Coastal wetlands as sources of Escherichia coli to the nearshore of Lake Ontario

Abstract

Beaches within the Laurentian Great Lakes Basin are frequently contaminated with fecal bacteria such as Escherichia coli (E. coli), leading to beach closures due to potential health risks. Climatic drivers and landscape sources of E. coli to nearshore areas have been established, the role of coastal wetlands as sources of coliform bacteria remains a major knowledge gap. We investigated E. coli abundance and dynamics in four coastal wetlands (3 drowned river mouth 33 and 1 barrier beach lagoon) on the north-shore of Lake Ontario for over a decade (2009-2018). A general additive model revealed significant seasonal, interannual, and spatial trends for E. coli concentrations across the study area. Additionally, we observed celled sites (i.e., natural ponds in wetlands) to have a higher partial effect on E. coli concentrations across the study period, indicating that these wetland features may be an important reservoir for endogenously sourced E. coli. To determine whether increased wave activity, indicative of seiche events, influenced E. coli concentrations in the wetland and the nearshore of Lake Ontario, we developed a local structural equation model to represent the entire study area. Seiche events were identified to significantly increase suspended solid loadings, which in turn resulted in significantly (p-value < 0.05) elevated E. coli concentrations in both the coastal wetlands and Lake Ontario. In addition to watershed inputs, our results demonstrated that coastal wetlands significantly (p-value < 0.05) influence E. coli concentrations in the nearshore of Lake Ontario, and should be considered in nearshore water quality assessments and mitigation efforts.