Vertical distribution of mesozooplankton in the central and eastern Arabian Sea during the winter monsoon

The vertical distribution of mesozooplankton in the central and eastern Arabian Sea was investigated during the winter monsoon in 1995. Samples were analysed from discrete depth zones defined according to oxygen and temperature profiles of the water column. Zooplankton had higher standing stock in the mixed layer compared to the strata below. The mixed layer had 78.5% of the total column biomass, while the deepest (500-1000 m) layer accounted for only 0.9%. The stratum between 500 and 1000 m had the lowest abundance of copepods as well as other zooplankton. A notable feature was that zooplankton biomass and density did not show much variations between coastal and offshore regions. Copepoda were the dominant group. Herbivores were generally more abundant at all depths. A total of 94 species of calanoid copepods were identified. Based on vertical distributions, they were assigned to four groups: (i) species restricted to the upper 200 m; (ii) predominantly surface-living species with tails to deeper waters; (iii) sparser deeper-living species generally confined below 300 m; (iv) species occurring throughout the water column. Diversity was fairly high in all strata with equitability being higher in the deeper strata.