Effects of water level fluctuation and sediment–water nutrient exchange on phosphorus biogeochemistry in two coastal wetlands

Freshwater coastal wetlands are sensitive to drying and wetting events (DWE), associated with water level fluctuations. Prior studies have shown that DWE influence sediment–water nutrient exchange, but the fate of these nutrients has received much less attention. To address this information gap, we investigated how microbial (i.e., bacteria and phytoplankton) structure and function respond to DWE-induced release of nutrients from the sediment of mesotrophic and eutrophic coastal wetlands. Our approach was three-fold: (1) sediment from each wetland was subjected to a desiccation (2 month) and re-wetting (2 days) period in the laboratory; (2) the overlying water column from the DWE was measured for nutrient concentration; and (3) we conducted a microbial response experiment using the water from either DWE experiment (containing the nutrients released during the DWE) or with added nutrients obtained from commercially available sources (but at similar concentrations to those released from the sediments), to test the effect of nutrient concentration and source on microbes inhabiting the water column and their role in the fate of the released nutrients, in particular phosphate (Pi). In both coastal wetlands, the microbial community structure in the water column (absolute and relative abundance of different microbial groups) was modified by bacteria resuspended from the sediment after re-wetting. However, results from the microbial response experiments showed that the microbial communities did not grow following inputs of commercially available nutrients or release of sediment nutrients. In Pi amended treatments, Pi uptake rates increased but not enough to reduce Pi turnover time to values measured in the control. As a result, Pi concentration and turnover remained high. In coastal wetlands with limited hydrologic exchange, these elevated nutrients will accumulate and lead to further eutrophication. However, in open coastal wetlands, with substantial hydrologic exchange, advected nutrients may influence biological activity in the nearshore zone.