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The results of an investigation carried out during June 2005 to May 2007 on physico-chemical parameters,species composition and community structure of phytoplankton including Chlorophyll a(Chl-a)at the Coleroon coastal waters(Southeast coast of India)are reported.Air and surface water temperatures(℃)varied from 25.1 to 30.1 and from 24.5 to 28.5 respectively.Salinity values varied from 6 to 28.5 and the pH ranged between 7.0 and 8.3.Variation in dissolved oxygen content was from 3.1 to 7.5 mg/dm3 while the light extinction coefficient values(LEC)ranged between 3.1 and10.1cm.The ranges of inorganic nutrients(μmol/dm3)viz.,nitrate,nitrite,phosphate and silicate were 10.1-23.4,1.2-8.9,0.2-3.1 and 55-125,respectively.The ranges of Chlorophyll a(μg/dm3)values was 2.0-7.5.Presently,124 phytoplankton species representing different classes viz: Bacillariophyceae(77),Dinophyceae(19),Cyanophyceae(15),Chlorophyceae(10)and Chrysophyceae(3)were recorded.The phytoplankton cell abundance varied from 0.290 to 111.662 cells/cm3,with peak diversity(3.38 bits/ind.)during summer season.The maximum abundance was found during summer season coinciding with the stable hydrographical conditions.The seasonal distribution and abundance of phytoplankton are discussed in relation to hydrographical parameters.Canonical Correspondence Analysis(CCA)was applied in this paper for discriminating environmental factors having effect on phytoplankton community at species level.Coleroon coastal water is subjected to long term fluctuations in physico-chemical parameters depending upon the seasonal tidal amplitude and freshwater influx resulting in a continuous exchange of organic,inorganic,plant and animal matters.