Abstract
Abstract Primary productivity was measured in the Indian Sector of the Southern Ocean (30° to 80°E) as part of a multi-disciplinary study during austral summer; Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West Survey, 2006). Gross integrated (0–150 m) productivity rates within the marginal ice zone (MIZ) were significantly higher than within the open ocean, with averages of 2110.2±1347.1 and 595.0±283.0 mg C m−2 d−1, respectively. In the MIZ, high productivity was associated with shallow mixed-layer depths and increased Pmax up to 5.158 mg C (mg chl a)−1 h−1. High Si:N drawdown ratios in the open ocean (4.1±1.5) compared to the MIZ (2.2±0.79) also suggested that iron limitation was important for the control of productivity. This was supported by higher Fv/Fm ratios in the MIZ (0.50±0.11 above 40 m) compared to the open ocean (0.36±0.08). As well, in the open ocean there were regions of elevated productivity associated with the seasonal pycnocline where iron availability was possibly increased. High silicate drawdown in the north-eastern section of the BROKE-West survey area suggested significant diatom growth and was linked to the presence of the southern Antarctic Circumpolar Current front (sACCF). However, low assimilation numbers (12.8–23.2 mg C mg chl a−1 d−1) and Fv/Fm ratios indicated that cells were senescent with initial growth occurring earlier in the season. In the western section of the survey area within the MIZ, high NO3 drawdown but relatively low silicate drawdown were associated with a Phaeocystis bloom. NO3 concentrations were strongly negatively correlated with column-integrated productivity and chlorophyll biomass which was expected given the requirement for this nutrient by all phytoplankton groups. Regardless, concentrations of both NO3 and silicate were above limiting levels within the entire BROKE-West survey area (N>15.7 μM, Si>18.3 μM) supporting the high-nutrient low-chlorophyll status of the Southern Ocean.
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