184
N. M. SHAH:
III. Discussion
The onset of the SW monsoon in May causes turbulence and increased wave activity
in the shallow coastal region and helps to bring the regenerated nutrients into the upper
water column; this in turn probably triggers off a new cycle of production, as sufficient
light is available down to the sea bottom.
JAYARAMAN and SHESHAPPA (1957) have found high phosphate and plankton content
along the coast north of Cochin during the SW monsoon.
A seasonal feature of the sea off Cochin is the alternation between upwelling, which
brings up nutrients, and sinking. The present study confirms that, off Cochin, intense
upwelling takes place during June, July and August, and the residual effects of upwelling
are discernible in September and October. Sinking starts in November and continues
until the end of February. Stable conditions are seen in March and April when the upper
layers warm up, leading to the formation of weak thermal gradients. Here the upwelling
is not, as in most other coastal areas, the direct effect of the wind systems but an indirect
effect (BANSE, 1959; SHARMA, 1966). A rough estimate of the rate of upwelling can be
made from the vertical movement of the temperature and O2 saturation curves shown in
Fig. 3 and 5. The water at about 22.5° C present at 10 m depth on 24 July had moved up
to 5 m depth by 12 August; similarly the O2 saturation curve also seemed to be displaced
upwards by about 5 m during this period. This gives a rate of upwelling of approximately
8 m per month.
So far as land drainage is concerned, the sea off Cochin is influenced by backwater
run-off. The hydrography of the backwater system, especially of the port area, has been
studied by a few workers (SHAH, 1961; RAMAMIRTHAM and JAYARAMAN, 1963;
CHERIYAN, 1967; QASIM, 1973).
In the area under study, nitrite, present during the SW monsoon, is absent during
the sinking months (Fig. 6). Nitrate, on the other hand, is present in very low concentrations from July to November. Both nitrite and nitrate show abnormally high values in
April coincident with Trichodesmium bloom; this organism may have the ability to fix
nitrogen. Though both nitrite and nitrate occur sparingly and for short periods, nitrogen
is unlikely to be a limiting factor. Inorganic phosphate values, which fall almost to nil
during January, pick up after February and fairly high concentrations are recorded
during the SW monsoon. In culture experiments the rate-limiting concentration has been
found to be approximately 0.25 ,ug-at 1- 1 (GOLDBERG, WALKER and WHISENAND, 1951),
so that phosphate may well be the limiting factor since the concentration in the sea falls
well below this value. Moreover, the seasonal distribution curve for phytoplankton
pigments (Fig. 6) seems to follow the curve for phosphate with a lag of a few weeks, a
relationship not seen with the other nutrients studied. Therefore, it is reasonable to
assume that the seasonal variations in the phytoplankton pigments are controlled by
phosphate concentration in the sea off Cochin.
With regard to the quantity of light available for photosynthesis, bright sunlight is
available throughout the year, except on certain days in the monsoon season when the sky
may be overcast. Even then, enough sunlight may fall on the sea surface for considerable
production to take place in the upper layers. However, the depth of the euphotic zone
could be limited by the presence of phytoplankton themselves, or other dissolved and
particulate material. A clear inverse relationship between Secchi disk depth and the
quantity of phytoplankton pigments has been noted in this study (Fig. 4).
N. M. SHAH:
III. Discussion
The onset of the SW monsoon in May causes turbulence and increased wave activity
in the shallow coastal region and helps to bring the regenerated nutrients into the upper
water column; this in turn probably triggers off a new cycle of production, as sufficient
light is available down to the sea bottom.
JAYARAMAN and SHESHAPPA (1957) have found high phosphate and plankton content
along the coast north of Cochin during the SW monsoon.
A seasonal feature of the sea off Cochin is the alternation between upwelling, which
brings up nutrients, and sinking. The present study confirms that, off Cochin, intense
upwelling takes place during June, July and August, and the residual effects of upwelling
are discernible in September and October. Sinking starts in November and continues
until the end of February. Stable conditions are seen in March and April when the upper
layers warm up, leading to the formation of weak thermal gradients. Here the upwelling
is not, as in most other coastal areas, the direct effect of the wind systems but an indirect
effect (BANSE, 1959; SHARMA, 1966). A rough estimate of the rate of upwelling can be
made from the vertical movement of the temperature and O2 saturation curves shown in
Fig. 3 and 5. The water at about 22.5° C present at 10 m depth on 24 July had moved up
to 5 m depth by 12 August; similarly the O2 saturation curve also seemed to be displaced
upwards by about 5 m during this period. This gives a rate of upwelling of approximately
8 m per month.
So far as land drainage is concerned, the sea off Cochin is influenced by backwater
run-off. The hydrography of the backwater system, especially of the port area, has been
studied by a few workers (SHAH, 1961; RAMAMIRTHAM and JAYARAMAN, 1963;
CHERIYAN, 1967; QASIM, 1973).
In the area under study, nitrite, present during the SW monsoon, is absent during
the sinking months (Fig. 6). Nitrate, on the other hand, is present in very low concentrations from July to November. Both nitrite and nitrate show abnormally high values in
April coincident with Trichodesmium bloom; this organism may have the ability to fix
nitrogen. Though both nitrite and nitrate occur sparingly and for short periods, nitrogen
is unlikely to be a limiting factor. Inorganic phosphate values, which fall almost to nil
during January, pick up after February and fairly high concentrations are recorded
during the SW monsoon. In culture experiments the rate-limiting concentration has been
found to be approximately 0.25 ,ug-at 1- 1 (GOLDBERG, WALKER and WHISENAND, 1951),
so that phosphate may well be the limiting factor since the concentration in the sea falls
well below this value. Moreover, the seasonal distribution curve for phytoplankton
pigments (Fig. 6) seems to follow the curve for phosphate with a lag of a few weeks, a
relationship not seen with the other nutrients studied. Therefore, it is reasonable to
assume that the seasonal variations in the phytoplankton pigments are controlled by
phosphate concentration in the sea off Cochin.
With regard to the quantity of light available for photosynthesis, bright sunlight is
available throughout the year, except on certain days in the monsoon season when the sky
may be overcast. Even then, enough sunlight may fall on the sea surface for considerable
production to take place in the upper layers. However, the depth of the euphotic zone
could be limited by the presence of phytoplankton themselves, or other dissolved and
particulate material. A clear inverse relationship between Secchi disk depth and the
quantity of phytoplankton pigments has been noted in this study (Fig. 4).