Surface phytoplankton composition and carbon biomass distribution in the Crozet Basin during austral summer of 1999: variability across frontal zones

Surface phytoplankton assemblages were studied in January/February 1999 in the Crozet Basin (43°50S–45°20S; 61°E–64°30E) between the northern Polar Zone and the Agulhas Front. Cell concentrations increased several fold northwards from the SubAntarctic Zone (SAZ) and reached peak numbers (average 2×10⁶ cells l^–l ) in the central and western Subtropical Zone (STZ). The most spectacular increase in cell numbers occurred at the Subtropical Front (STF) and was attributed to dinoflagellates and diatoms. Nanoflagellates and picoplankton were dominant in the entire area (average 2.8×10⁵–1.6×10⁶ cells l^–l). In the SAZ they were followed by coccolithophorids, dinoflagellates and diatoms. In the STZ coccolithophorids were often outnumbered by dinoflagellates. Diatoms were dominated by Pseudonitzschia delicatissima and were generally the least abundant algae, but reached peak densities of 1.2–4×10⁵ cells l^–l at, and north of the STF. Coccolithophorids contained mainly Emiliania huxleyi, but in the SAZ and STF Gephyrocapsa oceanica was a co-dominant species. Dinoflagellates were dominated by nano-sized species of Gymnodinium, Gyrodinium and Prorocentrum. The numbers of dinoflagellate and coccolithophorid species increased considerably in the convergence zone (STZ), which suggests their in-situ development. Heterotrophic dinoflagellates and ciliates were mainly present in the subtropics. Cell carbon biomass was attributed chiefly to auto- and heterotrophic dinoflagellates (av. 23–72 g C l^–l; 68–87%), showing their important contribution to the carbon flow. Variations in cell concentrations across the fronts and water masses, and the distribution of major species were most likely controlled by the combined effect of such factors as nutrient renewal in the convergence zone, availability of iron, increased water-column stability at fronts, and high horizontal gradients in surface-water temperature.