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Akashiwo sanguinea, a cosmopolitan red tide-forming dinoflagellate, is reported for the first time as a bloom from the Cochin estuary along the South Eastern Arabian Sea. Monthly sampling showed the continuous presence of A. sanguinea with occasional blooms. Blooms of A. sanguinea with the highest abundance of 2.8 × 105 cells l−1 were observed during the monsoon season (June 2019) with chlorophyll a of 17.2 mg m−3. The bloom period was characterized by higher concentrations of nitrate (26 μmol l−1) and phosphate (3.8 μmol l−1). Routine monitoring of the bloom region showed the survival of the Akashiwo sanguinea population in the temperature (26 to 30°C) and salinity (22 to 24 PSU) range indicating its eurythermal and euryhaline characteristics.

Cyanobacteria are Gram-negative, oxygen-evolving photoautotrophic bacteria found in almost every conceivable habitat around the world. Cyanobacteria are extremophiles identified from geothermal springs to cold deserts (Amarouche-Yala et al., 2014;Mataloni & Komárek, 2004) and existed for about 2.8 billion years on earth (Olson, 2006). As the oldest oxygen-evolving photosynthetic microorganisms on earth, cyanobacteria significantly influenced ambient atmospheric and biosphere status (Demoulin et al., 2019;Paerl & Huisman, 2009). Planktic species have been the subject of numerous investigations in terms of risk assessment; however, little is known about benthic cyanobacterial forms (Gaget et al., 2017). Although reports on the prevalence of benthic cyanobacteria have increased