₄^2-) and phosphorus (P). The data revealed several trends: 1) pH decreases as depth increases, 2) temperature decreases as depth increases and does not indicate stratification (Surface: 19.5 °C; 3 meters below surface: 15.5 °C), 3) DO decreases as depth increases, yet remains suitable for aquatic life (Surface: 11 mg/L; 3 meters below surface: 4 mg/L), and 4) specific conductance remains relatively constant with changing depth (633 μS/cm). The anion data illustrated that the lake is chemically well-mixed vertically and horizontally. NO₃-N and P concentrations where low, 0.43 mg/L and 23.4 mg/L, respectively. Currently, the causes of fish kills, which includes phytoplankton die-off, lake destratification, high concentrations of nitrate or phosphate, do not appear to be apparent in the gathered data. Additional data will be collected in the winter months to determine temporal variations and gain an insight into DO variations.