Low oxygen levels caused by Noctiluca scintillans bloom kills corals in Gulf of Mannar, India.

Coral reefs around the world are undergoing severe decline in the past few decades. Mass coral mortalities have predominantly been reported to be caused by coral bleaching or disease outbreaks. Temporary hypoxic conditions caused by algal blooms can trigger mass coral mortalities though are reported rarely. In this study in Gulf of Mannar (GoM), southeast India, we report a significant coral mortality caused by a bloom of the ciguatoxic dinoflagellate Noctiluca scintillans during September-October 2019. Dissolved oxygen levels declined below 2 mg l-1 during the bloom causing temporary hypoxia and mortality (up to 71.23%) in the fast growing coral genera Acropora, Montipora and Pocillopora. Due to global climate change, more frequent and larger algal blooms are likely in the future. Hence, it is likely that shallow water coral reefs will be affected more frequently by episodic hypoxic conditions driven by algal blooms. More studies are, however, required to understand the mechanism of coral mortality due to algal blooms, impacts on community composition and the potential for subsequent recovery.

Marine debris - An emerging threat to the reef areas of Gulf of Mannar, India.

Underwater survey was conducted to assess the accumulation and impact of marine debris in the reef areas of Gulf of Mannar in southeast India. A combination of roving diver technique and belt transect method was applied for the assessment, which was conducted during the period between February 2018 and March 2019. An estimated total reef area of 1152 m2 has been affected by marine debris. Abandoned fishing nets were found to constitute the major portion of 43.17 ± 5.48% of the marine debris. Live corals were found to be dominant substrates for marine debris with 39.11%. The average prevalence of coral colonies in contact with marine debris was 3.28 ± 0.27%. Prevalence of corals in contact with debris was very high in genus Acropora with 8.23 ± 1.29% followed by Montipora with 4.63 ± 1.29% due to their complex growth form. Of the corals in contact with debris, 47.56% were fragmented and 34% were found with tissue loss.

Seagrass restoration in Gulf of Mannar, Tamil Nadu, Southeast India: a viable management tool.

Seagrass beds are important marine ecosystems that provide significant ecological services. The global decline of seagrass beds is becoming severe due to the increasing pressure of human-induced factors and changing climatic conditions. Restoration of seagrasses is an evolving science that started in 1939. In this study, we report a remarkably successful restoration activity carried out in the Gulf of Mannar (GoM), Southeast India. This is the first wide-scale effort in Indian waters. After the initial experimentation, manual transplantation of seagrass sprigs was carried out near Vaan and Koswari islands in GoM. Transplantation was performed with PVC quadrats and jute twines in areas of 800 m2 in both the islands during February to May 2014. An increase from 16.4 ± 0.3 to 32.3 ± 0.6% in Vaan and from 15.1 ± 0.2 to 35.1 ± 0.9% in Koswari was observed in seagrass percentage cover during the period from June 2014 to May 2016. Area cover, shoot density, macrofaunal density and fish density increased at the restoration sites after the transplantation. Bottom trawling was found to be the most serious threat to the seagrass beds in these islands. This method of transplantation can be replicated in other areas of degraded seagrass in India to carry out wide-scale restoration of seagrasses.