Seasonal variations of plankton diversity in the Kaduviyar estuary, Nagapattinam, southeast coast of India.

The results of an investigation carried out during October 2005 to September 2006 on hydrography composition and community structure of phytoplankton and zooplankton including chlorophyll 'a' (Chl-a) content and primary productivity (PP) at the Kaduviyar estuary (Southeast coast of India) are reported. Air and surface water temperatures (degrees C) varied from 23 to 29 and from 24 to 31 respectively while the light extinction coefficient values (LEC) ranged between 0.15 and 0.59. Salinity values (per thousand) varied from 4 to 34 and the pH ranged between 7.8 and 8.3. Variation in dissolved oxygen content was from 3.06 to 5.63 mg l(-1). The ranges of inorganic nutrients (microM) viz., nitrate, nitrite, phosphate, silicate and ammonia were: 0.36-12.65; 0.06-1.86; 0.06-4.57; 17.96-235.06 and 0.001-0.75 respectively. The ranges of Chlorophyll 'a' (mg m(-3)) and the primary productivity (mg Cm(-3) hr(-1)) values were: 3.4-12.8 and 55-119 respectively. Presently, 85 phytoplankton species representing different classes viz: Bacillariophyceae (58); Dinophyceae (16); Cyanophyceae (7); Chlorophyceae (3) and Chrysophyceae (1) were recorded. The phytoplankton density varied from 14,135 to 74,697 cells l(-1), with peak diversity (3.46 bits ind.(-1)) during premonsoon season. The maximum density was found during summer season coinciding with the stable hydrographical conditions. Totally 92 species of zooplankton besides 18 larvae were recorded and the copepods formed the dominant group. The occurrence of most of the zooplankton species showed a distinct seasonal pattern, which was closely associated with the species-specific environmental conditions. That way the environmental factors exert major influence on the species composition, abundance and diversity of zooplankton. The zooplankton density (org. l(-1)) ranged between 4342 and 14,002 and between 4867 and 15,816 at stations 1 and 2 respectively. Kaduviyar estuarine waters were very rich in zooplankton diversity (bits ind.(-1)) viz: 5.29 at Station 2 which was slightly higher than 5.27 at Station 1. Higher values of zooplankton density and species diversity were found during premonsoon and summer seasons and which showed positive correlation with salinity. The seasonal distribution and abundance of plankton are discussed in relation to hydrographical parameters.