Stratification of zooplankton in the northwestern Indian Ocean

Abstract

Study on stratification of zooplankton in the north western Indian Ocean was carried out with special reference to its relative abundance and distribution. Samples were collected using multiple plankton net, during first cruise of ORV Sagar Kanya. Sixty samples from 12 stations were collected from the northwestern Indian Ocean, between lat. 10 degrees N and 4 degrees S and long. 52 degrees - 63 degrees E from 5 depth zones during the latter part of the southwest monsoon from 25th July to 30th August, 1983. In most of the samples, the dominant groups in the order of their numerical abundance were copepods, chaetognaths, foraminifera, ostracods and decapods. The study has revealed a pronounced diurnal vertical migration in almost all areas, showing a two-fold increase in numerical abundance in night collections (10755/100 m sup(3)) compared to day collections (5688/100 m sup(3)). The equatorial region was found to be relatively rich in total numbers of organisms (8405/100 m3) and taxa (16) when compared to the areas between lat. 5 degrees N - 10 degrees N. It was also noticed that collections made between 800-600 m depth were mostly constitutEd. by organisms of larger size and of mesopelagic habitat