Sea-surface distribution of coccolithophores, diatoms, silicoflagellates and dinoflagellates in the South Atlantic Ocean during the late austral summer 1995
The sea-surface distribution of four selected fossilizable phytoplankton groups (coccolithophores, diatoms, silicoflagellates and dinoflagellates) has been studied along a transect from Cape Town (34°S) to South Sandwich Islands (57°S) during the late austral summer. The observed distribution of these groups shows that their biogeographical distribution is significantly constrained by the water masses and associated frontal systems of the Southern Ocean. Coccolithophores are the dominant group and show cell abundances up to 51×103 cells/l down to 57°S. Three restricted areas are marked by particularly high cell densities: the continental shelf of South Africa, the area between the Sub-Tropical Convergence and the Sub-Antarctic Front, and the southern border of the Antarctic Polar Front, where the highest abundances are recorded (>650×103 cells/l). The species composition of the various assemblages representative of the four groups defines distinct biogeographical zones bounded by marked sea-surface temperature gradients. This biogeographical distribution is confirmed by factor analysis of the coccolithophore (5 factors, 85% of the total variance) and diatom and silicoflagellate (7 factors, 87.5% of the total variance) populations. When compared with the distribution pattern of siliceous fossil assemblages in surface sediments, our data show a more accurate coupling between the various water-masses of the South Atlantic Ocean and the living siliceous population.