Insight into the nitrogen cycling in the North Sea

Fixed nitrogen (N) is an important element which may limit marine primary production. Nitrogen inputs to coastal waters have increased putting pressure on this ecosystem. Supply and removal of N depends on a series of N-cycling processes including canonical denitrification and anammox which remove N, while dissimilatory nitrate reduction to ammonium (DNRA) recycles N allowing it to remain available for primary producers. The sediments in the coastal zone are a key site for these processes but the environmental factors regulating them are still poor understood and the fluxes poorly quantified. This study investigated sedimentary N-cycling at 5 sites in the open North Sea in August 2013, and at one station in the Wash estuary in May, June, September and October 2013 using 15N tracer techniques, pore water studies and direct sediment flux measurements. All sites had relatively low sedimentary organic carbon content (95%) with on average >80% associated with coupled denitrification. The average rates of denitrification were higher in the North Sea (7.62 mol m-2h-1) than in the Wash (4.4 mol m-2h-1). Anammox was not detected at the Wash sites and contributed only 6.6% to total N2 production at the North Sea sites. DNRA was observed during three of the months studied at the Wash sites but only at one North Sea site and, where measurable, was responsible for between. 6.5 and 30% of nitrate reduction. Temperature was identified as an important control on the overall rates of denitrification. The results indicate that denitrification is a major sink for nitrate in the North Sea