Bimonthly servations on cell density and distribution indins of phytoplankton were made at Thengapattanam estuary (8°12'-8021' N;7?8'-77"1S1E) in the south west coast of India. 'The system was marked by the near total elimination of salinity in the momoon and strong vertical salinity tratification
in pre-monsoon season. Phytoplankton cell density in the study sites varied between 5.2 and 52.6 x ld cells/m3. Blooms or single species dominance was not recorded. Phytoplankton species diversity index varied between 1.72 and 5.01; Species Richness Index between 0.55 and 5.86; Species Evemess Index fluctuated fmm 1.4 to 2.96. Phytoplankton similarity Index was studied for the whole group. Salinity distribution and mixing apparently played a crucial role in phytoplankton cell density and diversity in the estuary.

keywords