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The investigations in the western Bay of Bengal (BoB) during summer, winter and spring intermonsoon periods evidenced lack of pronounced seasonal variation in phytoplankton standing stock (chlorophyll a) and primary production. The supply of riverine materials to the coastal bay is witnessed by the presence of major nutrients (nitrate -NO 3 , phosphate -PO 4 & silicate -SiO 4 ) in the upper layers (up to 20meters) during monsoonal periods. However, it was not found to enhance the phytoplankton production rates to a large extent.The strong stratification due to enormous quantity of fresh water discharge, acts as a barrier to vertical transport of nutrients from the subsurface layers. The average primary production in the inshore and the were observed at 20-50m in the spring intermonsoon, whereas in summer and winter, the chlorophyll maxima occurred in the upper layers of the euphotic zone (<20m). The vertical distribution of phytoplankton (>5µm) was primarily restricted by the pycnocline, and the bulk of phytoplankton assemblage existed within the surface.The seasonal change in cell numbers, however, did not coincide closely with the changes in the chlorophyll a concentration. Diatoms formed the major group (>85%) in the total phytoplankton abundance irrespective of seasons. During spring intermonsoon, large patches of Trichodesmium blooms were observed in the coastalas well as open waters of the western BoB.

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Abstract. Cochin backwaters, a micro tidal estuary, undergo a characteristic transformation from a riverdominated system during summer monsoon to a tide-dominant system during pre-monsoon season. The present study observes that as the river flow weakens after monsoon, the flushing of the estuary diminishes and the nitrogen and phosphorous loadings through anthropogenic activities (industries) and sediment re-suspension alter the nutrient stoichiometry substantially. The increased tidal activity during pre-monsoon changes the estuary into a vertically mixed, eutrophic and flow-restricted system supporting an enhanced organic production. This implies that monsoon-induced hydrology plays an important role in regulating the nutrients, secondary production and even the migrant fauna of the estuary. The system is delicately poised, as continuous release of pollutants including nutrients into this estuary would suppress fish and shell fish production, where only pollution tolerant species can exist.

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