Zonal variability in primary production and nitrogen uptake rates in the southwestern Indian Ocean and the Southern Ocean

Abstract

Hydrographic parameters along with the primary and new production measurements were carried out during the austral summer, 2009, in the southwestern Indian Ocean and Indian sector of the Southern Ocean (SO). The production varies from 185 to >900mgCm−2d−1 in different zones of SO. The zonal variations in production accompany variations in SST, salinity and nutrients. Further, the new production (0.3 to 4.1mmolNm−2d−1) covaries with the overall production, while the uptake of reduced forms of nitrogen (both NH4 and urea) show opposite trends. In the NO3 limiting environment (north of subtropical convergence), NH4 uptake dominates the total regenerated production, whereas, urea uptake dominates the regenerated production under Si, light and micronutrient (e.g., Fe) limiting conditions (found between the subtropical convergence and Antarctica). On the basis of the C and N uptake data, the studied region can be divided into five zones (from the south to the north) viz., located between (i) the Antarctic continent and the polar front (Antarctic zone; ANZ), (ii) the polar and subantarctic fronts (SAF) (Polar frontal zone; PFZ), (iii) SAF and Agulhas Retroflection fronts (ARF) (South Subtropical front; SSTF), (iv) subtropical frontal zone (STFZ), and (v) ARF and the north subtropical front (Subtropical zone; STZ). Except at SSTF, regenerated production dominates in all the zones. From the south to the north, this could be due to different reasons e.g., light, grazing by zooplankton, supply of key micronutrients (probably Fe), Si-limitation, or NO3-limitation. In the absence of such limitations, the maximum possible f-ratio in SO could be as high as 0.78±0.12 and under such conditions the region could export most of the total production to the deep. Supply of micronutrients through the Agulhas return current and from the Crozet Island supports the higher chl a, C uptake and new production at the 48°E transect relative to the 57.5°E transect. The C:N assimilation ratio is found to be 5.64, marginally lower than the canonical Redfield ratio. This slight difference is likely due to the variation in the composition of phytoplankton and NO3-limitation in some zones. A comparison with earlier results shows that seasonal and spatial variations in f-ratios in these zones are much higher than its inter-annual variability.