Nearshore-offshore exchanges by enhanced turbulent mixing along the north shore of Lake Ontario

Abstract

Seasonal nearshore-offshore exchanges by coastal upwelling events in large lakes can play a significant role in nearshore nutrient dynamics, affecting lake productivity and water quality. We analyzed field observations along the north shore of Lake Ontario, collected in the summer of 2018, focusing on the littoral zone and specifically the Cladophora habitat zone (<15 m), to investigate episodic enhanced vertical mixing by coastal up/downwelling events. Vertical turbulent diffusivity (Kz) based on the buoyancy Reynolds number above the metalimnion layer during downwelling events was generally higher than those below the metalimnion layer during upwelling events; while Kz at the metalimnion layer can increase by ∼ two orders of magnitude during upwelling events. Our results suggest that Kz based on the Richardson number parameterization, which only accounts for large-scale current shear and stability, and incorporates an adjustable parameter is ∼ ten times higher than the Kz based on the buoyancy Reynolds number. Analysis of historical wind records indicates that the frequency of coastal upwelling favorable winds on the north shore of Lake Ontario has increased by > 45% over the last thirty years - suggesting an increasing trend of nearshore-offshore nutrient exchanges as a contributing factor for the nearshore water quality management.