WAXING AND WANING INTERDUNAL WETLANDS/SLACKS IN AN EVOLVING AEOLIAN LANDSCAPE ON LAKE MICHIGAN’S EASTERN COAST

Interdunal wetlands/slacks form where the wind scours the sand to the water table within the dune complexes along Lake Michigan’s eastern shore. Our review of historic aerial photograph/satellite imagery of these slacks shows they expand in times of higher water levels in Lake Michigan-Huron (MH) and contract, sometimes disappearing, with lower water levels, suggesting a link between the slacks’ water tables and those of Lake MH. Numerous slacks lie in a coastal dune complex at Saugatuck Harbor Natural Area, Allegan County, Michigan. We have studied the largest slack, lying between the arms of a parabolic dune, ~200m east of Lake MH, and smaller incipient slacks in blowouts in the dune’s arms since 2016. Pool water chemistry and vegetation indicate the large slack is a minerotrophic fen vegetated predominantly with the emergent sedge, Cladium mariscoides. Lake MH water levels have risen ~1m from 2016–2020. Water levels within and monitoring wells around the slack have also risen ~1m, rewetting and significantly expanding the main slack and creating incipient slacks in the smaller blowouts. Geologic cross-sections based on water well logs show fine to medium sand 60–80 ft thick with some gravel towards the bottom underlain by grey clay, providing a hydraulic connection to Lake MH. Four years of monitoring well data show slack water levels rise in the spring as Lake MH levels rise. However, these levels drop as the growing season begins while Lake MH levels continue to rise through the summer. During summer small increases in slack water levels occur in response to rain events. However, significant water level drops (~.7 cm/day) follow due to evapotranspiration by the wetland vegetation, especially during periods with no rain. Slack water levels begin to rise in the late summer and into the fall. The approaching end of the growing season slows plant growth, reducing and eventually ending evapotranspiration. Rain events are also heavier and more frequent in the fall. Together, these factors push slack water levels to their highest point of the year while Lake MH levels are beginning to decrease. In late fall to winter, slack water levels drop in concert with Lake MH levels. Hence, the existence and extent of interdunal wetlands/slacks is influenced by Lake MH levels and by seasonal processes such as evapotranspiration and precipitation.