The Distribution and Abundance of Zooplankton during Postmonsoon and Premonsoon Periods 271
of the fertile water, as a result of withdrawal of subsurface water from the shelf (BANSE,
1968).
The biomass values recorded by FRONTIER (1963 a) from stations RG 203 and RG 204,
which were close to our station 67/4 in March, when converted into values per m 2 yielded
128 and 72 ml for a 50 m column of water for the respective stations. These were much
higher than those obtained in March from 6714 (7 ml m -2).
There is some evidence of marked spatial variations in zooplankton abundance in
geographically limited areas. For instance, notes of high zooplankton abundance in
shallower areas along the southern bank of the Swatch (Stns 64/11, 64/12, 64/17) are
confirmed by records (Cruise Report "Zulun" I, 1964) of a rich phytoplankton bloom
in this area, as well as a high phosphate content. These and the presence of aerated water
in the southern region would offer ideal conditions for zooplankton production.
ELIZAROV (1968) reported ascent of deep oceanic water inside the Swatch in January and
excluded any possibility of wind-induced upwelling. How far this phenomenon is a
regular feature of the area is not known. A further example was found near Astola
Island (western sector) on the west coast, where rich zooplankton was associated with a
reduced surface isothermal layer. Similarly, in the slope water in March 1968, rich
zooplankton coincided with reduced surface isotherms. Such a situation is reported by
BRANDHORST (1958) from the eastern Pacific.
Total zooplankton expressed in numerical terms has often shown lack of agreement
with the corresponding values of biomass. This may be accounted for by the presence or
absence, or restricted distribution, of certain large organisms such as siphonophores,
salps, medusae etc., reported to be responsible for the depletion of phytoplankton and
copepods (FRASER, 1961; TRANTER, 1962).
Most of the early work on copepods of the shelf and slope water area in the Indian
Ocean deals only with taxonomy. The principal species referred to here are also reported
from other parts of the Indian Ocean (MENON, 1945; SEWELL, 1947; VINOGRADOV, 1968),
but there is a lack of quantitative data. Comparative studies of various sectors show that,
whereas epiplanktonic species formed the bulk of the copepod stock in the shelf and
neighboring waters, the oceanic species occupied a significant fraction of the sub-surface
region both in the slope water and on the shelf, and therefore may contribute substantial
food energy for higher trophic levels.
The composition of oceanic species is distinguished by the numerical abundance of
relatively few species, principally Pleuromamma indica and Euchaeta wolfendeni, the
former far exceeding the latter. VINOGRADOV and VORONINA (1961) reported vertical
distribution of a few species in April 1960 in a transect extending from the Seychelles, off
the African Coast, to Bombay. They describe 6 species which include 3 members of the
genus Pleuromamma (P. gracilis, P. xiphias and P. indica). According to these workers,
whereas other species are limited in their distribution by higher O2 content (0.5 mll-l),
P. indica is associated with an O2 content of less than 0.1 mll-l. This is supported by
our present study. Oxygen depletion immediately below the surface thermocline is
probably the rule in the Arabian Sea (RYTHER and MENZEL, 1965; BANSE, 1968). Where
there is vertical ascent of this layer, as during the SW and NE monsoon, or in areas of
divergence (RYTHER and MENZEL, 1965) and when massive displacement of surface water
results in reduced surface isothermal layers, Pleuromamma and other-oceanic species may
form a significant fraction of the subsurface plankton. This is borne out by our studies
of the fertile water, as a result of withdrawal of subsurface water from the shelf (BANSE,
1968).
The biomass values recorded by FRONTIER (1963 a) from stations RG 203 and RG 204,
which were close to our station 67/4 in March, when converted into values per m 2 yielded
128 and 72 ml for a 50 m column of water for the respective stations. These were much
higher than those obtained in March from 6714 (7 ml m -2).
There is some evidence of marked spatial variations in zooplankton abundance in
geographically limited areas. For instance, notes of high zooplankton abundance in
shallower areas along the southern bank of the Swatch (Stns 64/11, 64/12, 64/17) are
confirmed by records (Cruise Report "Zulun" I, 1964) of a rich phytoplankton bloom
in this area, as well as a high phosphate content. These and the presence of aerated water
in the southern region would offer ideal conditions for zooplankton production.
ELIZAROV (1968) reported ascent of deep oceanic water inside the Swatch in January and
excluded any possibility of wind-induced upwelling. How far this phenomenon is a
regular feature of the area is not known. A further example was found near Astola
Island (western sector) on the west coast, where rich zooplankton was associated with a
reduced surface isothermal layer. Similarly, in the slope water in March 1968, rich
zooplankton coincided with reduced surface isotherms. Such a situation is reported by
BRANDHORST (1958) from the eastern Pacific.
Total zooplankton expressed in numerical terms has often shown lack of agreement
with the corresponding values of biomass. This may be accounted for by the presence or
absence, or restricted distribution, of certain large organisms such as siphonophores,
salps, medusae etc., reported to be responsible for the depletion of phytoplankton and
copepods (FRASER, 1961; TRANTER, 1962).
Most of the early work on copepods of the shelf and slope water area in the Indian
Ocean deals only with taxonomy. The principal species referred to here are also reported
from other parts of the Indian Ocean (MENON, 1945; SEWELL, 1947; VINOGRADOV, 1968),
but there is a lack of quantitative data. Comparative studies of various sectors show that,
whereas epiplanktonic species formed the bulk of the copepod stock in the shelf and
neighboring waters, the oceanic species occupied a significant fraction of the sub-surface
region both in the slope water and on the shelf, and therefore may contribute substantial
food energy for higher trophic levels.
The composition of oceanic species is distinguished by the numerical abundance of
relatively few species, principally Pleuromamma indica and Euchaeta wolfendeni, the
former far exceeding the latter. VINOGRADOV and VORONINA (1961) reported vertical
distribution of a few species in April 1960 in a transect extending from the Seychelles, off
the African Coast, to Bombay. They describe 6 species which include 3 members of the
genus Pleuromamma (P. gracilis, P. xiphias and P. indica). According to these workers,
whereas other species are limited in their distribution by higher O2 content (0.5 mll-l),
P. indica is associated with an O2 content of less than 0.1 mll-l. This is supported by
our present study. Oxygen depletion immediately below the surface thermocline is
probably the rule in the Arabian Sea (RYTHER and MENZEL, 1965; BANSE, 1968). Where
there is vertical ascent of this layer, as during the SW and NE monsoon, or in areas of
divergence (RYTHER and MENZEL, 1965) and when massive displacement of surface water
results in reduced surface isothermal layers, Pleuromamma and other-oceanic species may
form a significant fraction of the subsurface plankton. This is borne out by our studies