Examining the Relationship between Lake Michigan Water Levels and the Ecohydrology of an Interdunal Wetland/Slack on the Southeast Coast of Lake Michigan
Prof. Suzanne DeVries-Zimmerman, Adjunct Assistant Professor, Department of Geological and Environmental Sciences, Geological and Environmental Sciences, Dr. Edward Hansen, Professor, Department of Geological and Environmental Sciences
Interdunal wetlands or slacks are an imperiled ecosystem along the Great Lakes. We initiated an ecohydrological study of an interdunal slack/wetland in the deflation basin of a large parabolic dune ~200m inland from Lake Michigan near Saugatuck, Michigan. The slack formed where wind unevenly scoured the sand to the water table, creating a series of pools and ridges. We reviewed historic aerial photographs together with the Lake Michigan water level curve to examine the relationship between slack features and the lake’s hydrology. The earliest photo, 1938, shows three pools along the northernmost edge and emergent wetland vegetation in the northeast corner and along the western edge of the slack. In subsequent photos, larger pond and emergent wetland vegetation areas occurred in years with higher water levels, while smaller pond and emergent vegetation areas occurred in years with lower water levels. Low lake levels from 1998 – 2014 significantly shrank the wetland areas with upland dune vegetation replacing wetland vegetation in many places. Even the northernmost pools were dry during a portion of this time. Rising lake levels (2014 – present) reflooded the wetland. Vegetation quadrat sampling in the summer of 2016 showed dominant wetland species in the slack. Thus, water levels in the slack generally reflect Lake Michigan levels with the ecological communities responding accordingly. In addition, different wetland plant communities and zonation of these communities were noted in the various pools and lobes of this slack, apparently due to different water depths in the respective pools and lobes. This phenomenon is under additional study. Groundwater levels obtained from a slack/dune monitoring well network show that local rain events significantly raise the slack’s water levels while not impacting those of Lake Michigan. Hence, water levels within the slack are reflective of both Lake Michigan’s levels and local precipitation events.
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