Episodic nearshore-offshore exchanges of hypoxic waters along the north shore of Lake Erie

Abstract

We investigate the nearshore-offshore exchange of hypoxic waters during episodic coastal upwelling events in the nearshore waters of northern Lake Erie using intensive field observations and a validated hydrodynamic and water quality model. We observe wind-induced coastal upwelling events to be the dominant nearshore physical process in the lake which are energized every 5–10 days. When the winds were predominantly blowing from the west or south-west, epilimnetic waters were transported to the offshore bringing in hypolimnetic waters with low temperature (8–10 °C), dissolved oxygen (DO: 0–6 mg L−1) and pH (6–7) to the nearshore zones. During these events, vertical diffusivity coefficients decreased from 10−2 m2 s−1 to values as low as ~ 10−7 m2 s−1. In late summer, the coastal upwelling events in the nearshore waters lower the near bottom DO to hypoxic levels (DO < 2 mg L−1). Lake-wide observations of DO and pH show that they are positively and linearly correlated while in the nearshore DO and pH experience spatial and temporal variability where upwelling events were developed, which were further assessed using a three-dimensional model. The model accuracy to reproduce offshore hypoxia was first assessed on a lake-wide basis using a coarse resolution model for a five-year period (2008–2012) and in nearshore waters using a higher resolution model for 2013. We use the model results to delineate the near bottom areas experiencing hypoxia at time scales longer than 48 h.