The Mandovi estuary is a prominent water body that runs along the west coast ofIndia. It forms an estuarine network with the adjacent Zuari estuary, connected via the Cumbharjua canal. The physico-chemical conditions seen in the Mandovi estuary are influenced by two factors : the fresh water runoff during the monsoon season (June-September) and the tidal influx of coastal seawater during the summer (October to May) season. However, the effects of monsoon related changes on the phytoplankton of the Mandovi estuary are not yet fully understood. An attempt to understand the same has been made here by applying the process of daily sampling at a fixed station throughout the monsoon season. It was noticed that the onset of the monsoon is responsible for an increase in nitrate levels upto 26 μM from <1 μM during pre-monsoon and enhancement of chlorophyll a (chl a ) as high as 14 μg·L - 1 during the same period. The phytoplankton population was observed through both chemotaxonomy and microscopy and was found to be composed mainly of diatoms. CHEMTAX analysis further uncover s the presence of several other groups of phytoplankton, the presence of which is yet to be reported in many other tropical estuar ies . It include s chrysophytes, cyanobacteria, prasinophytes, prymne siophytes and chlorophytes. The appearance of phytoplankton groups at various stages of the monsoon w as recorded, and this data is discussed in relation to environmental changes in the Mandovi estuary during the monsoon season.
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