Distinctive zonation of planktonic ostracods assemblages in the dynamic Southeastern Atlantic

Abstract

Species diversity, abundance and spatial distribution of planktonic Ostracoda in the frontal waters of the South Atlantic were studied in summer 2009. Mesozooplankton materials were collected using WP-2 net in the top 300 m layer, from 32 stations located along the profile designated from the southern coast of Africa towards the Antarctic. Twenty-one species of ostracods were recorded. Ostracoda species diversity generally decreased with increasing latitude. Their total abundance showed a parabolic pattern of distribution, with lower numbers at the edges of the transect and higher numbers in the middle. We identified three distinctive areas occupied by different assemblages of Ostracoda. The first area, the Benguela Current (BENG), was inhabited mainly by subtropical ostracods. The second area - Southern Subtropical Convergence, Subantarctic Front, Subantarctic Zone and Polar Front (SSTC, SAF, SANT and PF) - was characterized by the presence of a mixture of subtropical, subantarctic, and Antarctic species. The third area, the Antarctic Zone (ANTA), was inhabited mostly by subantarctic and Antarctic ostracods. Moreover, we confirmed that the SSTC was an almost impassable hydrographical barrier for subtropical-tropical Ostracoda species. This study demonstrated the distribution of some species extending further north (e.g., Metaconchoecia australis) than previously recorded. Morphometric analysis of the most common ostracod species, Discoconchoecia elegans, showed an increasing trend in its mean individual size from the BENG zone to the ANTA zone.