Research Highlight

Algal bloom kills corals, other marine animals in Gulf of Mannar

doi:10.1038/nindia.2021.51 Published online 9 April 2021

Green tide of Noctiluca scintillans in the Gulf of Mannar.

© Diraviya Raj, K. et al.

An algal bloom has decimated some species of fast-growing corals, fish and other marine animals in the Gulf of Mannar in south-east India1.

The bloom, coupled with marginally high water temperature, triggered temporary hypoxia, a condition during which levels of dissolved oxygen drop considerably, an international research team has found.

Such low oxygen levels affected the productivity and respiration of the corals, eventually causing their death, the researchers say.  

The Gulf of Mannar has 21 uninhabited islands where a variety of coral species thrives. To assess the threats of climate change and human activities on these corals, the scientists, including researchers from the Suganthi Devadason Marine Research Institute in Tamil Nadu, India, monitored and studied the islands.

During a trip across the islands in September 2019, they encountered a drifting mass of algal bloom that killed thousands of fish.  

The researchers found that the bloom, caused by toxin-producing algae Noctiluca scintillans, affected corals severely along the shoreline of two islands, Shingle and Krusadai.  

To probe the matter further, the researchers made visual inspections of the corals through scuba diving, collected algal cells from the surface water and measured dissolved oxygen levels in water.

The algal cells were found to have settled on the dead corals and carcasses of other marine organisms such as crustaceans, mollusks and echinoderms. The bloom hit hard the fast-growing coral species that belong to the genera  Acropora, Montipora and Pocillopora.

The fast-growing corals with faster metabolism rates require high-oxygen levels. This makes such corals succumb easily to low oxygen levels created by the algal bloom, the researchers say. 



1. Raj, K. D. et al. Low oxygen levels caused by Noctiluca scintillans bloom kills corals in Gulf of Mannar, India. Sci. Rep. 10, 22133 (2020)