Production of nitrogen gas via anammox and denitrification in intact sediment cores along a continental shelf to slope transect in the North Atlantic

Abstract

We measured the production of N2 gas from anammox and denitrification simultaneously in intact sediment cores at six sites along a transect of the continental shelf (50 m) and deeper slope (2000 m) in the North Atlantic. Maximum rates of total N2 production were measured on the shelf and were largely due to denitrification, with anammox contributing, on average, 33% of this production. On the continental slope, the production of N2 gas decreased but the proportion due to anammox reached a maximum of 65%. This change in both amount and dominant pathway of N2 production could be explained largely by the concentration of organic carbon at each site. With increasing carbon the total production of N2 increased rapidly while the response of anammox was not significant. On the continental slope, total N2 production fell below 2 µmol N m‐2 h‐1 and anammox was strongly related (r = 0.95) to denitrification but the relative magnitude of anammox to denitrification (1.65 : 1) suggested that anammox could not be fuelled by NO2‐ from denitrification alone. On the shelf, however, where total N2 production was predominantly greater than 2 µmol N m‐2 h‐1, no relationship between anammox and denitrification was found and anammox remained constant at 1.4 µmol N m‐2 h‐1. Despite the constancy and greater availability of NO3‐ and lower temperatures on the continental slope, the significance of anammox to the total production of N2 appears primarily controlled by the overall rate of N2 production.