Benthic Foraminiferal Mediated Denitrification as the Major Nitrogen Sink in a Semi-Enclosed Estuary

Bedford basin, located in the inner portion of Halifax, NS, Canada, is a eutrophic, partially ventilated, coastal basin. Although several bio-physical studies in the water column have been carried out, biogeochemical studies specifically focusing on the sediments are rare. To investigate the major diagenetic processes in the basin, we measured porewater geochemical profiles and conducted ex-situ flux experiments during the winter of 2019. These were used to parameterize a reaction-transport model for the benthic system. Porewater profiles revealed a 10 cm suboxic zone that was free of oxygen and H₂S. While a spatial separation between oxygen and free sulfide is a common in coastal sediments, such a wide zone was surprising. The basin receives high inputs of iron and manganese which, according to our model results, are responsible for the thick suboxic zone. Whole-core incubations revealed an efflux of ammonia (41.8±4.7 μmol cm^-2 yr^-1) and an influx of nitrate (42.8±0.4 μmol cm^-2 yr^-1), leading to a balanced DIN budget during the winter. Inverse modelling of porewater profiles and fluxes showed that 18% of the total organic carbon remineralization takes place through denitrification (58 μmol N cm^-2 yr^-1), and further indicated that the high nitrate influx is only possible via motile nitrate storing organisms. Foraminifera, Fursenkoina fusiformis, were reported previously from this site (and also observed during field work), and are known to perform denitrification. The modeling study indicates foraminiferal denitrification can contribute >63% of the total denitrification taking place in these sediments. Such foraminiferal denitrification is often reported from sediments beneath oxygen deficit zones (ODZs) in the open ocean, but reports from the coastal ocean are rare. This work suggests similarities between open oceans ODZs and sediment biogeochemistry in poorly ventilated coastal basins.