168
F. J. R. TAYLOR:
from coastal upwelling correlated with the NE monsoon and the clockwise circulation
within the Bay (LAFOND, 1957; JAYARAMAN, 1965; MOJUMDER, 1968). JAYARAMAN
disputed LAFOND's interpretation of upwelling, but MO]UMDER found further evidence
for the occurrence of upwelling in the April- June period.
In Fig. 3 the isotherms for the upper 600 m on the 75° E meridian (cruise V) have
been plotted. This corresponds also to the NE monsoon period during which the South
Equatorial Current is situated at its most southerly latitudes between 10° and 20° S. The
eastwardflowing Equatorial Countercurrent is situated at approximately 2° to 6° S,
and the westward-flowing North Equatorial Current is also developed close to the
southern tip of India and Ceylon. The figure clearly shows a strong subsurface rising
of the isotherms within the upper 600 m, reaching to approximaterly 80 m at station 322
and 323. A secondary subsurface upsloping also occurred near 20° S, with the main
sloping associated with the northern edge of the Subtropical Convergence region south
of 40° S.
Considering only the occurence of Ceratium gravidum (G) and Histioneis dolan (D),
it can be seen that these species occurred at stations to either side of the isothermal
"hump" on the right-hand flank of the Equatorial Counter Current and not directly
over the upward moving water. Station 315, where H. dolan was recorded, showed
little direct signs of upwelling.
One of the richest stations for dinoflagellates, station 294 just north of the Seychelles
Platform, was situated just south of the Equatorial Countercurrent and here, again,
C. gravidum was present.
Stations 283 to 287 south of Saudi Arabia, at which very high productivity was
recorded, did not reveal any of the "deep" species. Thus the "Anton Bruun" material
has some suggestion of an association of the deep species with stations where an upwelling
influence might be effective, as well as some contra-indications, even though the sampling
extended to the presumed deep range of the species. It may be noted that the most
extensive work on the dinophysoid species, including Histioneis and Citharistes, was
based by KOFOID and SKOGSBERG (1928) on samples collected from depths greater
than 1000 - 0 m. The relative richness of these rare genera within their Pacific material
suggests that for those genera, at least, their normal distribution may extend below
200 m to an unknown depth.
In summary, the position with regard to a tropical "shade flora" of dinoflagellate
species is still not entirely unequivocal. The "Anton Bruun" material was not suitable
for critically evaluating the problem although the depth of sampling permitted relatively
frequent collection of the suspect species. There is a possibility that the rare dinophysoid
genera may extend normal distribution below 200 m. One possible application of the
established recognition of deep water species would be their use as indicators in shallow
samples of deeper subsurface upwelling. HALIM (1967) has employed this principle in
the study of the dinoflagellates from the upwelling areas of the Cariaco Trench in the
SE Caribbean Sea. He found many of these species in surface samples collected over the
trench in October, 1960.
The study has revealed a great richness of dinoflagellates in the Andaman Sea and
Bay of Bengal during and after the NE monsoon. This relates well to the observations of
ZERNOVA (1962) and ZERNOVA and IVANOV (1964) who also found a great abundance
of phytoplankton in the Andaman Sea during the same season. Coverage of this region
at other seasons was not made by the "Anton Bruun", although from other studies it
F. J. R. TAYLOR:
from coastal upwelling correlated with the NE monsoon and the clockwise circulation
within the Bay (LAFOND, 1957; JAYARAMAN, 1965; MOJUMDER, 1968). JAYARAMAN
disputed LAFOND's interpretation of upwelling, but MO]UMDER found further evidence
for the occurrence of upwelling in the April- June period.
In Fig. 3 the isotherms for the upper 600 m on the 75° E meridian (cruise V) have
been plotted. This corresponds also to the NE monsoon period during which the South
Equatorial Current is situated at its most southerly latitudes between 10° and 20° S. The
eastwardflowing Equatorial Countercurrent is situated at approximately 2° to 6° S,
and the westward-flowing North Equatorial Current is also developed close to the
southern tip of India and Ceylon. The figure clearly shows a strong subsurface rising
of the isotherms within the upper 600 m, reaching to approximaterly 80 m at station 322
and 323. A secondary subsurface upsloping also occurred near 20° S, with the main
sloping associated with the northern edge of the Subtropical Convergence region south
of 40° S.
Considering only the occurence of Ceratium gravidum (G) and Histioneis dolan (D),
it can be seen that these species occurred at stations to either side of the isothermal
"hump" on the right-hand flank of the Equatorial Counter Current and not directly
over the upward moving water. Station 315, where H. dolan was recorded, showed
little direct signs of upwelling.
One of the richest stations for dinoflagellates, station 294 just north of the Seychelles
Platform, was situated just south of the Equatorial Countercurrent and here, again,
C. gravidum was present.
Stations 283 to 287 south of Saudi Arabia, at which very high productivity was
recorded, did not reveal any of the "deep" species. Thus the "Anton Bruun" material
has some suggestion of an association of the deep species with stations where an upwelling
influence might be effective, as well as some contra-indications, even though the sampling
extended to the presumed deep range of the species. It may be noted that the most
extensive work on the dinophysoid species, including Histioneis and Citharistes, was
based by KOFOID and SKOGSBERG (1928) on samples collected from depths greater
than 1000 - 0 m. The relative richness of these rare genera within their Pacific material
suggests that for those genera, at least, their normal distribution may extend below
200 m to an unknown depth.
In summary, the position with regard to a tropical "shade flora" of dinoflagellate
species is still not entirely unequivocal. The "Anton Bruun" material was not suitable
for critically evaluating the problem although the depth of sampling permitted relatively
frequent collection of the suspect species. There is a possibility that the rare dinophysoid
genera may extend normal distribution below 200 m. One possible application of the
established recognition of deep water species would be their use as indicators in shallow
samples of deeper subsurface upwelling. HALIM (1967) has employed this principle in
the study of the dinoflagellates from the upwelling areas of the Cariaco Trench in the
SE Caribbean Sea. He found many of these species in surface samples collected over the
trench in October, 1960.
The study has revealed a great richness of dinoflagellates in the Andaman Sea and
Bay of Bengal during and after the NE monsoon. This relates well to the observations of
ZERNOVA (1962) and ZERNOVA and IVANOV (1964) who also found a great abundance
of phytoplankton in the Andaman Sea during the same season. Coverage of this region
at other seasons was not made by the "Anton Bruun", although from other studies it