Indian Journal of Marine Sciences Vol. 38(2), June 2009, pp. 184-190 Seasonal variations of phytoplankton in Mahanadi estuary, east coast of India Subrat Naik, B C Acharya* & Anil Mohapatra Institute of Minerals and Materials Technology, Bhubaneswar-751 013, India [Email: bcacharya@yahoo.com] Received 14 January 2008; revised 1 September 2008 Seasonal variations of phytoplankton and chlorophyll-a along with its environmental variations including nutrients were studied from June-2004 to March-2007. Samples were collected in three different seasons pre-monsoon (May-June), post-monsoon (October-November) and summer (February-March) from the Mahanadi estuary. Seventy seven species of phytoplankton were encountered during the study period of which, diatoms (Bacillariophyceae) was the dominant group followed by dinoflagellates (Dinophyceae) and cyanobacteria (Cyanophyceae). Higher phytoplankton counts and chlorophyll-a concentrations were recorded in post-monsoon season. Nutrient concentration was also very high in the above season as compared to other two seasons. Cyanobacteria population was more during post-monsoon season. Positive correlations of phytoplankton, with chlorophyll-a and dissolved oxygen were recorded in all three sampling seasons. Positive correlation of phytoplankton population with Chl-a, DO, NO 2 -N, NH 4 -N indicated that the phytoplankton population in the estuary is controlled by above nutrients. Diatoms are dependent on NO 2 -N and NH 4 -N, dinoflagellates depend on NO 2 -N and SiO 4 . Cyanobacteria in the estuary depend on NO 2 -N, NH 4 and PO 4 . [Keywords: Phytoplankton; species abundance; chlorophyll-a; water quality; Mahanadi estuary] Introduction Estuaries may be specially enriched by nutrients from river water, organic pollution (locally within the estuary or remotely through runoff) and by the entrainment of coastal waters in a subsurface counter- current, transporting nutrients into the estuary 1 . The phytoplankton composition is affected by various environmental factors such as pH, light, temperature, salinity, turbidity and nutrients 2 . Besides, their importance as the primary producers in food webs and ensuring ecological balance, species of phytoplankton are useful indicators of water quality 3,4 . The relative availability of nutrients plays a major role in inducing the community structure of phytoplankton 5-7 . Mahanadi river system is the third largest in the peninsula of India and the largest river in Orissa state. The basin extends over an area of approximately 141,600 km 2 , with a total length of 851 km and peak discharge of 44,740 m 3 s -1 water 8 . There is heavy industrial activity in Paradip, and up stream of Mahanadi estuary. It also receives a large amount of agricultural run-off along its course. Human influences are pronounced at Sambalpur, Cuttack and Paradip where the proliferation of industries and sewage discharges are prominent. The nutrient rich water after traveling all the distances enters Bay of Bengal through the Mahanadi river mouth at Paradip. A lot of work has been carried out on the water quality of the Mahanadi estuarine system 9-12 , however, literatures on phytoplankton composition and Chlorophyll-a from Mahanadi are very scanty. The present investigation deals with the seasonal variations of phytoplankton, chlorophyll-a and their relationship with physico-chemical parameters in the estuary. Materials and Methods Water samples were collected from Mahanadi river mouth as a station (Lat. 20° 17′ 16″ and Long. 86° 42′ 28″) during 2004-05 to 2006-07 (Fig. 1). The surface sampling was carried out at an interval of 3 hrs in three different seasons such as, pre-monsoon (May- June), post-monsoon (October-November) and summer (February-March). Surface water temperature, pH were measured in situ by using WTW Kit. Salinity, total suspended solid (TSS), dissolved oxygen (DO) were measured by standard methods 13 . The samples for the analysis of nutrients and Chl-a were preserved in icebox and brought to the laboratory and analysed immediately. For phytoplankton estimation, the samples were preserved with lugol’s iodine solution and brought to laboratory for identification and counting under microscope. The