ABSTRACT:The dynamic relationship of water physico-chemical characteristics with phytoplankton has long been of great interest in both experimental ecology and environmental management. This study was carried out at the Brahmani Estuary (East coast, Bay Of Bengal, India) from March 2007 to November 2007. Based on the collected samples from six stations, phytoplankton population, DO, BOD, salinity and nutrient (o-PO4, NO3+NH3) were examined. The phytoplankton abundance varies with the seasonal variation. Due to wave action and circulation patterns DO distribution shows a marginal variation at different sampling points but the DO are relatively higher in the upstream compared to downstream, as oxygen solubility is higher for fresh water. In tropical climates higher light energy and temperature allow faster cycling of nutrients, thus seasonal maxima and minima of nutrient level are more difficult to asses and in general the nutrient concentration are low. Phytoplankton abundance increased through the increasing trend in salinity to the extent found in this investigation. A regression plot analysis exhibit a positive value i.e. R 2 = 0.7897, or correlates with each other up to 78.97%.. @ JASEM Moving from a river through an estuary to the sea demonstrates the major changes in depth, energy (current, turbulence), water clarity, salinity, sediments (organic contents) nutrients and biota. Estuarine communities are some of the most productive and diverse of costal communities, which is a transition zone between freshwater and marine water world. Estuarine habitat can be divided into a marine region (lower) with a direct connection to the seawater, a middle region where salt water and fresh water mix, and an upper or fluvial region dominated by fresh water but subjected to tidal action (Ketchum, 1983; Day et al., 1989). The Brahamani River with Subarnarekha and Mahanadi River grouped together as Central River basins of East coast of India. The river flows with a total length of 480 km, winding around the south & southeast part Bonaigarh and Talcher and then turning towards the east to unite with northern branches of Mahanadi River. It finally empties into the Bay of Bengal at the palmyras point. The significant tributaries of Brahmani river are the Karo, the Sankha and Tirka. The Brahmani, the Mahanadi and the Baitarani rivers together form a big river delta on the Orissa coastal plain. The survey data comprises of physico-chemistry & biology of this large estuarine region. Phytoplanktons, among the primary producers of aquatic ecosystems, depend upon sunlight, water, and nutrients to survive. Besides the availability of nutrients, the physico-chemical variables such as flushing rate, salinity and turbidity also influence the distribution and abundance of plankton communities in estuaries (McLusky, 1971;Cleorn, 1987;Ferreira et al., 2005;Madhu et al., 2007). The dynamic relationship between phytoplankton growth with respect to salinity, turbidity, DO, BOD and nutritional changes in the Brahmani Estuarine region was stud...