The role of sediment denitrification in reducing groundwater-derived nitrate inputs to Nauset Marsh estuary, Cape Cod, Massachusetts

The Nauset Marsh estuary is the most extensive (9.45 km(2)) and least distu rbed salt marsh/estuarine system within the Cape Cod National Seashore, eve n though much of the 19 km(2) watershed area of the estuary is developed fo r residential or commercial purposes, Because all of the Nauset watershed i s serviced by on-sire individual sewage disposal systems, there is concern over the potential impact of groundwater-derived nutrients passing from the se systems to the shallow receiving waters of the estuary. The purpose of t his study was to determine whether denitrification (the bacterial conversio n of nitrate to gaseous nitrogen) in estuarine sediments could effectively remove the nitrate from contaminated groundwater before it passed from the watershed to the estuary. Rates of denitrification were measured both in si tu and in sediment cores, in areas of active groundwater discharge, in rela tively pristine locations, and in areas situated down-gradient of moderate to heavily developed regions of the watershed. Denitrification rates for 47 sediment cores taken over an annual cycle at 5 stations ranged from non-de tectable to 47 mu mol N-2 m(-2) h(-1). Mean denitrification rates were posi tively correlated with sediment organic content, and varied seasonally due to changes in sediment organic content and to the effect of water temperatu res on sediment oxygen penetration depths. There was no correlation between observed denitrification rates and corresponding nitrate concentrations in groundwater. A comparison of in situ denitrification rates (supported by g roundwater nitrate) with denitrification rates observed in sediment cores ( supported by remineralized nitrate) showed that groundwater-driven denitrif ication rates were small, and not in excess of denitrification rates suppor ted by remineralized nitrate. Most of the denitrification in Nauset sedimen ts was apparently fueled by remineralized nitrate through coupled nitrifica tion/ denitrification. Denitrification did not contribute significantly to the direct loss of nitrate from incoming groundwater at Nauset Marsh estuar y. Groundwater flow was rapid, and much of it occurred in freshwater spring s and seeps through very coarse, sandy, well-oxygenated sediments of limite d organic content. There was little opportunity for denitrification to occu r during groundwater passage through these sediments. These results have im portant management implications because they suggest that the majority of n itrogen from contaminated groundwater crosses the sediment/water interface and arrives at Nauset Estuary, where it is available to primary producers. Preliminary budget calculations suggest that while denitrification was not an effective mechanism for the direct removal of nitrate in contaminated gr oundwater flowing to Nauset Marsh estuary, it may contribute to significant nitrogen losses from the estuary itself.