Abstract: The present study was carried out to determine the water quality in terms of physicochemical characteristics and plankton
distribution in the coastal waters of Kattumavadi, Palk Bay for a period of one year from April 2002 to March 2003. Air and surface
water temperatures varied from 28ºC to 32.50ºC and from 27.5 to 32.0°C while light extinction coefficient (LEC) varied between 0.95
and 1.85. Salinity ranged from 26.0 to 34.5‰ and the pH ranged between 7.95 and 8.35. Variation in dissolved oxygen content was
from 4.15 to 7.18 mll-1, and the particulate organic carbon (POC) content varied between 0.49 and 2.28 mgCl-1. Concentrations of
nutrients viz. nitrate (2.15 to 8.28μM), nitrite (0.12 to 0.62μM), inorganic phosphate (1.28 to 2.15μM) and reactive silicate (5.15 to
12.52μM) also varied independently. Chlorophyll a content ranged from 0.28 to 1.48 mg m-3 and the primary productivity, from 4.19
to 16.08 mgCm-3hr-1. The present study recorded a total of 43 species of planktonic diatoms and two species of blue-green algae.
Population density of phytoplankton varied from 18,000 to 34,000 cells l-1. Percentage composition of the diatoms showed minimum
values during the monsoon season and the maximum values during the premonsoon season.