Heatwaves and storms contribute to degraded water quality conditions in the nearshore of Lake Ontario

As extreme climatic events, such as heatwaves and storms, become more frequent in response to changing climates, understanding the role climatic events play on water quality is essential. Here, we use water quality monitoring data collected from the nearshore of Lake Ontario between 2000 and 2018 to ask: i) which sites in the nearshore of Lake Ontario have statistically extreme water quality conditions?; ii) do water quality conditions differ in extreme versus non-extreme climate years?; and iii) what are the significant antecedent extreme weather drivers of water quality in the nearshore of Lake Ontario? Three sites with the highest chlorophyll a concentrations and eutrophic conditions, two of which are in Areas of Concern, exhibited the strongest responses to climate extremes. Antecedent weather conditions explained 87.2% of the variation in extreme chlorophyll a concentrations. In particular, warmer temperatures and heatwaves corresponded with statistical extremes in chlorophyll a concentrations. Precipitation accounted for 35.5% of the variation in extreme conditions of turbidity, including storm events the day prior to sampling. When considering site-specific extreme conditions, antecedent weather conditions explained 66.8% of the variation in turbidity. We illustrate the strong role that heatwaves and storm events play on spatial and temporal patterns in extreme water quality conditions, highlighting the importance of incorporating climate change adaptation plans into ecosystem management strategies to preserve water quality in the highly important and iconic nearshore regions of the Laurentian Great Lakes.