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Not AvailableFrom 2.10.08 to 12.10.08 an intense bloom of Noctiluca scintillans (Macartney) was observed for the first time in the coastal areas of Gulf of Mannar (southeast coast of India) near Appa Island, Thalaiyari Island and Valai Island and subsequently it intensified into a dense bloom in Muthupettai area and spread from Kilakarai to Pamban. The coastal waters appeared dark green and microscopic examination revealed the presence of N. scintillans. The organism is bioluminescent, inflated and sub-spherical. The size of the organism ranged from 400-1200 microns. Though the species is colourless, the presence of photosynthetic green endosymbionts make the water green. The high temperature, salinity and low pH aided the spread of the bloom to adjacent areas of Muthupettai. During the intense period of bloom, the concentration of N. scintillans was around 13.5 lakh cells /l, the dissolved oxygen level was below detectable level and the total suspended solids was 510 mg/l, thereby increasing the turbidity and cutting of light to the bottom. This resulted in biodiversity loss in the intensely affected area from Valai Island to Muthupettai coast. The environmental parameters during the waning phase of the bloom were, surface water temperature 29.5° C, salinity 34.2 ppt, dissolved oxygen 4.86 ml/l, phosphate 8.28μ g at/l and ammonia 85 μ g at/l. Further investigations indicated the resilience of the ecosystem to recover from the sudden natural damage.Not Availabl