HYDRAULIC AND GEOMORPHIC INFLUENCES ON NITRATE RETENTION IN A FRESHWATER TIDAL MARSH

Freshwater tidal marshes border stream channels near the upstream end of the tidal limit. These marshes are significant sites for nitrogen processing, which may occur in the marsh groundwater, on marsh surfaces, or in channels. Freshwater tidal marshes often contain extensive channel networks that deliver nitrate-rich water from the main channel into the marsh ecosystems. The goal of this study is to determine geomorphological, hydrological, and biogeochemical processes that influence nitrogen retention in freshwater tidal marshes. Nitrogen retention was examined by using a mass balance approach. The geomorphic and hydrologic characteristics of the marsh network create a systematic scheme for sampling water and nutrient fluxes. Tidal stage, velocity, discharge, and the concentration of nitrogen species were measured for tidal cycles at the mouth of tidal channels of various sizes and stream orders (1-7). Nitrate was the main species that showed net loss. Nitrate mass balance calculations indicate an exponential increase in net nitrate retention with channel order. This nitrate retention relationship is similar to the relationship between marsh area and stream order, but high order (large) tidal marshes process proportionally less nitrate per surface area than small marshes. This suggests that biogeochemical processing may be limited by the amount of water and nitrate that can be brought into the marsh, which is controlled by the marsh channel hydraulics. The mass balance calculations of nitrate retention were compared with rates of marsh denitrification measured at different sites within the system. Measurements of groundwater fluxes indicate that groundwater denitrification in this system is limited to regions near the tidal channels, and it is less significant than marsh surface nitrogen processing. These data suggest that marsh surfaces are the main nitrate processing sites, but the efficiency of processing decreases with an increase in marsh area.