Monsoonal impact on planktonic standing stock and abundance in a tropical estuary (Cochin backwaters – India)

Environmental studies in the Cochin backwaters (CBW), a tropical estuary along the southwest coast of India showed that seasonal fluctuations in salinity created by the monsoonal rainfall and associated run off is a 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 postmonsoon seasons. The enormous input of nutrients (nitrate, phosphate and silicate) into the estuary from various sources (industries, agriculture and domestic) was responsible for the high phytoplankton biomass (av. 10.4 ± 10.1 mg m−3) irrespective of seasons. The phytoplankton community was, in general, dominated by diatoms (av. 88 ± 12%), and proliferation of multiple species of diatoms (Skeletonema costatum, Thalassiosira subtilis and Nitzschia closterium – 1600 × 103 cells L−1) at different locations were observed especially during high saline condition. In case of zooplankton (micro- and meso), high standing stocks (micro av. 81.4 ± 48.1 mg C m−3; meso av. 88 ± 125 mg C m−3, respectively) were recorded during the premonsoon season. Copepods (e.g. Calanoids) formed the abundant group (av. 75 ± 18%) in the mesozooplankton community irrespective of seasons. The ratio of carbon content between phytoplankton and zooplankton (P:Z) was quite high (>100) during monsoon and postmonsoon 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).