Simulations of Seasonal Water Temperature Cycles and Stratification in Large Lakes in Minnesota

The importance of temperature as a controlling variable at all levels of ecological systems is well known (Christie et al., 1988). Temperature is one of the most significant factors to determine, for example, where fish can live and thrive. Most of the time, fish will select to inhabit temperatures near their growth optimum, pending other conditions being favorable (food, dissolved oxygen, etc.). Each species has a temperature tolerance range (Eaton et al., 1995). If water temperatures are in excess of this range, fish will not survive. In temperature stratified lakes, fish are forced to find water layers where their requirements are met. It is also known that temperature is a major controlling factor affecting the growth rate of fishes. Different species of fish have different optimum temperature for growth. This means that the pattern of environmental temperature in lakes affects the production and yield of individual species populations. Therefore, correctly simulating temperature structures in lakes is important for estimating optimal thermal habitat and yields for fish species. The purpose of this study is to simulate seasonal water temperature cycles and stratification in ten large lakes in Minnesota, excluding Lake Superior. The simulations are made to supplement sparse field measurements of lake water temperatures. Measured or simulated water temperatures are needed to estimate thermal habitat and fish yield. The simulations have been requested by the Minnesota Department of Natural Resources. The simulations also fulfill two other purposes. One is to extend the application of the MINLAKE 96 model to lakes of surface areas considerably larger than 30 km2 (the upper limit of most previous applications). This includes comparisons of simulation results with whatever measurements are available. The other purpose is to generate baseline information on the thermal regime of the largest Minnesota lakes under past climate conditions for comparison with projected warmer climate scenarios.. The thermal regime of a lake is characterized by extracting from the simulation results maximum and minimum water temperatures in different seasons; strength and duration of stratification; onset, duration, and thickness of ice covers; and other parameters. Simulated water temperature information for ten large Minnesota lakes will be presented.