THERMAL STRATIFICATION AND CHEMICAL DISPERSION IN WARM MONOMICTIC RESERVOIRS WITH AND WITHOUT INDUCED MIXING

Each year, warm monomictic lakes undergo a period of thermal stratification during warmer months, and when the temperature drops, the lake undergoes turnover and mixes. During periods of stratification, the water separates into three distinct thermal layers – the upper epilimnion, the metalimnion, and the lower hypolimnion. Stratification may alter the dispersion of nutrients in the water column. In order to reduce anoxic conditions and prevent eutrophication in the hypolimnion, solar-powered mixers are often utilized to facilitate water movement in the top 10 feet of a reservoir. Much of the published literature focuses on the beneficial effects of mixing on controlling algae blooms, but does not address the potential larger implication on overall lake thermoclines, oxyclines, and biogeochemistry. Two warm monomictic reservoirs were investigated in this study – one that is younger (Hard Labor Creek Reservoir established in 2018) and does not utilize solar mixers, and one that is older (Lake Varner established in 1993) and does utilize solar mixers. Water samples have been collected throughout the water column from both reservoirs using a horizontal water sampler to investigate the potential effects of induced mixing on nutrient dispersion in the water column, as well as the locations of the thermoclines and oxyclines. Nutrients analyzed include total nitrogen, nitrate, phosphate, and sulfate. Additional water chemistry variables, such as dissolved oxygen and pH, have been collected using a multiparameter water probe. Preliminary results suggest that induced mixing does not affect the depths of the thermocline or the oxycline. Additionally, the dispersion of some nutrients also do not appear to be affected by induced mixing as the total nitrogen concentration is similar between the two reservoirs, with the epilimnion and metalimnion having lower total nitrogen concentrations, and a higher concentration is found at a depth of 9 meters. Additional nutrient dispersion data from the two reservoirs will be analyzed, as well as the timing of the lake turnover.