270
s. M. HAQ, J. ALI KHAN, and S. CHUGTAI:
periods. The considerable variations in zooplankton abundance may be regarded as
temporal and spatial in nature. The months of October through December in the area
of the present studies are periods of high zooplankton abundance, while in March a
period of low zooplankton abundance is found particularly in the main area of the shelf.
The results obtained during these periods may be compared with those obtained from
other parts of the Indian Ocean. Recently the Indian Ocean Biological Centre at Cochin
published the HOE Plankton Atlas (IOBC 1968, 1970), which provides the best source
for comparison. Our observations on the zooplankton biomass for various months
referred to earlier fall within the period of the NE monsoon (October through April) as
shown in the atlas. The average biomass based on day and night samples recorded for
the NE sector of the Arabian Sea shows the values 15 - 25 ml m- 2 for a water column
of 200 m. This is much lower than the average biomass computed for the 53 stations
(55 ml m- 2 ). The values obtained for this area are very close to those recorded from the
coast of Arabia. Comparison with other regions of the Indian Ocean including the
coast of Somalia, SW coast of India and the Bay of Bengal show values from present
studies to be 3 to 4 times as high as those recorded for these regions. On the other hand,
the results are of the same order of magnitude as those recorded during the SW monsoon
from the above region. The point worthy of mention here is the spectacularly high zooplankton production during November and December, noted earlier. The December
records, averaged over 6 stations from the area immediately south of Karachi, showed a
value of 166 ml m- 2 for a water column ranging up to 80 m depth, which to the author's
knowlegde is the highest ever recorded from the Indian Ocean. Earlier records for March
from the open sea of the northern Arabian Sea by BOGOROV and VINOGRADOV (1961)
show a value 100 ml per 1000 m 3 for a column of water 100 m deep which translated
into values per m 2 were lower (10 ml) than those obtained for this month (37 ml m- 2 )
in the entire area including the shelf and slope water. Because of different nets employed,
some discrepancies might be expected in this comparison. The various aspects of
studies so far carried out tend to suggest a close relationship between zooplankton
abundance and hydrographic conditions. Some insight into this may be derived from
other work done in this region. The months of November and early December are
generally calm. The zooplankton abundance noted in these months is associated with
the presence of cool water highly deficient in oxygen water which has penetrated into the
shelf as a part of a large-scale movement under the influence of the SW monsoon (BANSE,
1968). The dissolved phosphate contents recorded from various depths in this area
(Cruise Report "Zulun" I, 1964) have values ranging from 1 to 1.2 ,ug-at 1- 1 at 50 m or
above at most of the stations. This nutrient-laden water near the surface would account
for the high organic production, as noted in other areas (RYTHER and MENZEL, 1965;
BRANDHORST, 1958). This is supported by earlier studies of KABANOVA (1964) who
recorded high phytoplankton production rates (30.6 to 83.3 mgC m- 2 day-1) during
October and November on this shelf. The spectacular increase in zooplankton noted in
December must have been supported by increased primary production. Similarly the
observations carried out along the western shelf in October 1962 showed very high
numbers of micro crustaceans. RYTHER et al. (1966) found a very high organic production
rate (more than 1 gC m- 2 day-1) in November 1963 in the area extending from the
coast of Arabia to that of Pakistan. Perhaps this is a regular annual feature.
During November and March the phytoplankton production is probably too low
to support the larger zooplankton due to deepening of the thermocline, and consequently
s. M. HAQ, J. ALI KHAN, and S. CHUGTAI:
periods. The considerable variations in zooplankton abundance may be regarded as
temporal and spatial in nature. The months of October through December in the area
of the present studies are periods of high zooplankton abundance, while in March a
period of low zooplankton abundance is found particularly in the main area of the shelf.
The results obtained during these periods may be compared with those obtained from
other parts of the Indian Ocean. Recently the Indian Ocean Biological Centre at Cochin
published the HOE Plankton Atlas (IOBC 1968, 1970), which provides the best source
for comparison. Our observations on the zooplankton biomass for various months
referred to earlier fall within the period of the NE monsoon (October through April) as
shown in the atlas. The average biomass based on day and night samples recorded for
the NE sector of the Arabian Sea shows the values 15 - 25 ml m- 2 for a water column
of 200 m. This is much lower than the average biomass computed for the 53 stations
(55 ml m- 2 ). The values obtained for this area are very close to those recorded from the
coast of Arabia. Comparison with other regions of the Indian Ocean including the
coast of Somalia, SW coast of India and the Bay of Bengal show values from present
studies to be 3 to 4 times as high as those recorded for these regions. On the other hand,
the results are of the same order of magnitude as those recorded during the SW monsoon
from the above region. The point worthy of mention here is the spectacularly high zooplankton production during November and December, noted earlier. The December
records, averaged over 6 stations from the area immediately south of Karachi, showed a
value of 166 ml m- 2 for a water column ranging up to 80 m depth, which to the author's
knowlegde is the highest ever recorded from the Indian Ocean. Earlier records for March
from the open sea of the northern Arabian Sea by BOGOROV and VINOGRADOV (1961)
show a value 100 ml per 1000 m 3 for a column of water 100 m deep which translated
into values per m 2 were lower (10 ml) than those obtained for this month (37 ml m- 2 )
in the entire area including the shelf and slope water. Because of different nets employed,
some discrepancies might be expected in this comparison. The various aspects of
studies so far carried out tend to suggest a close relationship between zooplankton
abundance and hydrographic conditions. Some insight into this may be derived from
other work done in this region. The months of November and early December are
generally calm. The zooplankton abundance noted in these months is associated with
the presence of cool water highly deficient in oxygen water which has penetrated into the
shelf as a part of a large-scale movement under the influence of the SW monsoon (BANSE,
1968). The dissolved phosphate contents recorded from various depths in this area
(Cruise Report "Zulun" I, 1964) have values ranging from 1 to 1.2 ,ug-at 1- 1 at 50 m or
above at most of the stations. This nutrient-laden water near the surface would account
for the high organic production, as noted in other areas (RYTHER and MENZEL, 1965;
BRANDHORST, 1958). This is supported by earlier studies of KABANOVA (1964) who
recorded high phytoplankton production rates (30.6 to 83.3 mgC m- 2 day-1) during
October and November on this shelf. The spectacular increase in zooplankton noted in
December must have been supported by increased primary production. Similarly the
observations carried out along the western shelf in October 1962 showed very high
numbers of micro crustaceans. RYTHER et al. (1966) found a very high organic production
rate (more than 1 gC m- 2 day-1) in November 1963 in the area extending from the
coast of Arabia to that of Pakistan. Perhaps this is a regular annual feature.
During November and March the phytoplankton production is probably too low
to support the larger zooplankton due to deepening of the thermocline, and consequently