Wetland vegetation response to record-high Lake Ontario water levels

Abstract

Lake Ontario water levels were the highest in recorded history in 2017 and 2019, resulting in significant impacts to shoreline properties and observable (but not previously quantified) changes in coastal wetland vegetation. In this study, we assessed differences in coverage of five plant community guilds (submerged aquatic vegetation, Typha, meadow marsh, shrub, and upland) along the shoreline elevation gradient from 12 Lake Ontario coastal wetlands surveyed from 2009 to 2019. This time period included a span of relatively stable water levels (2009–2016), followed by the two high-water years. In general, we found that extreme high water levels led to a decrease in vegetation coverage, most notably at the lower extent of the elevation ranges for the meadow marsh, shrub, and upland vegetation guilds. We also found a modest increase in Typha coverage at the upper extent of its elevation range in 2019, indicating that Typha advanced into the meadow marsh zone during the study period. These findings can be used to calibrate and validate predictive models that inform adaptive management of the new outflow regulation plan for Lake Ontario and can aid in modelling the dynamics of wetland vegetation in relation to predicted changes in Great Lakes water levels due to climate change.