Vegetation species richness is associated with daily water-level fluctuations in Lake Ontario coastal wetlands

The effects of seasonal and multi-year water-level fluctuations on coastal wetland vegetation communities have been intensively studied in the Great Lakes, but there is relatively little information on how daily water-level fluctuations (driven by wind tides and seiches) influence them. We examined the relationship between daily water level fluctuation intensity and species richness for five vegetation guilds (i.e., submerged aquatic vegetation (SAV), emergent, meadow marsh, shrub and upland) at 13 Lake Ontario wetlands sampled from 2010 to 2015. At each site, portable water-level loggers were used to determine the mean daily fluctuation intensity (range: 12.20-106.85 cm), and vegetation data were collected along an elevation gradient from 0.8 m below to 1.2 m above the long-term average shoreline. Linear mixed-effects models were fitted for each vegetation guild to assess the relationship between fluctuation intensity and species richness. We found an elevation-dependent relationship with fluctuation intensity for all guilds except SAV. Higher daily fluctuation intensity (i.e., more frequent significant changes in water levels) causes periodic de-watering of elevations below the shoreline and flooding of elevations above the shoreline which inhibits the establishment of shrub and upland species while maintaining rich emergent and meadow marsh communities. Increased emergent richness was evident at the 0.4 m, -0.2 m, 0 m, and +0.2 m depth and increased meadow marsh richness was evident at the +1.0 and +1.2 depth, whereas decreased richness was evident for shrubs at +0.4 and + 0.6 m and for upland vegetation at +0.4 m. Our results suggest that daily fluctuation intensity is an important factor influencing wetland vegetation community biodiversity. Crown Copyright (C) 2019 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved.