Pelagic community structure of the subtropical convergence region south of Africa and in the mid-Atlantic ocean

Abstract

Cross-frontal changes in the microphytoplankton, zooplankton and micronekton species composition and biomass were investigated in two sectors of the Subtropical Convergence region (STC) to evaluate patterns in the pelagic community in areas of contrasting hydro- dynamic structure. The first sector was south of Africa (±20°E, winter 1993) where the frontal zone is relatively permanent and intense. The other sector was in the mid-Atlantic ocean (±2°E, summer 1994) where the STC is ephemeral and weak. Higher biological diversity and weaker zonation patterns were observed in the mid-Atlantic sector, relative to the sector south of Africa. This indicates that the boundaries of the STC were more relaxed in the former region, suggesting that the structure in the mid-Atlantic community is less controlled by hydrodynamic forcing. In both sectors, species of Antarctic and subtropical origin were present on both sides of the convergence, suggesting that cross-frontal mixing was prevalent. Changes in the relative proportion of microphytoplankton, micro- and mesozooplankton in both regions appear to reflect the seasonality of sampling, rather than regional differences in the pelagic food web structure. Despite the marked contrast in the intensity of the hydrographic front between the two sectors, higher phytoplankton, zooplankton and mesopelagic fish abundances were consistently associated with the Subtropical Convergence, reflecting the importance of this region in the pelagic production of the south Atlantic Ocean.