Water level fluctuation and sediment-water nutrient exchange in Great Lakes coastal wetlands

The influence of water level fluctuation on sediment-water nutrient exchange in coastal wetlands of Lakes Michigan and Huron was investigated using controlled, laboratory experiments. At each wetland, sediment cores were collected from 5 locations along a transect perpendicular to the shoreline, desiccated for 8. weeks, and then re-wetted with original site water for 24. h to simulate water level fluctuation. Soluble reactive phosphorus release declined exponentially along transects, with highest release rates from sediments collected at the ordinary high water mark (OHWM), and lowest rates from sediments underlying water > 0.25 m in depth. Nitrate exchange showed no obvious pattern in the Lake Michigan wetlands but nitrate was lost at all locations in the Lake Huron wetlands, suggesting denitrification. Ammonium was released at all sites, but with no obvious pattern along transects. Sulfate release was low at the OHWM locations and increased in a lakeward direction, plateauing by the 0.25. m water depth. Based on phosphorus (P) release rates, we modeled water level scenarios based on climate change projections to estimate the effects of 3 different water level recessions and 1 water level increase on system-level P exchange. The impact of the P release could be ecologically significant in the localized area of release given the high P concentrations measured in the overlying water during sediment incubations (> 500. μg/L). The long-term ecological significance of sediment nutrient flux under fluctuating water levels deserves further attention.