Environmental implications of stratification and turbulent mixing in a shallow lake basin

The extent of stratification and vertical mixing in the water column (7–11 m deep) was investigated over an offshore reef in the western basin of Lake Erie. Measurements reveal that the vertical transport of oxygen and heat is controlled by the complex interaction of several physical mechanisms. Generally, when the wind speed (W) was >7 m·s–1 and the air was cooler than the water (Tair< Tw), the water column was well mixed due to turbulent mixing. However, when W < 7 m·s–1 (~65% of the summer), turbulence was too weak to overcome the stratification and mix the water column. An analysis of 25 years of meteorological data revealed that a period of 4.5 ± 1.9 days of calm, warm weather (W < 7 m·s–1 and Tair > Tw) occurs every year. Results indicate that there is strong probability of hypoxia due to stratification (i.e., when diffusivities < 10–6 m2·s–1) and sediment oxygen demand (i.e., 0.1–1.0 g·m–2·day–1) during these periods. The environmental implications of stratification to water quality and its effects on benthic organisms, such as the burrowing mayfly (Hexagenia spp.), require further considerations in large temperate lakes and basins that are sufficiently shallow that there is no permanent seasonal stratification.