Mesoscale distribution of zooplankton in the Sub-Antarctic Frontal system in the Indian part of the Southern Ocean: a comparison between optical plankton counter and net sampling

(Antartic Research (ANTARES) IV Cruise, January–February 1999). Distribution of zooplankton biovolume/biomass in the Sub-Antarctic Frontal system is described for the Indian Ocean sector, northwest of Kerguelen Islands, with a comparison of two types of sampling techniques: WP2 net and optical plankton counter (OPC). During the ANTARES IV Cruise (January–February 1999), three zones were sampled: the Polar Frontal Zone (PFZ), south of the Sub-Antarctic Front; the Frontal Zone between the Sub-Tropical Front; and the Agulhas Front and the Sub-Tropical Zone, north of the main stream of the Agulhas Return Current. Copepods were the dominant group in all net samples regardless of the area considered. Maximum values were recorded in the PFZ with both net catches and OPC. A strong contrast in terms of population structure and biomass was observed between the zone south of the Sub-Antarctic Front and the zones to the north. The patterns of the zooplankton biovolume distribution by size confirmed this relationship. Biovolume 2D maps showed in this south area a more patchy distribution than in the other areas where a vertical gradient dominated. The OPC data are in fair agreement with net sample data and are spatially heterogenous in both size structure and biomass.