Benthic nutrient fluxes on a basin-wide scale in the Skagerrak (North-Eastern North Sea)

Abstract

Benthic ammonium, nitrite, nitrate, phosphate and silicate fluxes were measured on a basin-wide scale (14 locations) in the open Skagerrak. Fluxes were measured in situ at two of the locations using a benthic chamber lander. The benthic flux measurements revealed patterns of geographic variation of nutrient fluxed in the Skagerrak. Nitrate fluxes generally reflected sediment deposition patterns and were mainly directed into the sediment in the high accumulation areas, and out of the sediment in areas with relatively little import of allochtonous organic matter. The nitrate fluxes were related to C/N-ratios of sediments. Low C/N-ratios were associated with high (in relative terms) nitrate effluxes and high C/N-ratios were associated with high nitrate fluxes into the sediment, suggesting that the fastest net regeneration (and nitrification) of nitrogen occurred in nitrogen-rich (low C/N) sediments. The nitrate flux into nitrogen-poor (high C/N) sediments appeared to be due to denitrification in the main sediment deposition areas, where mainly allochtonous organic matter is thought to accumulate. Areas of net benthic nitrification were thus on the Danish shelf and in the western part of the Norwegian Trench. Areas of net denitrification were in the eastern, northeastern and also central deep part of the Skagerrak. In these areas virtually all of the dissolved inorganic nitrogen fluxes were directed into the sediment and they are suggested to constitute considerable sinks for non-gaseous nitrogen. Phosphate fluxes were highest in the central deep part and lower on the margins of the Skagerrak. They appeared to be positively related to clay contents of sediments. Silicate fluxes varied little: almost all fluxes were between 1 and 2 mmol·m −2·d −1. Sediment oxygen uptake did not correlate any of the nutrient fluxes except with those of silicate. It is suggested that benthic silicate fluxes reflected the deposition of a large proportion of the fast-sinking, autochtonously produced diatoms on the sea-floor of the Skagerrak, whereas a great deal of other fresh in situ produced algal material may be flushed out of this sea. There was no relation between either organic carbon contents of sediments or water depths and benthic fluxes of any of the nutrients. The phosphate and silicate fluxes measured did not correlate with any of the other nutrient fluxes, nor with each other. The ammonium fluxes were, however, generally inversely related to the nitrate fluxes with high (in relative terms) influxes of ammonium correlating with high effluxes of nitrate. We suggest this was due to nitrification, and that the nitrifying bacteria could not meet their ammonium demand with what was regenerated in the sediment, so that additional ammonium had to be taken up from the overlying water. High nitrite influxes were associated with both high nitrate influxes and effluxes. Uptake of nitrate from the overlying water in association with nitrate effluxes is interpreted as a consumption of nitrate during nitrification, and uptake of nitrate in association with nitrate uptake was probably mainly due to nitrite consumption during denitrification. The observed relation between nitrite and nitrate fluxes indicates that the rate-limiting step in benthic nitrification was the oxidation of ammonium to nitrite, and in dentrification it probably was the reduction of nitrate to nitrite.