Microbial mediation of 'reactive' nitrogen transformations in a temperate lagoon

Abstract

Coastal lagoons positioned along the land margin may play an important role in removing or transforming 'reactive' nitrogen during its transport from land to the ocean. Hog Island Bay is a shallow, coastal lagoon located on the ocean-side of the Delmarva Peninsula in Virginia (USA). External nitrogen inputs are derived primarily from agriculturally enriched groundwater, and these support, in part, the high production of benthic macroalgae and microalgae as the dominant primary producers. This study focuses on processes in the water column (phytoplankton and bacterial) and in the sediments (microalgal and bacterial) responsible for transformations of dissolved inorganic and organic nitrogen (N). Sediment-water exchanges of dissolved inorganic and organic N were measured as well as sediment gross and net mineralization of organic N. Net changes in dissolved inorganic nitrogen concentrations were greater in the water-column incubations than in the incubations including sediment and water. In the water column, metabolism resulted in net uptake of NH 4 + during all seasons and in net uptake of NO 3 -during most seasons. In the sediments, gross mineralization, which ranged from 0.9 to 6.5 mmol N m -2 d -1 , resulted in short turnover times (<1 d) for the sediment NH 4 + pool; however, sediment-water fluxes of both NH 4 + and NO 3 -were either negligible or directed into the sediments. The NH 4 + produced by gross mineralization was rapidly consumed in the dark. Biological processes potentially responsible for removal of sediment NH 4 + and NO 3 -include coupled nitrificationdenitrification, dark uptake by benthic microalgae, and immobilization by heterotrophic bacteria. In the absence of dark uptake of NH 4