Nitrification and dissimilatory ammonium production and their effects on nitrogen flux over the sediment-water interface in bioturbated coastal sediments

Nitrification and dissimilatory reduction of nitrate to ammonium were measured concomitantly with nitrogen release from marine coastal sediment samples from 2 fjord sill stations. Dissimilatory ammonium production (DAP) and nitrification were measured using core injections of "NO; and H1"CO;, respectively. DAP was detected in all segments of the cores by tracing "NH: evolved from the added "NO;. 15NH: recovery increased with increasing core depth, ranging from 1.6 to 10 % for Stn H and from 0.3 to 2.9 % for Stn L. Nitrification activity at Stn L was in the order of 10 nmol cm-3 h' in the upper 2 cm but was not demonstrated in deeper strata. Consequently, DAP was most pronounced in the upper few centimeters although it was anticipated that more than 97 % of the nitrate produced was denitnfied. Mean fluxes of ammonlum out of the sediment were 24 and 12 pm01 m-2 h-' for Stns H and L, respectively, and corresponding nitrate fluxes were 2 and -24 pm01 m-2 h-'. The sum of ammonium and nitrate release in the individual cores did not reach the rates expected from their oxygen consumption rates, which implies that inorganic nitrogen was lost, probably due to coupled nitrification and denitrification. This estimated loss was about half of the obtained nitrification rate at Stn L. Furthermore, the estimated loss was larger in cores with a rich macrofauna and especially with high numbers of Arnphiura spp. (brittlestars). It is suggested that these animals stimulate both nitrification and dissimilatory nitrate reduction.