SEDIMENT-WATER FLUXES OF NUTRIENTS IN AN ANTARCTIC COASTAL ENVIRONMENT - INFLUENCE OF BIOTURBATION

Rates of exchanges of nitrate and ammonium across the sediment-water i nterface were measured in an inshore marine environment at Signy Islan d, South Orkney Islands, Antarctica, over 6 months from August 1991 to February 1992. The sediment was a source of ammonium to the water col umn but a sink of nitrate, although nitrate exchange rates were very v ariable. Concentration profiles of nitrate and ammonium in the sedimen t porewater corroborated the measured vertical exchanges. Bioturbation , by a largely amphipod benthic infauna which was confined to the top 2 cm of sediment, was investigated experimentally. Removal of bioturba tion depressed sedimentary O2 uptake by 33% and sedimentary release of NH4+ by 50%. In contrast, in the absence of bioturbation, the removal of NO3- from the water column by the sediment increased in rate. The measured fluxes of ammonium and nitrate from the sediment did not matc h with the amount of nitrogen mineralised within the sediment, and ure a may account for the difference. It is suggested that the export of n itrogen from the bottom sediment may be significant in sustaining prim ary production in the Antarctic inshore environment. Ammonium and urea are preferred to nitrate as a nitrogen source by phytoplankton. The n itrate concentrations in the sediment porewater were low, but a large pool of nitrate was identified in the top 0-2 cm layer, which was rele ased by KCl extraction or by freezing of the sediment. This extractabl e pool of nitrate did not equilibrate with the soluble nitrate pool in the sediment, but seemed to be released from components of the benthi c infauna, which were also largely confined to the top 0-2 cm. The phy siological role of this nitrate is unknown.