Characterizing daily water-level fluctuation intensity and water quality relationships with plant communities in Lake Ontario coastal wetlands

Seasonal and multi-year water-level fluctuations in the Laurentian Great Lakes maintain wetland vegetation diversity and wildlife habitat value, but little is known about the relationship between daily water-level fluctuations (wind tides and seiches) and plant community attributes. We used portable water-level loggers to determine mean daily fluctuation intensity (FI) at different timescales in selected Lake Ontario wetlands of various hydrogeomorphic types. Among years, FI varied across sites but was more consistent within sites and was significantly higher in open embayments than in drowned rivermouths or protected embayments. Of the time-scales considered (15, 30, 60, and 120 min), 15-minute intervals gave the best resolution in FI among wetlands. We examined associations between FI and vegetation coverage of seven guilds: submerged and rooted floating-leaved vegetation (SAV), free-floating, non-persistent emergent (NPE), Typha spp., meadow marsh (MM), shrub, and upland at 20-cm elevation increments along transects from approximately 0.8 m deep to 1.2 m upland while accounting for poor water quality, which is known to decrease vegetation coverage. Wetlands with higher conductivity had less SAV, free-floating, and NPE coverage, while wetlands with high turbidity had marginally less SAV and Typha spp., but more free-floating coverage. Wetlands with higher FIs had more NPE vegetation in shallow depths and more MM but less shrub and upland coverage at elevations above the long-term average water level, suggesting that FI is a hydrologic factor in vegetation establishment. These results can be used for modeling Lake Ontario plant communities and support adaptive management monitoring following a water-level regulation change.