154
s. Z. QASIM: Productivity of Backwaters and Estuaries
There was no substantial increase in the rate of production when the water was rich in
nutrients. This indicates that the nutrient requirement of different phytoplankton species
is highly variable and that the high concentration of nutrients alone is not conducive to a
substantial increase in phytoplankton productivity. This was experimentally verified by
taking cultures of the dinoflagellate Ceratium and the diatom Biddulphia and exposing
them to different concentrations of phosphate and nitrate. Their rates of photosynthesis
were measured by the 14C uptake as a function of time (QASIM, BHATIATHIRI and DEVASSY,
1972b). The concentrations of P04 3 --P and N03 --N used were 0.5,1.0,2.0,5.0 and
10.0 ,ug-at I-I. In Ceratium the rate of photosynthesis increased with higher P04 3 --P
concentrations and reached its maximum at a _concentration of 10 ,ug-at I-I in about
10 days. A similar situation was found with Biddulphia where maximum photosynthesis
was attained within 10 days with increasing concentrations of phosphate. However, when
different concentrations of nitrate were used in Ceratium, maximum photosynthesis was
obtained at 0.5 ,ug-at I-I and any further increase in the concentration of nitrogen
inhibited photosynthesis. In Biddulphia an increase in the concentration of nitrate led to a
progressive increase in photosynthesis. Similar results were obtained when phosphorus
and nitrogen were used in a combined state. In Ceratium maximum photosynthesis
occurred when the concentration of each was 0.5 ,ug-at I-I, whereas in Biddulphia peak
photosynthesis was obtained when the concentration of each was 5.0 -10.0 ,ug-at I-I.
These experiments indicate that the nutrient requirement of the two species is very different. From these results it can also be postulated that the changing concentrations of
nutrients may lead to a succession in the growth of phytoplankton organisms. Such a
succession is of common occurrence in the backwater.
VI. Food Chain
The average gross production in the estuary is 280 gC m- 2 year- 1 and the net
production for days is approximately 195 gC m- 2 year- 1 and for days and nights (24 hrs)
about 124 gC m- 2 year-I. The estimated annual rate of consumption of the daily net
production by the zooplankton herbivores is 30 gC m- 2 year- 1 (QASIM, 1970). The rate
of consumption is greater during the premonsoon season (46%) than in other seasons
(QASIM et al., 1969). This gives rise to a large surplus of primary production which falls to
the bottom as detritus. The shallow euphotic zone increases the fallout of basic food
material which forms an important link in the food chain (QASIM, 1970). The detritus is
consumed by a variety of benthic animal communities and thus leads to several alternate
pathways in the food chain (QASIM, 1971).
Continuous collections of detritus falling on the bottom were made throughout the
year (QASIM and SANKARANARAYANAN, 1972). The quantity of detritus in the backwater
was much greater than that previously reported from the Southampton waters (TREVALLION, 1967) or from the North Sea and Kiel Bay (KREY, 1961). The values in the backwater
ranged from 67 g m- 2 day-l in March to 1013 g m- 2 day-l in May and amounted to
90 - 99% of the total phytoplankton productivity. The biochemical composition of
detritus and its calorific value were determined (QASIM and SANKARANARA YANAN, 1972).
Dried pellets of detritus offered as food to a penaeid prawn (Metapenaeus dobsoni) in the
laboratory were readily eaten. The prawns lived exclusively on detritus for a long time and
moulted several times in aquarium tanks.
s. Z. QASIM: Productivity of Backwaters and Estuaries
There was no substantial increase in the rate of production when the water was rich in
nutrients. This indicates that the nutrient requirement of different phytoplankton species
is highly variable and that the high concentration of nutrients alone is not conducive to a
substantial increase in phytoplankton productivity. This was experimentally verified by
taking cultures of the dinoflagellate Ceratium and the diatom Biddulphia and exposing
them to different concentrations of phosphate and nitrate. Their rates of photosynthesis
were measured by the 14C uptake as a function of time (QASIM, BHATIATHIRI and DEVASSY,
1972b). The concentrations of P04 3 --P and N03 --N used were 0.5,1.0,2.0,5.0 and
10.0 ,ug-at I-I. In Ceratium the rate of photosynthesis increased with higher P04 3 --P
concentrations and reached its maximum at a _concentration of 10 ,ug-at I-I in about
10 days. A similar situation was found with Biddulphia where maximum photosynthesis
was attained within 10 days with increasing concentrations of phosphate. However, when
different concentrations of nitrate were used in Ceratium, maximum photosynthesis was
obtained at 0.5 ,ug-at I-I and any further increase in the concentration of nitrogen
inhibited photosynthesis. In Biddulphia an increase in the concentration of nitrate led to a
progressive increase in photosynthesis. Similar results were obtained when phosphorus
and nitrogen were used in a combined state. In Ceratium maximum photosynthesis
occurred when the concentration of each was 0.5 ,ug-at I-I, whereas in Biddulphia peak
photosynthesis was obtained when the concentration of each was 5.0 -10.0 ,ug-at I-I.
These experiments indicate that the nutrient requirement of the two species is very different. From these results it can also be postulated that the changing concentrations of
nutrients may lead to a succession in the growth of phytoplankton organisms. Such a
succession is of common occurrence in the backwater.
VI. Food Chain
The average gross production in the estuary is 280 gC m- 2 year- 1 and the net
production for days is approximately 195 gC m- 2 year- 1 and for days and nights (24 hrs)
about 124 gC m- 2 year-I. The estimated annual rate of consumption of the daily net
production by the zooplankton herbivores is 30 gC m- 2 year- 1 (QASIM, 1970). The rate
of consumption is greater during the premonsoon season (46%) than in other seasons
(QASIM et al., 1969). This gives rise to a large surplus of primary production which falls to
the bottom as detritus. The shallow euphotic zone increases the fallout of basic food
material which forms an important link in the food chain (QASIM, 1970). The detritus is
consumed by a variety of benthic animal communities and thus leads to several alternate
pathways in the food chain (QASIM, 1971).
Continuous collections of detritus falling on the bottom were made throughout the
year (QASIM and SANKARANARAYANAN, 1972). The quantity of detritus in the backwater
was much greater than that previously reported from the Southampton waters (TREVALLION, 1967) or from the North Sea and Kiel Bay (KREY, 1961). The values in the backwater
ranged from 67 g m- 2 day-l in March to 1013 g m- 2 day-l in May and amounted to
90 - 99% of the total phytoplankton productivity. The biochemical composition of
detritus and its calorific value were determined (QASIM and SANKARANARA YANAN, 1972).
Dried pellets of detritus offered as food to a penaeid prawn (Metapenaeus dobsoni) in the
laboratory were readily eaten. The prawns lived exclusively on detritus for a long time and
moulted several times in aquarium tanks.