Differential environmental responses of tropical phytoplankton community in the southwest coast of India

Phytoplankton characteristics (biomass, primary productivity, species composition and community structure) were studied in a tropical estuary (Cochin estuary) and adjacent coastal marine region for understanding the factors controlling their productivity patterns and community dynamics. The Cochin estuary (CE) sustained a high level of inorganic nutrients supporting very high phytoplankton biomass, productivity, and abundance. Though the CE exhibited a 2-3 fold increase in the annual mean of chlorophyll a (14.6±8.5mgm−3) and primary production (1288±999mgCm−3d−1) than the coastal waters, both regions sustained a substantial dominance of small-sized phytoplankton, in particular nanoplankton (2-20 μm), which apparently contributed >70% of total chlorophyll a and primary production. Diatoms were the dominant phytoplankton functional group (>75% of total abundance) prevailed over both sampling locations almost throughout the year. However, the sporadic dominance of certain species of dinoflagellates (during premonsoon) and green and blue-green algae (during monsoon) were also encountered in the estuary. The SIMPER analysis, based on phytoplankton species abundance data, revealed the presence of certain characterizing species exclusive for both estuary and coastal waters, and most of them belonged to smaller diatoms e.g., Skeletonema costatum, Thalassiosira subtilis, Nitzschia longissima, Cylindrotheca closterium etc. The incessant proliferation of these small sized diatoms, especially in the CE, can be considered as a potential source of consistently higher phytoplankton biomass and primary productivity, and which can even survive in the recurring short-term (tides) and long-term (seasonal) hydrographic changes of the Cochin estuary.