Winter Limnological Conditions in a PRAIRIE Pothole and the Application of Molecular Oxygen

The physical, chemical, and biological conditions in Round Lake, Oakwood Lakes State Park, South Dakota were monitored during the 1977-1978 winter, and reaeration utilizing molecular oxygen was evaluated as a practical method to prevent winterkill. Ice and snow conditions after 20 December, 1977 allowed little light penetration, and phytoplankton numbers (6174/ml to 1668/ml) and chlorophyll concentration (46.6 mg/L to 14.1 mg/L) decreased throughout the winter. Aquatic bacteria and fungi comprised from 87.0% to 93.7% by number of the total cells identified (phytoplankton, bacteria, and fungi) in the lake water under ice. Dissolved oxygen declined to anoxic conditions after 19 January, 1978, and hydrogen sulfide gas reached a mean maximum (6.0 mg/L) on 12 February. Analysis of chemical parameters indicated increased chemical concentrations attributable to the freezing out of ions. A mean free carbon dioxide level increase from 0.95 mg/L to 59 mg/L was caused by increased respiration not counterbalanced by photosynthesis Community respiration measured by the diel oxygen method (community metabolism) varied from 1.4 mg O2/L/24 hrs 1-2 December, 1977, to a low (0.2 mg O2/L/24 hrs) 11-12 January, 1978. Water biochemical oxygen demand (BOD) measurements from samples incubated below the ice indicated that mean lake BOD increased from 0.8 mg O2/L/24 hrs 12 February, 1978, to 8.5 mg O2/L/24 hrs 16 March, 1978. Sediment BOD was 10 to 15 times that of the water BOD during the same period. Liquid oxygen stored in 127,440 L o2 gas capacity tanks, rubber welders hose, and graphite air stones comprised the molecular oxygenation apparatus. Oxygenation employing 3 different strategies, resulted in 711,400 liters of oxygen gas being expended under the ice with few positive effects noted in reducing anoxic conditions in the lake. The total lake oxygen demand for the 4 month period was calculated to be 1.05 X 10 8 L O2. Depending on molecular oxygen dissolution efficiency, an expenditure of $70,000 to $350,000 for oxygen alone, might have been required to prevent winterkill in Round Lake during 1977-1978