DISSOLVED-OXYGEN MODEL FOR REGIONAL LAKE ANALYSIS

A deterministic, one-dimensional, unsteady dissolved oxygen (D.O.) model has been formulated to simulate summer D.O. conditions (stratification) in a wide range of lakes of the north central United States and to study potential impacts of global climate change. The simulation includes 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 morphometry, trophic status and daily weather parameters have to be specified as input, and daily profiles of water temperature and dissolved oxygen are obtained as output. The model finds the onset of summer-stratification 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) have been used to calibrate and validate the D.O. model. The simulations for dimictic lakes with strong stratification are better than for weakly stratified polymictic lakes, i.e. the model works better for deep lakes than for shallow lakes. The average standard errors for calibration and validation are 1.4 and 1.9 mg D.O. l(-1), respectively. The temperature simulations, especially the mixed-layer depth, affect the D.O. simulation results. A sensitivity analysis to model coefficients was also conducted. The model is most sensitive to sedimentary oxygen demand (SOD). Twenty seven classes of lakes (3 depths X 3 areas X 3 trophic states) in the north central U.S. were analyzed with the model. Simulated mean daily dissolved oxygen concentrations in the epilimnion are near saturation, those in the hypolimnion vary considerably depending particularly on length of time since the onset of stratification and the sedimentary oxygen demand.