Thermal Structure of the Lower Great Lakes in a Warm Year: Implications for the Occurrence of Hypolimnion Anoxia

In this paper, surface heat flux, thermal structure and dissolved oxygen concentrations for the lower Great Lakes are examined for an anomalously warm year, 1983. The year was characterized by large reductions in surface heat losses in winter and above-average surface heat flux gains in summer. On an annual basis, the lakes buffered large surface heat gains in summer months through losses in other months. Observations indicated higher surface water temperatures, significant reductions in duration and extent of ice cover, and an earlier disappearance of the 4°C isotherm, signalling an earlier start to thermal stratification. In response to greater surface heating and low wind conditions, the thermocline formed higher in the water column, and stratification lasted longer than in other years. These conditions contributed to slight hypolimnetic anoxia in the central basin of Lake Erie in the latter half of September.