Accounting for water levels when using water quality to assess the health of Great Lakes coastal marshes

Abstract

Coastal marshes are at the unique interface between land and lake water, which are home to unique wildlife. There are numerous freshwater coastal marshes along the Great Lakes coast that each have distinct geomorphologies which are home to different vegetation and fauna communities. The determination of coastal marsh health conditions is a necessity to assess influences of environmental stressors such as human-induced disturbances upstream of wetlands. Water quality indices (WQIs) are used to indicate the impacts of watershed alterations on the health of coastal marshes. It typically uses nutrient and sediment variables in the water column to assess to relate that to marsh health conditions. As coastal marshes are subjected to the constant fluctuation of water levels, it was found to be a significant factor that impacted WQI scores, likely due to the dilution effect. With increasing water levels, WQI scores also generally increased which indicated improved wetland health. Thus, it is a confounding variable against land-use and land-cover (LULC) alterations within the watershed. This study is the first to evaluate the dilution effect by relating the change of WQI scores to the change in wetland volume and wetland area. As well, it is the first to test the confounding effects of varying watershed coverage in LULC, long-term lake level ranges, and the hydrogeomorphology of coastal wetlands.