182
N. M. SHAH:
between Secchi disk depth and phytoplankton pigments. It was also noted that, while the
colour tended to be greenish when Secchi depths were less than about 9 m, it was bluish
whenever Secchi depth exceeded 9 m.
5. Phytoplankton Population and Pigments
A numerical analysis of the phytoplankton population was made with a view to
providing supplementary information on the nature of the population, especially the
larger forms. No evaluation of shape, size, chlorophyll content per cell etc., of each
species was attempted. The composition of the phytoplankton stock changed considerably
with time and depth, the total count being between 1.2 X 10 3 and 2.9 X 10 5 cells per liter.
The only plankter present throughout the year was Thalassiosira sp. and the fluctuation in
its abundance was reflected, to a certain extent, in the plant pigment content, both at
surface and at 15 m depth. The total pigment content is, of course, the combined
contribution of all other organisms, especially the smaller, delicate ones.
Chlorophyll a values range between almost nil and about 8 mg m- 3 at the surface and
between 0 and 6 mg m- 3 at 15 m. If total column (0-16 m) pigments are considered,
values range between approximately 5 and 210 mg m- 2 • Fig. 7 shows the vertical
distribution of pigments. Values from 0, 5, 10 and 15 m depth were used in constructing
this diagram. Pigments are generally higher in the surface layer, but sometimes the
maximum occurs at 5 m. In May 1967, distribution was peculiar in that the pigment
content at 20 m depth was much higher than at 10 m off the surface. About 25% of the
total at 20 m was phaeopigment. The possibility of in situ production contributing
towards the pigment content cannot be ruled out because, with the setting in of the SW
monsoon, nutrients which have been regenerated in the bottom sediments are likely to be
stirred up into the water layer immediately above the bottom and photosynthetic production could take place if light is available. Since the Secchi disk is visible even at 16 m
depth, sufficient light is, in fact, available near the bottom. Moreover, diel studies showed
a significant increase in pigments at 20 m towards noon, which would indicate the
presence of chlorophyll in living cells.
Chlorophyll c varies between 0 and about 8 mg m- 3 in the surface and nil and about
6 mg m- 3 at 15 m depth. Similarly plant carotenoids also vary between almost 0 and
7 mg m- 3 • The percentage of phaeo-pigments in the total varies between about 10 and
40% during the SW monsoon; during the other month it is about 10% at 15 m depth
while the upper layers have very little. Taking total pigment content, June to October are
the rich months and November to March the poor months, average values being about
150 and 20 mg m- 2 respectively. January is the poorest month with 5 -10 mg m- 2
(Fig. 6).
6. Annual Cycles
A comparison of the distribution of temperature, salinity, O2 saturation and phytoplankton pigments between December 1967 and December 1968 and also between
January 1968 and January 1969 (Fig. 8) show, that after undergoing very large changes
(Figs.3 and 5), they more or less revert to their orginal values in the succeeding year.
Clearly, the cycles are repeated year after year, thus maintaining a delicate dynamic
equilibrium in this complex environment.
N. M. SHAH:
between Secchi disk depth and phytoplankton pigments. It was also noted that, while the
colour tended to be greenish when Secchi depths were less than about 9 m, it was bluish
whenever Secchi depth exceeded 9 m.
5. Phytoplankton Population and Pigments
A numerical analysis of the phytoplankton population was made with a view to
providing supplementary information on the nature of the population, especially the
larger forms. No evaluation of shape, size, chlorophyll content per cell etc., of each
species was attempted. The composition of the phytoplankton stock changed considerably
with time and depth, the total count being between 1.2 X 10 3 and 2.9 X 10 5 cells per liter.
The only plankter present throughout the year was Thalassiosira sp. and the fluctuation in
its abundance was reflected, to a certain extent, in the plant pigment content, both at
surface and at 15 m depth. The total pigment content is, of course, the combined
contribution of all other organisms, especially the smaller, delicate ones.
Chlorophyll a values range between almost nil and about 8 mg m- 3 at the surface and
between 0 and 6 mg m- 3 at 15 m. If total column (0-16 m) pigments are considered,
values range between approximately 5 and 210 mg m- 2 • Fig. 7 shows the vertical
distribution of pigments. Values from 0, 5, 10 and 15 m depth were used in constructing
this diagram. Pigments are generally higher in the surface layer, but sometimes the
maximum occurs at 5 m. In May 1967, distribution was peculiar in that the pigment
content at 20 m depth was much higher than at 10 m off the surface. About 25% of the
total at 20 m was phaeopigment. The possibility of in situ production contributing
towards the pigment content cannot be ruled out because, with the setting in of the SW
monsoon, nutrients which have been regenerated in the bottom sediments are likely to be
stirred up into the water layer immediately above the bottom and photosynthetic production could take place if light is available. Since the Secchi disk is visible even at 16 m
depth, sufficient light is, in fact, available near the bottom. Moreover, diel studies showed
a significant increase in pigments at 20 m towards noon, which would indicate the
presence of chlorophyll in living cells.
Chlorophyll c varies between 0 and about 8 mg m- 3 in the surface and nil and about
6 mg m- 3 at 15 m depth. Similarly plant carotenoids also vary between almost 0 and
7 mg m- 3 • The percentage of phaeo-pigments in the total varies between about 10 and
40% during the SW monsoon; during the other month it is about 10% at 15 m depth
while the upper layers have very little. Taking total pigment content, June to October are
the rich months and November to March the poor months, average values being about
150 and 20 mg m- 2 respectively. January is the poorest month with 5 -10 mg m- 2
(Fig. 6).
6. Annual Cycles
A comparison of the distribution of temperature, salinity, O2 saturation and phytoplankton pigments between December 1967 and December 1968 and also between
January 1968 and January 1969 (Fig. 8) show, that after undergoing very large changes
(Figs.3 and 5), they more or less revert to their orginal values in the succeeding year.
Clearly, the cycles are repeated year after year, thus maintaining a delicate dynamic
equilibrium in this complex environment.