Phytoplankton variability in the Crozet Basin frontal zone (Southwest Indian Ocean) during austral summer

Size-fractionated chlorophyll a (Chl a) biomass and phytoplankton community distributions were investigated along two transects located in the Indian Ocean frontal region between 43–46°S and 62–65°E during late summer 1999. Chl a distribution was linked to the merged Subantarctic Front (SAF) and the Subtropical Front (STF) which marked the border between the cold and less saline subantarctic waters and the warm and more saline subtropical waters. Chl a concentrations increased from <0.3 μg l−1 in the subantarctic waters to 0.80 μg l−1 in the subtropical waters. In the northeastern region, a patch of high Chl a concentrations was associated with the meandering of the fronts which were in very close proximity. Pico-sized phytoplankton fractions were always dominant over the survey area contributing 54–71% to the total Chl a standing stock. In the subtropical waters, cyanobacteria were the major components of the pico-size fractions. Synechococcus spp. showed a northward positive gradient from 2–5×103 cells ml−1 in the subantarctic waters to 6×104 cells ml−1 in the subtropical waters. Prochlorococcus spp. were only present in the warmer subsurface waters north of the SAF/STF. They reached a maximum of 15.6×104 cells l−1 north of the Agulhas Front (AF). Except for the AF where the coccolithophorid Emiliana huxleyi formed 70% of the total carbon stocks, dinoflagellates <20 μm were the major contributors (50–90%) to the total carbon biomass. They were mainly composed of Gymnodinium/Gyrodinium, Prorocentrum spp. and Oxytoxum laticeps with peaks in the STF and subtropical waters. Nanoflagellates and diatoms formed together 0.2–3.9% of the entire cell carbon biomass. Diatoms (mainly, Pseudonitzschia delicatissima and Pseudonitzschia turgidula) were poorly represented in the entire survey area. The low biomass associated with low nutrient concentrations suggested a postbloom situation with a shift towards nonsiliceous pico-size communities. The low biological activity observed during this cruise was confirmed by the low RNA:DNA ratios.