Microbial mediation of ‘reactive’ nitrogen transformations in a temperate lagoon

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 NH4 during all seasons and in net uptake of NO3 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 NH4 pool; however, sediment-water fluxes of both NH4 and NO3 were either negligible or directed into the sediments. The NH4 produced by gross mineralization was rapidly consumed in the dark. Biological processes potentially responsible for removal of sediment NH4 and NO3 include coupled nitrificationdenitrification, dark uptake by benthic microalgae, and immobilization by heterotrophic bacteria. In the absence of dark uptake of NH4 by benthic microalgae, potential nitrification calculated as the difference between gross mineralization and NH4 fluxes, would range from 1.5 to 6.4 mmol N m–2 d–1, similar to rates observed in a range of other systems. Similarly, potential denitrification rates estimated as the difference between calculated nitrification rates and measured NO3 fluxes would vary from 1.88 to 5.16 mol N m–2 d–1 and fall within the range of rates reported for similar systems. However, since calculated benthic microalgal N demand (2.51 to 16.11 mmol N m–2 d–1) exceeded NH4 release by gross mineralization at all sites and during all seasons, this suggests that dark benthic microalgal uptake was likely to be an important sink for mineralized N. Finally, sediment bacterial N immobilization may also be important given the relatively high C/N of sediment organic matter. These estimates of the potential consumptive processes for mineralized sediment N indicate that the lagoon is likely to retard and or remove ‘reactive’ N during its transport to the coastal ocean.