Mixing events in eau galle lake

Abstract

Eau Galle Lake (Spring Valley, Wis.), a U.S. Army Engineers reservoir, is more susceptible to weather-related mixing events than most north temperate lakes. Bottom withdrawal at Eau Galle leads to considerable heat storage in the hypolimnion and by late summer there is only 6–8° C difference between surface and bottom temperatures. Steep metalimnetic gradients never develop and as a result the reservoir has a relatively low thermal stability, which implies a susceptibility to weather-induced (i.e., wind or cold front passage) mixing events. Two types of mixing events were observed in Eau Galle Lake in the summers of 1981 and 1982: (1) Small scale mixes similar to those observed in lakes. These events lead to net movement of nutrients into the epilimnion and a resultant increase in chlorophyll a concentrations. (2) Large scale mixes that function essentially as short-lived turnovers. These large scale mixes are preceded by a cooling trend in air temperature which results in heat loss from the surface of the reservoir. Surface cooling reduces the already small temperature differences between surface and bottom and sufficient wind can produce considerable mixing. These events lead to the introduction of oxygen into the previously anoxic hypolimnion and rather than increasing epilimnetic concentrations of nutrients, there is a general loss throughout the water column. Low thermal stability and susceptibility to mixing may be a common feature of bottom withdrawal reservoirs and may exert a considerable influence on nutrient and phytoplankton dynamics. © 1984 Taylor & Francis Group, LLC.