ZOOPLANKTON, ESPECIALLY CALANOID COPEPODS, IN THE UPPER 1000M OF THE SOUTH-EAST ARABIAN SEA

Abstract. Vertical distribution of zooplankton in the upper 1000 m was studied from the south-east Arabian Sea in order to determine the variations in zooplankton at different depths. The distribution and migration patterns of calanoid copepod species were given special attention. The mean zooplankton standing stock in the upper 1000 m was 2.1 g dry Wt m-2, of which 97.7% was concentrated in the upper 400 m. Herbivores were generally more abundant at all depths, but did not predominate. An increase in zooplankton at night occurred in the upper 200 m, as well as at the 600-1000 m stratum. The maximum diversity of calanoid copepods also coincided with these two depths. Based on vertical ranges, the calanoid copepod species were assigned to three groups: (i) species occurring predominantly in the epipelagic layer and forming the bulk of the calanoids; (ii) relatively sparser deeper living species confined below 200 m; and (iii) species occurring throughout the water column. Some amount of vertical niche separation among congeneric species was indicated. While some species showed active migration, a good number of species were non- migratory. © 1990 Oxford University Press.