Nutrient and biogenic particulate distributions, primary productivity and nitrogen uptake in the Weddell-Scotia Sea marginal ice zone during winter

During austral winter of 1988, we determined the distributions of inorganic nutrients (nitrate, silicic acid, phosphate, nitrite and ammonium) and particulate materials (chlorophyll, biogenic silica, particulate organic carbon and particulate organic nitrogen), as well as primary productivity and rates of nutrient (NO3− and NH4+) uptake in the upper 150 m of the marginal ice zone of the Weddell-Scotia Sea. Nutrient concentrations were high and particulate matter levels were low throughout the study area, but occasionally nutrient minima and particulate maxima occurred near the ice edge associated with warm-core eddies. Chlorophyll concentrations and primary productivity averaged 0.12 μg l−1 and 32 mg C m−2 d−1, respectively. Surface growth rates calculated from carbon uptake and total particulate organic carbon were very low (ca. 0.03 doublings d−1), but living phytoplankton only comprised about 10% of the POC in the surface layer. Thus, mean phytoplankton growth rates appear to have been between 0.1 and 0.2 doublings d−1. Although nitrate was about 40 times as abundant as ammonium, ammonium was consistently the preferred substrate of the plankton assemblages, accounting for over half of the nitrogen taken up. Paired samples from the same depth and vertically integrated f-ratios averaged 0.43. Both ammonium and nitrate were removed at rates that individually exceeded the apparent nitrogen demand of the phytoplankton, implying significant heterotrophic uptake of inorganic nitrogen