Can created tidal marshes reduce nitrate export to downstream estuaries?

Abstract

Constructed marshes have the potential to serve as a buffer between anthropogenic sources of nitrogen and nutrient sensitive estuaries. Yet, there is little information available on the reduction of nitrogen loads that created tidal marshes might provide. A 5.6ha brackish marsh was constructed between row crop agricultural production and an estuary in North Carolina. Nitrate-N fluxes in and out of the marsh were monitored multiple times per hour for over a year to capture tidal and storm dynamics. Out of the five storms when most of the fluxes occurred, nitrate was retained in four (up to 42%), and released in the fifth, such that over the monitoring period the marsh retained 9% (40kg) of the nitrate that entered. Nutrient addition tracer studies confirmed the potential nitrate retention (up to 45%) of the marsh. The residence time of water in the constructed brackish marsh was identified as the primary factor limiting nitrate retention through an examination of the factors that were related to percent nitrate retention. A retention model based on a mass transfer coefficient was applied during the storm events and tracer studies to further evaluate the marsh N retention capacity. The model adequately simulated the nitrate retention with calibrated mass transfer coefficients (0.1–0.56m day−1) that fell within the range of those reported in literature for other systems. These results indicate that constructed tidal marshes can serve as substantial nitrate sinks and can be an integral part of overall plans to reduce the nutrient export to estuaries.