The study was carried out in the coastal waters of Bay of Bengal off Rushikulya estuary, to evaluate the diversity of zooplankton and its distribution with respect to hydrographical parameters. A gradual increase in salinity from 27.12-34.78 and water temperature from 25.8-31.9 ^oC was noticed. The zooplankton volume ranged from 0.1-3.8 mL per 10 m³. The density of zooplankton varied from 0.34-6.55 x 10⁴ individuals 10 m³, out of which copepods were found to be the dominant group contributing 53.4-67.9% of the total zooplankton standing stock. In total, 93 species of zooplankters belonging to 46 genera and 33 families of six different phyla were identified. Copepods were represented by 49 species of 21 genera, belonging to 15 families and three orders. The lowest and highest contribution of copepods was recorded during January and June respectively. Chaetognaths appeared as the second dominant group and their contribution ranged from 9.0-10.7% of the total density. The share of meroplankton components accounted for about 9.4-15.7% of the bulk zooplankton density. A gradual increase in zooplankton volume and density from post-winter to summer showed a significant influence of salinity and temperature on zooplankton community at this locality.

Zooplankton plays a significant role in marine food web by contributing to the flow of energy and matter and biogeochemical cycling of elements and their vertical flux. They account for about one-tenth of the total marine biomass. They considerably affect the world marine fishery, both as primary and secondary consumers. In fact, their abundance is taken as a good index of the available fishery resources of different water masses (Nair, 1980). Zooplankton species are also considered as indicators of water mass (Rao, 1958), where chaetognaths were used to locate the current pattern of Indian seas. Later, Ganapati (1975), studied the abundance pattern of polychaetes in relation to water quality with special reference to organic pollution load in the coastal ecosystems. Recent studies on climate change which use zooplankton, mainly the copepods as the indicators (Anonymous, 2009), showed a significant correlation between Earth's changing climate and zooplankton community. Due to the above multidimensional economic and ecological utility, studies on zooplankton community have been an important area of research in all marine biological explorations. A great deal of attention has been given during the 19^th and 20^th centuries to improve the knowledge about taxonomy, biochemical composition, ecology of zooplankton with special reference to their abundance, distribution and impact on secondary and tertiary production regionally and globally.

Studies on marine zooplankton can be traced back to the late 17^th century, when it was identified for the first time by Leeuwenhoek as `little animals'. However, taxonomy, ecology and biology of marine zooplankton started only after the mid-19^th century as a part of different cruise programs, like the Novara Expedition (1857-1859), Valdivia Expedition (1898-1899), RMIC Investigator Expedition, Galathea Expedition (1950-1952) and the International Indian Ocean Expedition (IIOE) (1960-1965). A study on quantitative estimation of these denizens of the oceans by Hensen (1887) in the late 19^th century is such an instance of earliest attempts. In Indian waters it started only in the early 20^th century by Sewell (1913), followed by Annandale and Kemp (1915). Initially, the zooplankton studies here were limited to some estuarine milieu like Cochin backwaters (George, 1958; Qasim et al., 1969; Haridas et al., 1973 and Madhupratap, 1980), Vellar estuary (Seshadri, 1957; Subbaraju and Krishnamurty, 1962; and Shanmugam et al., 1986), Hooghly estuary (Dutta et al., 1954; Shetty et al., l961 and Sarkar et al., 1984) and Chilka lake (Devasundaram and Roy, 1954 and Patnaik, 1973). Zooplankton studies in the open ocean commenced with Sewell (1948), when he correlated the distribution of zooplankton with reference to the changes in surface and deep-water currents. It started advancing with qualitative and quantitative abundance studies in the coastal waters of Mandapam (Raghuprasad and Jayaraman, 1954) and Visakhapatanam (Ganapati and Rama Sharma, 1958). During the 1970s-1990s, a tremendous emphasis has been given to zooplankton research in the Arabian sea (Purushan et al., 1974; Panampunnayil and Desai, 1975; Madhupratap, 1978; Nair et al., 1978; Nair, 1980; Nair and Peter, 1980; Nair et al., 1981 and 1989; Govindan et al., 1982; Nair et al., 1983; Goswami, 1985; Tiwari and Nair, 1993; and Ramaiah and Nair, 1997), Lakshadweep islands (Achuthankutty et al., 1989; Madhupratap et al., 1991; and Santhakumari, 1996), Bay of Bengal (Achuthankutty et al., 1980; and Ramaiah et al., 1996) and Andaman Sea (Madhupratap et al., 1981a and 1981b). Zooplankton studies in offshore areas of the Bay of Bengal were carried out for the first time in the 1950s during Galathea expeditions and with some more detail during the IIOE. Qasim (1977) has given an account of the secondary productivity in the Arabian Sea and Bay of Bengal, based on the information collected during IIOE. Spatial distribution of zooplankton in the shelf and offshore waters of the Bay of Bengal has been depicted in a plankton atlas prepared using the IIOE data sets (Wyrtki, 1971). Subsequently, Rao (1973) described the geographical distribution of zooplankton biomass in the Indian Ocean with special reference to the Arabian Sea and Bay of Bengal.

Environmental Sciences Journal, Zooplankton Diversity, Meroplankton Components, Bay of Bengal, Fishery Resources, World Marine Fishery, Biochemical Composition, Coastal Ecosystems, Large Marine Ecosystems, Global Positioning System, Phytoplankton Community.