Faculty Mentor(s)

Suzanne DeVries-Zimmerman, Geological & Environmental Sciences; Dr. Edward Hansen, Geological & Environmental Sciences

Document Type


Event Date



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 (main slack) and smaller blowouts on its arms (incipient slacks). Rising Lake Michigan water levels have increased groundwater levels, monitored at twelve locations, >1m, rewetting the main slack from its previous dry state, expanding its extent, and deepening pools. Adjacent blowouts now have incipient slacks with pools up to .5m deep. The main slack is dominated by Cladium mariscoides and Spiraea tomentosa with deeper pools having Carex lacustris, C. aquatilis, and Juncus species. The incipient slacks have a microbial mat and are sparsely vegetated with Juncus balticus and J. brachycephalus. Pool water chemistry and vegetation indicate the slacks are minerotrophic fens. We analyzed groundwater and surface water samples from the main and incipient slacks for pH and dissolved oxygen (DO) in summer 2019. pH and DO values in the slack pools increase overall from morning to late afternoon and decrease in the evening, suggesting a photosynthetic influence. Incipient slack pools typically have higher DO and pH values than mature slack pools, potentially indicating that algae populations in incipient pools have a greater influence than in mature pools. Incipient slacks also lack a decaying vegetation reservoir to depress oxygen values. Seasonal differences in surface water pH and DO values were noted as pH values were lower in May than July. Cool spring temperatures delayed photosynthesis and the accompanying removal of carbonic acid from the water, keeping pH values low. Rising temperatures increased photosynthesis, removing carbonic acid from the water and adding oxygen to it. This decreased pH and raised DO values.


This research was supported by the Hope College Department of Geological and Environmental Sciences.