Episodic hypoxia in the western basin of Lake Erie

Hypoxic conditions continue to be an environmental concern in lakes, including those with shallow and well-mixed basins, such as the western basin of Lake Erie, in which hypoxia is not anticipated. We investigated the dynamics and causes of hypoxia using field measurements at two locations in the western basin during the late summer of 2017. Two hypoxic events (dissolved oxygen [DO] concentrations < 2 mg L-1) were recorded that were caused by upwelling of hypolimnetic water from the central basin of the lake following winds from the south and southwest. In this case, instantaneous stratification occurred when cool central basin water (i.e., 15.7 degrees C) intruded as a 2.5-m-thick layer above the bottom under the warm western basin waters (i.e., 23.9 degrees C). A third hypoxic event, which was associated with more typical thermal stratification from atmospheric warming, occurred during a calm and warm period near the end of the deployment. In this case, we observed a continuous decline in hypolimnetic DO from approximate to 8 to < 5 mg L-1, which likely declined to < 2 mg L-1 in 14 d using a one-dimensional model. Interbasin exchange flows generated instantaneous hypoxia multiple times within a year and were the dominate cause (63% of 11 cases) of hypoxia identified during August fishing trawls in the study area over the past 30 yr. Results of this work should help with the prediction and understanding of hypoxia in lakes with multiple basins, which will be informative for water quality and fisheries management.