Photointerpretation analysis of plant communities in Lake Ontario wetlands following 65 years of lake-level regulation

Abstract

Abstract Regulation of Lake Ontario water levels since the St. Lawrence Seaway began operation in the early 1960s reduced the range of lake-level fluctuations and resulted in alteration of wetland plant communities, especially invasion of sedge/grass meadow marsh by cattails. A new regulation plan was implemented in January 2017 that seeks to restore some of the natural range in lake levels to benefit the environment. An adaptive management process accompanied the new Plan 2014, requiring before-after data for evaluation. Aerial photographs from 2014/2015 were obtained from existing sources for 16 wetlands previously assessed from 2001 photographs, and photointerpretation was used to re-map the vegetation types from the earlier study. The updated data set provides further information on wetland response to lake-level regulation. Overall, area of Meadow Marsh decreased from 2001 to 2014/2015 in most wetlands. In some cases, losses were due to increased Cattail abundance and in others to Mixed Emergent abundance. Although area of Cattail did increase at half the sites, expansion landward into Meadow Marsh was likely limited to an upper elevation with drier soils that had not been flooded for at least 15 years. Waterward expansion of Cattail was slow because it occurs by formation of vegetatively-expanding floating mats. The updated data set can now serve as a baseline for future adaptive management evaluations, including response to extreme high lake levels in 2017 and future low lake levels that Plan 2014 should allow during periods of low water supply.