VERTICAL DISTRIBUTION OF MACROZOOPLANKTON BIOMASS AND ITS MAJOR CONSTITUENTS IN THE NORTHERN ARABIAN SEA DURING JANUARY TO MAY

In the tropical region of the Indian Ocean the distribution of the zooplankton in the euphotic zone of the upper 200 metres is known in great detail from the results of Oceanographic Expeditions. The zooplankton biomass distribution from depths below 200 metres and its zoo-constituents are known from (i) the results of the John Murray expedition (Sewell, 1948), (ii) observations at seven stations by R. R. S. 'Discovery' (Foxton, 1957) and (iii) the eastern sector of the Indian Ocean during the R. V. Vityaz Cruise in 1962 (Daniel and Prem Kumar, 1965). Therefore, studies were undertaken on zooplankton samples collected from 400 .. 0 m at 72 stations and from 900-0 m at 6 stations, during the Oceanographic Expedition on I. N. S. DARSHAK in the northern Arabian sea from January to May 1974. In this paper, the biomass and the constituents of macrozooplankton measuring more than 1.32 mm based on these data are presented.


INTRODUCTION
In the tropical region of the Indian Ocean the distribution of the zooplankton in the euphotic zone of the upper 200 metres is known in great detail from the results of Oceanographic Expeditions.The zooplankton biomass distribution from depths below 200 metres and its zoo-constituents are known from (i) the results of the John Murray expedition (Sewell, 1948), (ii) observations at seven stations by R. R. S. 'Discovery' (Foxton, 1957) and (iii) the eastern sector of the Indian Ocean during the R. V. Vityaz Cruise in 1962 (Daniel and Prem Kumar, 1965).Therefore, studies were undertaken on zooplankton samples collected from 400 .. 0 m at 72 stations and from 900-0 m at 6 stations, during the Oceanographic Expedition on I. N. S. DARSHAK in the northern Arabian sea from January to May 1974.In this paper, the biomass and the constituents of macrozooplankton measuring more than 1.32 mm based on these data are presented.

MATERIAL AND METHODS
Macrozooplankton samples were collected with Nansen pattern net of 80 cm mouth di.ameter, 225 cm total net length and terelene netting bolting cloth 7.5 meshes per line~r em.The net was hauled vertically at a speed of 1 m per second, over a davit from 400 m to the surface at 72 stations and from 900 m to surface at 6 stations (Text-fig.1).The total displacement volumes in ml were obtained following the techniques of Foxton (1957), Daniel and Premkumar (1965), and Daniel and Jothinayagam (1977).Following Prasad, (1969, P. 400) the displacement volumes have been considered equivalent to total biomass of the sam-pIes.The numbers of each zoo constituent occurring in the entire plankton samples from 400-0 m and from 900-0 m were analysed for estimating the numbers of organisms in a haul.Since, the net used for smpling was of a wider mesh, vide supra, only macrozooplankton measuring more than 1.32 mm were collected and the numbers of taxa like copepods were low.
Although, a comparison of the data on biomass and zooconstituents of plankton hauls from the surface (188 stations) (Daniel and Jothinayagam, 1977) 100 m depth (100 stations) (Daniel and Krishnamoorthy, 1977) 200 m depth (165 stations).(Paulinose and Aravindaksham, 1977) 400 m depth (72 stations) and 900 m depth (6 stations) would yield interesting results, since there is much diversity in the gear used for collecting the samples from the different stratified depths (i.e., diameter of the mouth of the sampler, mesh size and the length of the net and speed of haul were different) it is not possible to compare these results.
However, it appears from the present study and the papers published earlier, based on samples made from the same area and collected during the same cruises from the surface, 100 m depth, and 200 m depth (Daniel & ]othinayagam, 1977; Daniel & Krishnamoorthy (in press)  and Paulinose & Aravindaksh~m, 1977) that (i) zooplankton biomass is comparatively very high at the surface extending upto 100 m in the oceanic regions and upto the thermocline in the shallower inshore regions (Daniel and Jothinayagam, 1977, Daniel and Krishnamoorthy, (in press) (ii) the macrozooplankton below the euphotic zone is not very rich and (iii) swarming of certain meso pelagic macro-zooplankters contributed to the high biomass values in the deeper waters in some stations (see Text-figs.2-6).
I t is of in terest to note the occurrence of Dimophyes arctica (Chun) in both the 400 m and 900 m samples at station 2509, located at Latitude 22°09' N and Longitude 64°53' E. The occurrence of this species at this station suggests the upwelling of deep water masses in this region, during March, 1974. Further, this is the first record of this species north of 12° latitude in the Arabian Sea, the previous record being off Cochin at the lower boundary of the thermocline, in November (Daniel, 1977).This underlines the need for further investigations on deepwater macroplankton.
. 5. Longitudinal variations in the plankton biomass during Day/Night periods for 1 0 intervals.
. 6. La.titudinal variations in the plankton biomass during Day/Night periods for 1 0 intervals.