Spatio-temporal variations of phytoplankton assemblage in coastal waters of Maharashtra, north-eastern Arabian Sea

Seasonal variations in phytoplankton species and abundance along the coast of Maharashtra from two different locations (Mumbai and Ratnagiri) and depths were investigated during August 2014-May 2017. A total of 84 species of phytoplankton were recorded from the region out of which 67 were diatoms, 16 dinoflagellates and one blue green alga. Seventy-five and 60 species of phytoplankton were recorded from Ratnagiri and Mumbai respectively. Diatoms dominated the phytoplankton community (81.7-93.3%) followed by dinoflagellates (4.7-7.2%) and blue green algae in both locations. Maximum phytoplankton density was recorded during post-monsoon season followed by pre-monsoon and monsoon seasons at Mumbai and Ratnagiri. Highest average phytoplankton cell density recorded was 9.09 x 104 cells l-1 and 4.06 x 104 cells l-1 during post-monsoon season at Ratnagiri and Mumbai respectively. Maximum cell density was observed at 20 m depth from Mumbai and Ratnagiri. Thalassiosira subtilis, Trieres mobiliensis, Skeletonema costatum, Coscinodiscus centralis and Ditylum brightwelli were the most abundant phytoplankton species recorded at Mumbai, whereas Chaetoceros curvisetus, S. costatum, Trichodesmium erythraeum, Chaetoceros lorenzianus and Ditylum brightwelli were abundant in Ratnagiri waters. The blue green alga, T. erythraeum was abundantly recorded during pre-monsoon season. Species diversity index (H’) ranged from 4.72 to 5.52 and was higher during post-monsoon season compared to pre-monsoon and monsoon. Highest phytoplankton diversity was observed at 20 m depth compared to 40 m depth