MISSISSIPPI RIVER DIVERSIONS WETLANDS: NITROGEN REMOVAL IN A FLUCTUATING SALINITY ENVIRONMENT

Mississippi River Diversions provide a conduit to reconnect riparian wetland areas in the Mississippi River delta with the river. The Mississippi river contains nitrate concentrations as high as 2 mg/L during the spring flood. The Caernarvon diversion can supply up to 8000 cfs of river water with this nitrogen load to Breton Sound significantly freshening the basin. In addition, these low lying coastal wetlands can receive nearly instantaneous injections of high saline ocean water during the passage of hurricanes due to the high storm surge. The effects of these fluctuations on denitrification, the major N removal pathway of wetlands is generally unknown. Therefore, we investigated the denitrification rates of both freshmarsh and saltmarsh under different salinities to determine the effect of changing salinities on this microbial-mediated process. Rates were determined over short (1-2 days) and longer (>8 days) time intervals. Denitrification of salt marsh soil was an order of magnitude lower under fresh condition than 35 ppt. Denitrification rates were affected by changes in salinity over the short-term, and in a few cases, the negative effects lasted > 5 days. Salinity fluctuations that decrease nitrogen removal may lead to greater coastal water loading of nitrate, which is the primary driver of coastal eutrophication and consequent coastal hypoxia.