Pelagic community structure of the subtropical convergence region south ofAfrica and in the mid-Atlantic Ocean

Cross-frontal changes in the microphytoplankton, zooplankton and micronekto n species composition and biomass were investigated in two sectors of the S ubtropical Convergence region (STC) to evaluate patterns in the pelagic com munity in areas of contrasting hydrodynamic structure. The first sector was south of Africa (+/- 20 degrees E, winter 1993) where the frontal zone is relatively permanent and intense. The other sector plas in the mid-Atlantic ocean (+/- 2 degrees E, summer 1994) where the STC is ephemeral and weak. Higher biological diversity and weaker zonation patterns were observed in t he mid-Atlantic sector, relative to the sector south of Africa. This indica tes that the boundaries of the STC were more relaxed in the former region, suggesting that the structure in the mid-Atlantic community is less control led by hydrodynamic forcing. In both sectors, species of Antarctic and subt ropical origin were present on both sides of the convergence, suggesting th at cross-frontal mixing was prevalent. Changes in the relative proportion o f microphytoplankton, micro- and mesozooplankton in both regions appear to reflect the seasonality of sampling, rather than regional differences in th e pelagic food web structure. Despite the marked contrast in the intensity of the hydrographic front between the two sectors, higher phytoplankton, zo oplankton and mesopelagic fish abundances were consistently associated with the Subtropical Convergence, reflecting the importance of this region in t he pelagic production of the south Atlantic Ocean. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.