COMPOSITION AND COMMUNITY STRUCTURE OF PELAGIC COPEPODS IN THE INDIANSECTOR OF THE ANTARCTIC OCEAN DURING THE END OF THE AUSTRAL SUMMER

The present paper describes latitudinal and vertical changes in the co mposition, abundance and diversity of copepods in the Indian sector of the Antarctic Ocean, during the end of austral summer along a transec t on 66 degrees 30'E between 43 and 62 degrees S, within three layers (600-0, 200-0, 100-0 m). Highest copepod densities were noted in the c entral part of the transect, between the Antarctic Divergence and the Antarctic Convergence, with a maximum in the Antarctic Divergence zone , particularly in the upper levels of the water column. A total number of 80 copepod species were identified over the entire survey area. Th e south end and the central part of the transect comprised a small num ber of species. North of the Antarctic Convergence, this number increa sed markedly with the progressive disappearence of those species chara cteristic of Antarctic waters and their replacement by temperate and s ubtropical species. Generally, small copepods, particularly Oithona si milis, Oithona frigida and Ctenocalanus citer, dominated in numbers in both Antarctic and sub-Antarctic areas. The contribution of large spe cies to total copepod numbers was much lower, with Calanus simillimus in the central part of the transect, Pleuromamma borealis in the subtr opical zone and Calanus propinquus in the southern part. Correspondenc e analysis showed a marked latitudinal gradient in population structur e with four groups of samples and species corresponding to four latitu dinal zones. Community structure (species richness, relative dominance index, evenness, Shannon species diversity index) and species abundan ce patterns (as rank-frequency diagrams) suggested that the maturity a nd species richness increased gradually from south to north. A low div ersity index and evenness were observed in the area of the Antarctic D ivergence, whereas the convergence zone showed high diversity and even ness. Conversely, the frontal zone showed high diversity and evenness. Distribution appeared unrelated to chlorophyll concentrations and on the large scale was related to the hydrologic characteristics.