RÉSUMÉ
The Adyar estuary was once known for its unique ecology and endemic flora and fauna, has lost its pristine condition due to urbanization, discharge of untreated domestic wastes, industrial effluents and encroachments. Zooplankton were monitored for a period of one year from July 2009 to June 2010, following standard methods to evaluate the seasonal variations in diversity and density in relation to environmental parameters like temperature (28.6-33.6ºC), salinity (23.3-30.3‰), pH (7.3-7.8) and DO (4.4-7.1mg l-1). Highest diversity was observed during post-monsoon (20 species) and pre-monsoon (19 species), followed by summer (9 species) and monsoon (9 species). The zooplankton density was maximum during summer (1887167 m-3) followed by pre-monsoon (1843832 m-3), post-monsoon (1153333 m-3) and monsoon (182334 m-3). Zooplankton community structure and dynamics showed a differential pattern with dominance of harpacticoids and rotifers in pre-monsoon; cyclopoids and rotifers during post-monsoon and summer. The significance of monitoring zooplankton biodiversity as a base-line study for future investigations on environmental changes in this area is discussed.