The Role of Oligohaline Marshes in Estuarine Nutrient Cycling

Abstract

Oligohaline marshes, poised at the land-sea margin, often occur where the estuary is most enriched in inorganic particles and nutrients. Although light can limit the production of planktonic communities, high nutrient concentrations and regular tidal inundation results in highly productive macrophyte and algal communities. Despite potentially important water quality values, relatively few detailed studies of N and P cycling in oligohaline marshes are evident in the literature. Because of the temporal variability in marsh flux studies, the net annual retention of N and P is best assessed by measurement ofN and P burial in the sediment. In the Chesapeake Bay and other estuaries and subestuaries, high rates of tidal marsh N and P burial indicate an important water quality function. A recent study shows the marshes of a Chesapeake Bay tributary retain a large portion of nitrogen and phosphorus entering the river from above the fall line. The marshes trap 35% of the nitrogen and 81% of the phosphorus which would otherwise be recycled, exported, or buried in the subtidal sediments of the estuary. Although there are few studies, high nitrate supply rates, potentially high nitrification rates, and high rates of sediment metabolism can result in high rates of denitrification. More complete studies of tidal marsh nutrient cycling, particularly nitrogen cycling, are needed for a better understanding of the importance of these tidal freshwater marshes to estuarine nutrient balances. Alternative methodologies for denitrification measurement are needed for more accurate measurements, and more attention needs to be paid to scaling individual measurements to whole marsh ecosystems. A new method for the measurement of net N-2(g) exchange was applied to a Chesapeake Bay tributary to develop an annual estimate of net denitrification in the marsh sediments. Denitrification rates were similar to 60 mu mol N m(-2) h(-1) with high seasonal variability. Annual calculations were made based on a loose correlation to annual ambient nitrate concentrations. This preliminary calculation suggests that an additional 10% of the fall line nitrogen may be removed by such marsh systems. More measurements of net N-2(g) exchange and computer simulation models are required to determine the net removal of fall line nitrogen by the upper estuarine marshes.