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 gr oups (coccolithophores, diatoms, silicoflagellates and dinoflagellates) has been studied along a transect from Cape Town (34 degrees S) to South Sandw ich Islands (57 degrees S) during the late austral summer. The observed dis tribution of these groups shows that their biogeographical distribution is significantly constrained by the water masses and associated frontal system s of the Southern Ocean. Coccolithophores are the dominant group and show c ell abundances up to 51 x 10(3) cells/1 down to 57 degrees S. Three restric ted areas are marked by particularly high cell densities: the continental s helf 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 x 10(3) cells/l). The spe cies composition of the various assemblages representative of the four grou ps defines distinct biogeographical zones bounded by marked sea-surface tem perature gradients. This biogeographical distribution is confirmed by facto r analysis of the coccolithophore (5 factors, 85% of the total variance) an d diatom and silicoflagellate (7 factors, 87.5% of the total variance) popu lations. When compared with the distribution pattern of siliceous fossil as semblages in surface sediments, our data show a more accurate coupling betw een the various water-masses of the South Atlantic Ocean and the living sil iceous population. (C) 1999 Elsevier Science Ltd. All rights reserved.