Pumping of epilimnetic water into hypolimnion improves oxygen but not necessarily nutrient conditions in a lake recovering from eutrophication

Abstract

To mitigate deep water oxygen depletion and its consequences, epilimnetic water was pumped into deep water of a eutrophic, 26 km2 subbasin of Lake Vesijärvi, Finland. In winter, the mechanical mixing largely eliminated vertical differences in temperature, oxygen, and nutrients. Although ice cover prevented oxygen flux from the atmosphere, the high proportion of shallows in the basin with high oxygen concentration facilitated the avoidance of hypoxia by mixing. Despite the disappearance of anoxia in deep water, the volume-weighted mean concentrations of nutrients were not affected. In summer, the introduction of epilimnetic water into the hypolimnion improved oxygen conditions, but anoxia or hypoxia continued to develop. This condition favored the coexistence of nitrification and denitrification and, similar to winter, more than halved total and ammonium-nitrogen (NH4-N) concentrations in deep water. Despite the presence of nitrate or low concentration of oxygen, phosphorus concentrations continued to be high, and therefore deep water anoxia cannot have been the primary driver maintaining the basin in its eutrophic state. Consequently, in the management of Lake Vesijärvi, a major emphasis should be on external nutrient load and the leakage of nutrients from shallow sediments. A longer time series and attention to biological parameters is needed to determine the final outcome of pumping epilimnetic water into the hypolimnion.