PARTIAL AND FULL LIFT HYPOLIMNETIC AERATION OF MEDICAL LAKE, WA TO IMPROVE WATER-QUALITY

Hypolimnetic aeration of Medical Lake, WA using partial lift and full lift aerator designs resulted in enhanced water quality. The partial lift system significantly reduced hypolimnetic ammonia (NH3) and total phosphorus (TP) concentrations and increased hypolimnetic temperature. The system had no statistically significant effect on chlorophyll a (Chl a) concentration, phytoplankton biovolume, hypolimnetic nitrate (NO3-) and dissolved oxygen (DO) concentrations. The full lift system also significantly reduced hypolimnetic TP and NH3 concentrations, but increased hypolimnetic DO and temperature. This system likewise had no effect on Chl a concentrations. Although anoxia persisted with the partial lift system, it was viewed as effective because phosphorus was not released from the lake's bottom, demonstrating that measurable concentrations of DO were not required to hold phosphorus in the sediments. Further research is required on (1) the effect of hypolimnetic aeration on stimulating DO depletion rates and (2) long term effects on phytoplankton biovolumes and Chl a concentrations. Continued operation of the full lift system should further reduce in situ oxygen demands and eventually a new equilibrium and higher hypolimnetic oxygen concentrations can be attained.