Projections of Seasonal Water Temperature Cycles and Stratification in Five Large Lakes in Minnesota under a 2xC02 Climate Scenario

The objective of this study was to simulate seasonal water temperature cycles and stratification in five large lakes in Minnesota under a 2xCOz climate warming scenario and to compare the results with those under the present (1983 - 1990) climate. The five lakes studied were Mille Lacs Lake, Upper Red Lake, Lake Vermillion, Lake Pepin and Rainy Lake. MINLAKE 96, a one dimensional, unsteady water quality model, was employed. The values of some parameters which describe lake water temperature and ice characteristics are extracted from the simulation results for both present and 2xC02 climate scenarios. Results are presented for surface temperatures, bottom temperatures, and differences between surface and bottom water temperatures under the two climate scenarios. Time series for ice thickness are also presented. During the open water season, water temperatures are projected to increase in the entire lake. The maximum increase is usually in early spring. In the ice cover season, water temperatures may decrease slightly with climate change, especially near the lake bottom. In deep Rainy Lake the decrease is more than in the shallow Upper Red Lake. Bottom temperatures less than 8 DC are found during a shorter period under a 2xCOz climate scenario. This will affect the growth of fish. During the ice cover season the climate change tends to weaken the inverse stratification. In the open water season climate change strengthens stratification. Ice is thinner under a 2xCOz climate scenario most of the time, but not all the time. Thicker ice cover is caused by absence of snow. Snowfall and snow depths are smaller under a 2xCOz climate scenario. Ice-in dates are later and ice-out dates are earlier under a 2xCOz climate scenario. Thus ice cover duration is shorter by an average of 49 days (range from 34 to 73 days). Ice cover also becomes less continuous. The shorter ice cover duration will decrease the possibility of anoxic conditions (winterkill) in lakes during the ice cover season. Earlier ice-out leads to earlier spring overturn.