Nutrients in the Southern Ocean GLOBEC region: variations, water circulation, and cycling

As part of the US Southern Ocean Global Ecosystem Dynamics (SO GLOBEC) study on the West Antarctic Peninsula (WAP) continental shelf, four surveys were conducted along a grid of close-spaced stations lying within an area composed of Marguerite Bay (MB) and the adjacent continental margin. The surveys occurred in the autumn and winter of both 2001 and 2002. A detailed repetitive plan of the SO GLOBEC surveys permitted the delineation of the important features of nutrient processes. Horizontal intrusions of Upper Circumpolar Deep Water onto the WAP continental shelf do not appear to produce fluxes of nitrate and phosphate onto the shelf but appear to lead to a dilution of the silica inventory on the shelf. High silica concentrations (>115 μM) observed in the near-bottom waters on the shelf were most likely generated locally through dissolution of opal in sediments. nt deficits and net community production (NCP) were calculated by the integration of decreases in nutrient concentrations in the mixed-layer below corresponding values in the underlying remnant Winter Water layer. Waters in MB and other coastal areas were strongly depleted in nutrients—deficits of total inorganic nitrogen (TIN=NO3−+NO2−+NH4+) and silica were >0.6 mol m−2 and >2.5 mol m−2, respectively—and enriched in ammonium stocks (>0.25 mol m−2) relative to other parts of the grid, especially in the autumn of 2001. Autumn NCP values calculated from TIN deficits in MB equaled or exceeded autumnal NCP values from the Joint Global Ocean Flux Study Ross Sea study in 1997. Observed ΔN/ΔP and ΔSi/ΔN removal ratios were 12.4±1.6 and 3.9±1.2, respectively, in 2001; and 11.1±1.3 and 4.1±1.1, respectively, in 2002. These results strongly suggest the dominance of diatoms in the study region. Co-location of maxima in ammonium stocks with the areas of the highest TIN and silica deficits indicated a spatial coupling between primary and heterotrophic production in both years. This co-location highlights the finding that temporal trends in ammonium were usually the opposite of trends in the other nutrients.