| Summary: | Hydrographical parameters and phytoplankton assemblages were determined along the Pondicherry, Parangipettai and Nagapattinam coastal waters, southeast coast of India. All the hydrographical parameters such as sea surface temperature, salinity, pH, total alkalinity, dissolved oxygen and nutrients like nitrate, nitrite, inorganic phosphate and reactive silicate, chlorophyll a and phytoplankton assemblages were studied for a period of five months (May, August, September 2010, March and November 2011). Over 121 species of phytoplankton represented by 93 species of diatoms, 16 species of dinoflagellates, 9 species of blue-green algae, 2 species of greens and 1 species of silicoflagellate were recorded. High phytoplankton species diversity was found in March 2011 when salinity and nitrate concentrations were low and reactive silicate and inorganic phosphates were moderate. Species diversity was low during May 2010 when increased sea surface temperature, salinity and low nutrients availability were observed. Coscinodiscus centralis, Diatoma vulgaris and Proboscia alata were dominant, especially Coscinodiscus sp. distributed in all stations whereas Skeletonema costatum, Odontella sinensis and Ditylum brightwellii were abundant in August and September 2010. From principal component analysis and multiple regression analysis, it is evident that variables like sea surface temperature and dissolved oxygen are the most important factors influencing the seasonal pattern of phytoplankton population.
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