DISSOLVED-OXYGEN MODEL FOR REGIONAL LAKE ANALYSIS

A deterministic, one-dimensional, unsteady dissolved oxygen (D.O.) mod el has been formulated to simulate summer D.O. conditions (stratificat ion) in a wide range of lakes of the north central United States and t o study potential impacts of global climate change. The simulation inc ludes contributions by photosynthesis, plant respiration, reaeration, biochemical oxygen demand (BOD) and sedimentary oxygen demand (SOD) as source and sink terms. The one-dimensional vertical oxygen transport equation is solved in conjunction with the heat transport equation in daily time steps beginning 1 March and ending 31 October. Lake morphom etry, trophic status and daily weather parameters have to be specified as input, and daily profiles of water temperature and dissolved oxyge n are obtained as output. The model finds the onset of summer-stratifi cation from initially isothermal and constant D.O. conditions. Trophic status is related to Secchi depth, phytoplankton (chlorophyll-alpha) concentrations, BOD and SOD. Field data from seven lakes (11 years) ha ve been used to calibrate and validate the D.O. model. The simulations for dimictic lakes with strong stratification are better than for wea kly stratified polymictic lakes, i.e. the model works better for deep lakes than for shallow lakes. The average standard errors for calibrat ion and validation are 1.4 and 1.9 mg D.O. l(-1), respectively. The te mperature simulations, especially the mixed-layer depth, affect the D. O. simulation results. A sensitivity analysis to model coefficients wa s also conducted. The model is most sensitive to sedimentary oxygen de mand (SOD). Twenty seven classes of lakes (3 depths X 3 areas X 3 trop hic states) in the north central U.S. were analyzed with the model. Si mulated mean daily dissolved oxygen concentrations in the epilimnion a re near saturation, those in the hypolimnion vary considerably dependi ng particularly on length of time since the onset of stratification an d the sedimentary oxygen demand.