INFLUENCES ON THE GEOCHEMISTRY OF AN INTERDUNAL WETLAND/SLACK IN THE LAKE MICHIGAN COASTAL DUNES

We studied the geochemistry of a large, ~1.25ha, interdunal wetland/slack in a Lake Michigan coastal dune complex at Saugatuck Harbor Natural Area, Allegan County, Michigan. The slack has a subtle microtopography of low-relief ridges and pools of varying depths. We collected surface water samples from the pools (n=7) in summer 2019, winter, spring, and summer 2021. Summer 2019 samples were analyzed for dissolved oxygen (DO), temperature (T) and pH. Values presented are seasonal averages for the pools. 2021 samples were analyzed for these parameters as well as specific conductivity (SC) and total alkalinity. Cation and anion analyses of the 2021 samples are in progress. Low pH (6.6), T (0.6⁰C) and high DO (~11ppm) were noted in the slack pools in winter 2021. Pool pH (7.4) and T (~24⁰C) were higher and DO lower (~6ppm) in spring 2019 than in winter 2021. pH (8.9) continued to rise from May–July in 2019 while DO decreased (~4 ppm). Warmer temperatures increased photosynthesis, removing carbonic acid from the water and increasing pH. However, warmer temperatures also decreased the amount of oxygen the water could hold. Vegetation decomposition also increased, further decreasing the DO. In 2021, pH and T also rose. Alkalinity also increased slightly, 178.7 (winter 2021) to 191.2 mg/L as CaCO₃. The pH, SC and alkalinity then dropped from June–July. Heavy rains (17.3 cm), including ~11cm three days prior to sampling, raised slack water levels ~15 cm between June and July. This volume of rain diluted the pools, lowering SC from 246.6 to 194.4 µS/cm and the alkalinity from 191.2 to 143.2 mg/L as CaCO₃. The rain’s acidity decreased the pool pH from 8 (June) to 7.2 (July). Hence, the alkalinity drop may also have been due to the pools buffering the rain’s acidity. DO, pH, T and alkalinity values vary between the slack’s pools due to their bathymetry and location. For example, pools > 60 cm are stratified with DO, pH and T decreasing with depth while shallower pools (<30–40 cm) are not. Therefore, while seasonal differences are noted in the slack’s geochemistry, precipitation, especially large-scale events, can also appreciably influence the geochemistry of the slack’s pools. With climate change models predicting more frequent heavy rain events in the Great Lakes, the slack’s geochemistry could be impacted more significantly in the future.