Tidal freshwater wetlands receive and retain significant amounts of water, nutrients, and sediment loads from terrestrial watersheds. Wetlands retain nutrients, particularly nitrogen, through microbial processing (e.g. denitrification), plant uptake, and burial. Previous research has provided data on these processes through plot studies and laboratory experiments; however, in situ validation of these results is necessary. Extending the localized measurements to the ecosystem scale requires an understanding of external controls on ecosystem retention processes, such as the determination of whether nitrogen retention is controlled by supply, temperature, or hydrologic transport. These controls were examined through a multi-scale, mass balance approach to measure nitrate retention in tidal freshwater wetlands of the Patuxent River, Maryland. Mass balance measurements of hydrologic and nitrate fluxes were conducted over a 3-year period on a range of marsh sizes. These mass balance results indicate that nitrate retention is not limited by incoming nitrate supply, and is not sensitive to the range of temperatures encountered during the growing season. Nitrate retention data composed of all marsh sites and seasons can be expressed as a simple function of water volume. This result suggests that nitrate retention is principally controlled by hydrologic transport in this tidal freshwater marsh ecosystem.
