On October 6, 2008 fishermen from Keezhakarai coast of Gulf of Mannar (GoM) were panic stricken as they noticed a large number of dead fish and other organisms washed ashore. The surface seawater had turned greenish. Researchers ascertained that the green tide was the blooming of Noctiluca scintillans , which is non-toxic.
However, corals were found to be affected by the asphyxiation caused by the N. scintillans bloom in Valai and Thalaiyari islands that triggered the massive death of marine organisms.
Proliferation or outbreak of N. scintillans , a free-living marine dinoflagellate phytoplankton, results in millions of cells per litre of water, which is known as algal bloom. Massive fish mortality is often associated with algal blooms due to depletion of dissolved oxygen level caused by N. scintillans respiration. Further, the dense cells of N. scintillans can damage fish gills.
During a survey conducted recently by Suganthi Devadasan Marine Research Institute (SDMRI), fishermen said algal blooming was frequent in GoM, almost every year during October, but with very little density and with no related impact. However, after 10 years, in 2019 blooming again caused large-scale fish death and coral mortality and it recurred in 2020 and 2021. All the witnessed blooms of N. scintillans happened between southwest and northeast monsoons.
The Noctiluca bloom in 2021 has severely affected a larger area than before between Rameswaram and Punnakayal in Thoothukudi district in GoM and fish mortality is significantly higher. Heaps of dead fishes are seen along the shores of the islands and mainland. Crabs, prawns, squids, gastropods, bivalves, sea anemones, sea urchins, sea cucumbers, sea snakes and worms are other casualties. Coral mortality has been observed in Krusadai, Manoliputti and Manoli islands of Mandapam region by the Reef Research Team of Suganthi Devadason Marine Research Institute (SDMRI-RRT) in association with the Forest Department through rapid underwater monitoring during October and November this year.
“A total of 102 colonies in Krusadai (within 500 sq.m area), 211 colonies in Manoliputti (within 270 sq.m area) and 422 colonies in Manoli Island (within 1 sq.km area) were found dead due to the bloom. The branching coral, Acropora is the worst affected coral genus contributing more than 95% of the total dead corals,” said J.K. Patterson Edward, Director, SDMRI.
According to the SDMRI’s research paper ‘Low oxygen levels caused by Noctiluca scintillans bloom kills corals in Gulf of Mannar, India’ published in the journal ‘Scientific Reports’, the presumable causes of algal bloom in GoM are high temperatures, abundant nutrients, low tidal amplitude and little current.
All the growth forms of Acropora including the branching, table and digitate types were found dead. Likewise, all the size classes, from recruits (<5cm diameter) to larger colonies (>160 cm diameter), witnessed mortality. Montipora and Pocillopora were the other coral genera affected by the bloom. However, massive coral genera such as Porites, Dipsastraea and Favites did not face mortality, but were stressed with excessive mucus secretion.
“In general, factors like well-mixed and nutrient-rich waters, seasonal circulation, currents and nutrient enrichment of coastal waters by human activities influence the occurrence and intensity of algal blooms. Climate change has been reported to be the prime reason for such blooms and, predictably, blooms will recur in future as climate change worsens,” Dr. Patterson Edward observed.
Coral cover in GoM has already suffered significant decline mainly due to the coral mortality caused by the bleaching events in 2010 (10% mortality) and 2016 (16% mortality). The annual blooming phenomena inflicts a loss not merely to corals but also to coral-associated biodiversity particularly the fishery resources and ultimately to the livelihood of the dependent people. Focused research and monitoring will help to develop mitigation protocols for managing the reef areas from this emerging annual threat to corals and associated biodiversity, he noted.
