Monsoonal impact on planktonic standing stock and abundance in a tropical estuary (Cochin backwaters - India)
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Environmental studies in the Cochin backwaters (CBW), a tropical estuary along the southwest coast of India showed that seasonal fluctuation in salinity created by the monsoonal rainfall and associated run off is the major factor controlling the distribution and abundance of micro- and mesozooplankton. During premonsoon season, the CBW was characterized by warm waters (av. 32.6 ± 0.6 °C) with relatively high salinity (>23; except in the lower estuary). On the other hand, fresh water was found to dominate the entire area during monsoon and post monsoon seasons. The enormous input of nutrients (nitrate, phosphate & silicate) into the estuary from various sources (industries, agriculture and domestic) was responsible for the high phytoplankton biomass (av. 10.4±10.1 mgm-3) irrespective of seasons. The phytoplankton community was in general, dominated by diatoms (av. 88 ±12%), and proliferation of multiple species of diatom (Skeletonema costatum, Thalassiosira subtilis and Nitzschia closterium -1600 x 103 cells L-1) at different locations was observed especially during high saline condition. In case of zooplankton (micro- and meso), high standing stocks (micro. av. 81.4 ± 48.1 mgCm- 3; meso av. 88 ± 125 mgCm-3, respectively) were recorded during the premonsoon season. Copepods (eg. Calanoids) formed the abundant group (av. 75 ± 18%) in the meso zooplankton community irrespective of seasons. The ratio of carbon content between phytoplankton and zooplankton (P:Z) was quite high (>100) during monsoon and post monsoon seasons, but became low during premonsoon season (<5). Hence, it is suggested that during the periods of fresh water dominance, the trophic food web of Cochin estuarine system is characterized by substantial amount of unconsumed carbon at primary level owing to the reduction in phytoplankton grazers (zooplankton).
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