Dissimilatory nitrate reduction to ammonium (DNRA) seasonally dominates NO3 ^sup -^ reduction pathways in an anthropogenically impacted sub-tropical coastal lagoon

Sediments have an important role in nutrient dynamics by providing sites for denitrification, which, in conjunction with nitrogen (N) fixation and other processes such as dissimilatory nitrate reduction to ammonium (DNRA), can regulate ecosystem N availability. Little Lagoon, Alabama, USA, is affected by anthropogenic perturbations and has N inputs from nitrate (NO3 ^sup -^)-contaminated submarine groundwater discharge. N cycle pathways (N-fixation, denitrification, anammox, and DNRA) and benthic fluxes were measured at three sites over a year to investigate the hypothesis that DNRA, by producing ammonium (NH^sub 4^ ^sup +^) as a dissimilatory end product of NO3 ^sup -^ reduction, was a significant process retaining bioavailable fixed N in Little Lagoon. DNRA was found to be an important NO3 ^sup -^ reduction pathway, especially in the summer months (study average: 52.1 [mu]mol N m^sup -2^ h^sup -1^) compared to denitrification (study average 7.7 [mu]mol N m^sup -2^ h^sup -1^). Sulfidic sediments during the majority of the year interfere with denitrification and result in decreased N loss through denitrification. The conversion of NO3 ^sup -^ to the more biologically preferred form of N, NH^sub 4^ ^sup +^ via DNRA, means that NO3 ^sup -^ reduction has considerable nourishing potential and likely will affect both ecosystem function and services. These results indicate that NO3 ^sup -^ reduction may contribute to, rather than counteract, eutrophication during warm summers in estuaries receiving high N inputs. As estuarine sediments provide a key ecosystem function of N removal, excess anthropogenic N input jeopardizes their ability to effectively remove reactive N in Little Lagoon and likely in other impacted ecosystems.