Fate of Riverine Nitrate Entering an Estuary: I. Denitrification and Nitrogen Burial

Abstract

Fate of riverine nitrate entering a well defined turbid estuary receiving discharges from the Atchafalaya River, a distributary of the Mississippi River, was determined. Seasonal distribution of NO3 and its transformations were measured in Four League Bay (9,300 ha). Denitrification was estimated by incubating wet samples in the presence of acetylene and monitoring N2O production. The annual sediment accumulation of N was also determined within the bay and within the adjacent marshes. Nitrogen accumulation ranged from 6.0 to 23 gN per m2 per yr on the marsh and 6.1 to 11.2 gN per m2 per yr in the bay. Denitrification in this system was controlled by the availability of NO3 − with fluxes ranging from 2 to 70 ngN per g per hr. The annual (N2O +N2)-N emission was equivalent to 142 and 120 μg per g or 2.1 and 1.7 gN per m2 from the 5 bay and 5 marsh stations, respectively. Approximately 1.95×105 kgN, predominantly as N2, is being returned to the atmosphere via denitrification. We estimate this to be equivalent to 50% of the riverine NO3 − entering this estuary. A significant amount was also assimilated within the estuary.