Phytoplankton distribution in the Indian sector of the Southern Ocean during spring

During October 1995 the dynamics of phytoplankton were investigated in the Indian sector of the Southern Ocean along a 62°E meridian transect between 49°00′S and 58°50′S. During spring, chlorophyll a (chl a) concentrations did not exhibit a large spatial variability. The highest chl a concentrations (0.30–0.45 mg m−3) were encountered in the surface water of the marginal ice zone (MIZ) and the polar frontal zone (PFZ), while in the permanent open-ocean zone (POOZ) maximum values were always <0.30 mg m−3. Total integrated chl a progressively decreased from South to North and again increased at the margin of the PFZ. Chl a stock values were 61 mg m−2 at the ice edge, from 40 to 52 mg m−2 in the POOZ, and 57 mg m−2 in the PFZ. Chlorophyll a-size fractionation indicated that autotrophic communities were dominated by picoplankton (size <2 μm) in the permanent ice-free regions (POOZ and PFZ) where it contributed 50–61% of the chl a stock. The >10 μm size-fraction was dominant at the ice edge with a contribution of 42%, whereas in the POOZ and the PFZ it accounted for 21–31% of the total chl a stock. Over the transect, the nucleic acid stocks varied from 331 to 547 mg DNA m−2 and 190 to 681 mg RNA m−2. Their distribution exhibited the same general pattern as chl a, with the highest values in the MIZ and PFZ. Autotrophic phytoplankton abundance as measured by flow cytometry did not show spatial variability. Cell numbers ranged from 0.8 to 6×106 l−1 in the upper 200 m, and the maximum was observed in the PFZ surface waters. The comparison of the present spring data with summer 1994 data obtained at the same sites (J. Mar. Syst. 17 (1998) 179–194) indicates that the spatial distribution of chl a concentration showed the same pattern. In both seasons the highest chl a concentrations were encountered in the MIZ and PFZ. However, in the whole area spring chl a stock was on average 1.5–3 times as high as summer stock. Distribution of size-fractionated phytoplankton was similar during spring and summer. In the POOZ and PFZ the pico-size fraction was always dominant, accounting for more than 50% of chl a stock. In contrast cells >10 μm were always dominant in the MIZ. Despite vertical water stratification due to ice melting and low grazing pressure the chl a concentrations remained low (<0.5 mg m−3) during spring and summer in the MIZ. Our data show that a spatially complex phytoplankton field characterized the ice-edge. This suggests that in exposed open-ocean regions other environmental factors, such as wind forcing, could induce a rapid horizontal dispersion of meltwater that would prevent phytoplankton enhancement.