Frequency and duration of weak thermal stratification and hypoxia in the shallow western basin of Lake Erie

Polymictic lakes are not always continuously mixed; they often experience alternating periods of mixing and weak stratification. The shallow western basin of Lake Erie is one example of a polymictic basin where the frequency of mixing has important consequences for water quality. In this study, we capture high-frequency data from the summers of 2021, 2022, and 2023, including water temperature, dissolved oxygen (DO), and water current velocity. This dataset allows us to show that hypoxic events (<2 mg/L) are triggered by episodes of weak stratification in Pigeon Bay, in western Lake Erie. Our five sampling sites in Pigeon Bay were located 50 m to 20 km from an important nearshore municipal water intake that supplies drinkable water to ∼66,800 residents and to the second largest greenhouse cluster in the world. Between June and September in these 3 years, we found that Pigeon Bay was stratified with a vertical temperature difference above 2 °C, for respectively 45 % (2021), 54 % (2022), and 25 % (2023) of the time. Significantly, all the hypoxic events were associated with stratified events. During the sampling periods, stratified events and hypoxic events were induced by either 1) local surface heating or 2) advection of hypolimnetic water from central Lake Erie. The majority of the hypoxic events in Pigeon Bay (83 % in 2021, 86 % in 2022, and 67 % in 2023) were associated with the horizontal transport of cold water that originated over 20 km from the central basin of Lake Erie.