ECOHYDROLOGICAL CHANGES IN INTERDUNAL WETLANDS/SLACKS AS A RESPONSE TO RISING LAKE LEVELS IN AN EVOLVING EOLIAN LANDSCAPE ALONG LAKE MICHIGAN’S EASTERN COAST

We are conducting ecohydrological studies in mature and incipient interdunal wetlands/slacks in a coastal dune complex on Lake Michigan’s eastern coast. The slacks are located in the Saugatuck Harbor Natural Area, ~200m inland of Lake Michigan within a large parabolic dune (mature slack) and smaller blowouts on its arms (incipient slacks). Slack water levels reflect Lake Michigan’s. Rising Lake Michigan water levels for the last 5 years have increased groundwater levels, monitored at 12 locations, >1m, rewetted the mature slack from its previous dry state, reflooded pools (>.5m rise), and expanded its extent. Adjacent blowouts now have incipient slacks with pools up to .5m deep. Pool water chemistry and vegetation indicate the slacks are minerotrophic fens. pH and dissolved oxygen values in the slack pools increase from morning to late afternoon, suggesting photosynthetic processes. The incipient slacks have a higher pH and DO overall, a microbial mat, and are sparsely vegetated with Juncus balticus and J. brachycephalus. Vegetation quadrat sampling has been done annually in the mature slack since 2016. This slack has a high species diversity, especially in damp to shallow water (<10 cm depth) areas, with almost half of these species preferring calcareous conditions. From 2016–2017 shallow water levels (<30 cm) prevailed. Large areas of the slack would dry and have damp soils by mid-summer. These areas were dominated by Cladium mariscoides and Spiraea tomentosa. Deep pools (~.5 m depth) on the slack’s northern side remained wet. Carex lacustris, C. aquatilis, and Juncus species with Utricularia sp., a submergent species, vegetated these pools. Water levels have continued to rise from 2017–2019, expanding the slack. Much of it now remains wet throughout the season with standing water up to 40 cm deep (deep pools 60–70 cm). Vegetation coverage values have significantly decreased. Cladium and Spiraea are now absent from much of the slack. The vegetation is shifting to species tolerant of deeper water such as Calamagrostis canadensis and Juncus species. Utricularia occurs throughout much of the slack rather than being restricted to the deep pools. The slack’s wetland ecosystem is dynamic and reflective of water depth as wetland vegetation returns and then transitions to different wetland species as water levels rise.