Water-column remineralization in the Indian sector of the Southern Ocean during early spring

The vertical distribution of remineralization assessed through the respiratory activity of the electron transport system (ETS) in microbial communities (<200 μm) was studied from surface to 1000 m depth, at seven stations along a latitudinal gradient extending from the Polar Front region (49°S) to the ice edge (59°S) at 62°E, during October 1995. A net imbalance between primary production and water-column remineralization occurred in the area of influence of the Polar Front (from 49°S to 53°S), while near the ice edge both processes were in approximate equilibrium. In the Polar Front region, community respiration in the mixed layer (15–35 mmol C m−2 day−1) exceeded primary production (10–11 mmol C m−2 day−1), while remineralization in the 200–1000 m, although variable (2–9 mmol C m−2 day−1), was considerably higher than export production (<1 mmol C m−2 day−1) measured with drifting sediment traps during the same study. This imbalance can be explained by advection of organic matter from the Crozet Plateau where phytoplankton blooms are produced year round, presumably enhanced by iron enrichment. The surface transport would be secured by strong current jets created by fragmentation of the Antarctic Circumpolar Current at the Southwest Indian Ridge. Relatively high respiration rates found below 500 m support also the occurrence of a deep transport of organic matter. Our results suggest that dissolved organic carbon might be the main source of organic matter advected to the region during the period of study. The average value of export production of organic carbon calculated from ETS activity and extrapolated to an annual basis (∼1.5 mol C m−2 yr−1), represents about 25% of recent estimates of annual gross primary production in the Southern Ocean.