Simulation of dissolved oxygen profiles in a transparent, dimictic lake

Abstract

Thrush Lake is a small, highly transparent lake in northeastern Minnesota. From 1986 to 1991, vertical profiles of water temperature, dissolved oxygen, chlorophyll a concentration, underwater light irradiance, and Secchi depths were measured at monthly intervals during the ice‐free period. Average Secchi depth was 7.5 m. Metalimnetic oxygen maxima were observed every summer. The oxygen peaks were related to Chl a distribution and water transparency. A deterministic one‐dimensional model was developed to simulate vertical dissolved oxygen (D.O.) profiles on a daily time scale throughout the open‐water season. The model includes oxygen production by photosynthesis and surface reaeration as well as oxygen consumption by plant respiration and biochemical and sedimentary oxygen demand. The measurements suggest that the model must use different Chl a and sedimentary oxygen demand values in the surface mixed layer and below the surface mixed layer in conjunction with high transparency to obtain a metalimnetic oxygen maximum. Measured Chl a values are specified as input to the D.O. model. The model reproduces measured D.O. distributions with a standard error of 1.4 mg liter−1 and shows the observed metalimnetic oxygen maxima. Model output changes only slightly with starting date of simulation and initial D.O. values. Simulated water temperature and D.O. distributions in the lake, especially the surface mixed‐layer depth and the metalimnetic oxygen maxima, are sensitive to changes in water transparency.