Groundwater Controls on Wetland Vegetation of a Ridge-and-Swale Chronosequence in a Lake Michigan Embayment

A chronosequence of wetland swales between beach ridges in the Manistique/Thompson embayments of Lake Michigan contains plant communities that differ across the strandplain. We characterized vegetation in 33 swales and compared distribution with previously reported groundwater flow systems. Older swales near a groundwater divide created by the peak Nipissing ridge receive local flows and hold sedge/leatherleaf floating mats that transition to swamp. Farther lakeward, another groundwater divide is created by discharge of calcareous waters released by termination of an underlying clay confining layer, resulting in swales dominated by northern white cedar. Cedar swamp continues lakeward in swales having flow-through calcareous groundwater, but several swales are perched above those flows. Farther lakeward, a large amalgamated beach ridge creates another groundwater divide with discharges that again support cedar swamp. Calcareous discharge from the confined aquifer, with downslope flow-through waters, then supports more cedar swamp. Flow-through waters meet yet another calcareous discharge, resulting in ponding and development of floating mats. Finally, a deep regional aquifer discharges at the Lake Michigan shore and supports marsh/shoreline species. Our results have implications for assessing potential responses to climate change, interpretation of past climate changes in paleoecological studies, and management of wetlands facing future climate changes.