The influence of water-level fluctuations on water chemistry and invertebrate community composition in a Great Lakes coastal wetland

Fringing wetlands lie at the interface between terrestrial and lentic ecosystems. The interaction between lake and wetland influences the hydrology and creates habitat unique from other types of freshwater wetlands. Water level fluctuations are common to the Great Lakes and likely influence water quality and shape invertebrate communities, yet potential relationships have not been explored. In this study, I documented these changes in a Lake Huron coastal wetland over a four-year period that corresponded to a 1.04 m decline in water level.; A transect spanning a Scirpus-dominated wetland was established in 1997. Sampling stations were spaced at 20-m intervals and extended 280 m shoreward of the wetland/lake interface. Water chemistry (1997–2000) and invertebrate (1998–2000) samples were collected at approximate monthly intervals during the active growing season (June–September).; An increase in the total dissolved ion content of wetland surface water occurred in conjunction with declines in lake level. Changes for all of the major anions and cations were statistically significant between years, particularly in 1999 and 2000 when lake levels were below average. In 2000, water depth in the wetland averaged 0.09 m and much of the near shore region of the wetland was devoid of standing water. As depth declined, the combination of reduced lake/wetland mixing and a water table gradient of 10 cm per 100 m resulted in stronger interactions between wetland surface water and sediments. Seiche-related water level fluctuations increased connectivity between interstitial and surface water. Higher concentrations of SO₄−2, Mg +2, and Si in 2000 relative to previous years supported tighter coupling between surface and interstitial water.; A total of 60 invertebrate taxa were collected from the study wetland during the three-year investigation. Richness and diversity increased as lake stage declined. Low water levels stimulated the growth of benthic algae and likely excluded piscivorous predators from the wetland. Scrapers increased 22% between 1998 and 2000, and predators increased 6%. Invertebrate distributions within the wetland shifted in response to declining lake levels, changes in hydrology, and food resources. Twenty-one taxa were only collected during the low water year (2000) when water depth in the wetland averaged 0.09 m. All of these taxa are common to temporary wetlands.; The documented changes in surface water chemistry and invertebrate community composition illustrated the importance of water level fluctuations in the ecology of Great Lakes coastal wetlands. Climate change predictions for the Great Lakes basin suggest that the frequency and duration of below average lake levels will increase. Understanding how water level fluctuations influence wetland water quality, productivity, and habitat is critical for anticipating future climate-driven changes to the Great Lakes ecosystem.