Intense bloom of the diatom Hemidiscus hardmanianus (Greville) in relation to water quality and plankton communities in Tuticorin coast, Gulf of Mannar, India.

The present study reports a dense bloom of the marine-diatom Hemidiscus hardmanianus observed off the Tuticorin coast in the Gulf of Mannar (GoM), India. The surface water discoloration (pale green) was observed during a coastal survey conducted in the initial period of the northeast monsoon (October 2018). The bloom extended over an area of approximately 5 km2 around the Tuticorin harbor. Distribution and relative abundance of the phytoplankton and zooplankton species together with the water quality and Chlorophyll-a were studied in the area of bloom. H. hardmanianus density was maximum (10.57 × 104 cells L-1) in the bloom site, which was almost 97% of the total phytoplankton population. The present report is the first record of H. hardmanianus bloom in the Gulf of Mannar. The chain-forming diatom Biddulphia biddulphiana was also observed in strong numbers (802 and 432 cells L-1), which has been rarely reported from the Indian coastal waters.