Regeneration of nutrients and biological productivity in Antarctic waters

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Abstract
Watermass structures, productivity and nutrient characteristics are presented for Antarctic water adjoining the Indian Ocean, between 11 and 53.degree. E longitude, based on data collected during 4 expeditions from 1982 to 1986. From contours of potential temperature, salinity, and potential density and nutrients, the Subtropical Convergence (STC) zone is located around 42 to 38.degree. S and the Antarctic Convergence (AC) zone between 53 and 46.degree. S. In the Divergence region, which occurs around 65.degree. S, nutrient-rich Warm Deep Water is brought to the surface. Nitrate and phosphate isoplets indicate the dominance of physical mixing processes over biological processes. Markedly high chlorophyll (chl a) and primary productivity (PP) values in the coastal ice edge zone (mean surface values: chl a, 1.60 .+-. 1.03 mg m-3; PP, 1.61 .+-. 1.0 mg C m-3 h-1) compared to those in the oceanic region (mean chl a, 0.30 .+-. 0.09 mg m-3; PP, 0.3 .+-. 0.06 mm C m-3 h-1) suggest that the coastal waters are far more productive. Measurements on a transect from 69 to 30.degree. S indicate that the Antarctic region south of the Convergence (south of 58.degree. S) is a highly productive zone. A sharp decrease in the silica concentration from the divergence up to the STC indicates a higher demand of this nutrient by Antarctic diatoms. Positive anomalies of non-conservative parameters like nitrate, phosphate and silicate show that the atomic .DELTA.N/.DELTA.P ratio south of STC is 10, whereas .DELTA.Si/.DELTA.N shows 2 slopes in this region reflecting the difference in silica regeneration rates in subsurface and deep Antarctic waters. The .DELTA.Si/.DELTA.N oxidative ratio in the region south of STC shows a 5-fold increase as compared to the region north of it. The changes in the 2 regions may reflect the difference in the regeneration processes of these nutrients. From the .DELTA.N/.DELTA.P anomaly and oxidative ratios it can be concluded that the silica remineralisation is high in the region south of the STC and is mostly confined to the Antarctic subsurface waters.