Marsh nutrient export supplied by groundwater discharge: Evidence from radium measurements

We use Ra-228 and Ra-226 to determine the mass balance of dissolved inorgan ic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the North Inle t salt marsh estuarine system. While this system has only minor freshwater inputs of nutrients or radium, it is an extremely productive ecosystem. In addition, there are significant exports of these dissolved species to the c oastal ocean. Saline groundwater in this estuarine system contains nutrient and radium concentrations more than an order of magnitude greater than sur face waters. Using a radium mass balance, we estimate the groundwater disch arge necessary to support the export of radium to the coastal ocean and the corresponding flux of nutrients from the groundwater. From these calculati ons, we show that the underlying aquifer supplies nutrients sufficient to s upport the net primary productivity of the salt marsh ecosystem and to acco unt for the known export of nutrients from the marsh. We conclude that the major nutrient source to the North Inlet, South Carolina, salt marsh is the saline aquifer lying just beneath the surface of the marsh. Furthermore, e xtrapolation of the nutrient export to include other South Carolina marshes suggests that nutrient fluxes from salt marshes to the coastal ocean rival riverine nutrient fluxes for the region.