MODEL SIMULATIONS OF DISSOLVED-OXYGEN CHARACTERISTICS OF MINNESOTA LAKES - PAST AND FUTURE

A deterministic, one-dimensional, unsteady numerical model has been de veloped, tested, and applied to simulate mean daily dissolved oxygen ( DO) characteristics in 27 lake classes in the state of Minnesota. Reae ration and photosynthesis are the oxygen sources, while respiration, s edimentary, and biochemical water column oxygen demand are the sinks o f oxygen in the model. The lake classes are differentiated by surface area (A(s)), maximum depth (H(max)), and trophic status expressed as S ecchi depth (Z(s)). Because lake stratification is most important to l ake oxygen dynamics, simulated DO characteristics are plotted in terms of a stratification parameter A(s)/H(max)0.25 and Secchi depth Z(s). Simulations provide DO profiles on a daily time scale. Specific DO cha racteristics of ecological and environmental interest are epilimnetic DO, hypolimnetic DO, DO gradient from surface to bottom, and DO minima and maxima. Specific results are as follows: Simulated mean daily and weekly DO values in the epilimnion of all lakes for both past and fut ure climate scenarios are near saturation over the summer season. Hypo limnetic DO values depend strongly on lake morphometry, trophic status , and time throughout the summer season. Future climate conditions are specified as the historical records from 1955 to 1979, adjusted (mont hly) by the 2 x CO2 GISS model output to account for doubling of atmos pheric CO2. With this climate change, weekly averaged epilimnetic DO i s projected to drop by less than 2 mg/liter, and will remain above 7 m g/liter throughout the open water season. The hypolimnetic DO reductio ns after climate change are on the order of 2-8 mg/liter. Periods of a noxia are longer by as much as 80 days. Those changes would alter wate r quality dynamics in lakes and have a profound effect on lake ecoyste ms including indigenous fishes. The results presented are useful for e valuating environmental management options.