Latitudinal changes in copepod assemblages across the South West Indian Ridge

Abstract

In the South Atlantic Ocean, the Subtropical Front (STF) separates warm-water copepod assemblages of high diversity from cold-water assemblages of lower diversity. Comparable information from the southern Indian Ocean are scarce. Here we test observations made in the southwest sector of the Indian Ocean using samples collected during 2009 from along the South West Indian Ridge (SWIR, ∼27 - 42° S). Distinct assemblages were associated with the Agulhas Return Current (ARC), the Sub-Tropical Front (STF) and Sub-Antarctic Front (SAF). Sixty-nine percent of the structure of the assemblages could be attributable to the temperature at both the surface and 200 m, the salinity and the fluorescence signal of surface waters and the depth of the upper mixed layer. Forty-nine copepod genera and 132 species were recorded. While richness was greatest in waters of the ARC and lowest in the SAF, copepods were most abundant in the latter and scarcest in the former. Mean prosome length decreased with increasing latitude, and assemblages of the ARC included greater numbers of carnivorous taxa than those associated with either the STF or the SAF. As in the Atlantic, the STF markedly influenced the relative distribution range of copepod assemblages above the South West Indian Ridge.