BENTHIC NUTRIENT FLUXES ON A BASIN-WIDE SCALE IN THE SKAGERRAK (NORTH-EASTERN NORTH-SEA)

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 c hamber lander. The benthic flux measurements revealed patterns of geog raphic variation of nutrient fluxes in the Skagerrak. Nitrate fluxes g enerally reflected sediment deposition patterns and were mainly direct ed into the sediment in the high accumulation areas, and out of the se diment in areas with relatively little import of allochtonous organic matter. The nitrate fluxes were related to GIN-ratios of sediments. Lo w C/N-ratios were associated with high (in relative terms) nitrate eff luxes and high C/N-ratios were associated with high nitrate fluxes int o the sediment, suggesting that the fastest net regeneration (and nitr ification) of nitrogen occurred in nitrogen-rich (low C/N) sediments. The nitrate flux into nitrogen-poor (high C/N) sediments appeared to b e 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 wes tern part of the Norwegian Trench. Areas of net denitrification were i n the eastern, northeastern and also central deep part of the Skagerra k. In these areas virtually all of the dissolved inorganic nitrogen fl uxes were directed into the sediment and they are suggested to constit ute considerable sinks for non-gaseous nitrogen. Phosphate fluxes were highest in the central deep part and lower on the margins of the Skag errak. They appeared to be positively related to clay contents of sedi ments. Silicate fluxes varied little: almost all fluxes were between 1 and 2 mmol . m(-2). d(-1). Sediment oxygen uptake did not correlate w ith any of the nutrient fluxes except with those of silicate. It is su ggested that benthic silicate fluxes reflected the deposition of a lar ge 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 wa s no relation between either organic carbon contents of sediments or w ater depths and benthic fluxes of any of the nutrients. The phosphate and silicate fluxes measured did not correlate with any of the other n utrient fluxes, nor with each other. The ammonium fluxes were, however , generally inversely related to the nitrate fluxes with high (in rela tive terms) influxes of ammonium correlating with high effluxes of nit rate. We suggest this was due to nitrification, and that the nitrifyin g bacteria could not meet their ammonium demand with what was regenera ted in the sediment, so that additional ammonium had to be taken up fr om the overlying water. High nitrite influxes were associated with bot h high nitrate influxes and effluxes. Uptake of nitrite from the overl ying water in association with nitrate effluxes is interpreted as a co nsumption of nitrite during nitrification, and uptake of nitrite in as sociation with nitrate uptake was probably mainly due to nitrite consu mption during denitrification. The observed relation between nitrite a nd nitrate fluxes indicates that the rate-limiting step in benthic nit rification was the oxidation of ammonium to nitrite, and in denitrific ation it probably was the reduction of nitrate to nitrite.