The role of sediments on the Bering Sea shelf N cycle: Insights from measurements of benthic denitrification and benthic DIN fluxes

Continental shelves are hotspots for sedimentary denitrification, and the loss of N through denitrification can limit primary production in ecosystems. Spatial and seasonal trends in sedimentary denitrification and benthic nutrient fluxes are poorly characterized in the highly productive Bering Sea shelf ecosystem. Through the Bering Sea Ecosystem Study (BEST) program, we measured benthic fluxes of N2 and dissolved inorganic nitrogen (DIN: NH 4 + + NO 2 − + NO 3 − ), the extent of coupled sedimentary nitrification/denitrification, and the water column DIN deficit relative to phosphate, as indicated by a modified N* parameter (N**), on the Bering Sea shelf in the spring and summer 2009–2010. We found that sedimentary denitrification is widespread over the shelf, it is fueled mostly through coupled nitrification/denitrification, the net balance of sedimentary DIN flux is near zero over the shelf, and that the water column DIN deficit varies widely according to season and year. In the summer, N** in the surface layer appeared to be strongly affected by non-Redfieldian uptake of inorganic nutrients by phytoplankton in the spring bloom; in the winter, N** was strongly affected by sedimentary denitrification. Our findings indicate that the estimate of total N loss in Bering Sea shelf sediments should be revised upwards by at least 50% to 5.2–6.2 Tg N y−1. In addition, sediments are not a significant source of remineralized N for primary production over the shelf; hence sedimentary denitrification exacerbates N-limitation of the ecosystem.