SIMULATION OF DISSOLVED-OXYGEN PROFILES IN A TRANSPARENT, DIMICTIC LAKE

Thrush Lake is a small, highly transparent lake in northeastern Minnes ota. From 1986 to 1991, vertical profiles of water temperature, dissol ved oxygen, chlorophyll a concentration, underwater light irradiance, and Secchi depths were measured at monthly intervals during the ice-fr ee period. Average Secchi depth was 7.5 m. Metalimnetic oxygen maxima were observed every summer. The oxygen peaks were related to Chl a dis tribution and water transparency. A deterministic one-dimensional mode l was developed to simulate vertical dissolved oxygen (D.O.) profiles on a daily time scale throughout the open-water season. The model incl udes oxygen production by photosynthesis and surface reaeration as wel l as oxygen consumption by plant respiration and biochemical and sedim entary oxygen demand. The measurements suggest that the model must use different Chl a and sedimentary oxygen demand values in the surface m ixed layer and below the surface mixed layer in conjunction with high transparency to obtain a metalimnetic oxygen maximum. Measured Chl a v alues are specified as input to the D.O. model. The model reproduces m easured D.O. distributions with a standard error of 1.4 mg liter(-1) a nd shows the observed metalimnetic oxygen maxima. Model output changes only slightly with starting date of simulation and initial D.O. value s. Simulated water temperature and D.O. distributions in the lake, esp ecially the surface mixed-layer depth and the metalimnetic oxygen maxi ma, are sensitive to changes in water transparency.