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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.

Abstract The Gulf of Mannar (GoM) in southeast India is known for its coral reefs and associated biodiversity. Multiple coral bleaching events driven by global climate change and subsequent space competition have damaged the coral reefs of GoM during the past couple of decades. Certain marine sponges are space competitors to corals and can outgrow corals by overgrowing. Spongivore fishes play vital role in regulating sponge abundance in a reef. The present study was aimed to identify spongivore fishes at Pattinamaruthoor patch reef (55.25% coral cover) in Tuticorin coast of GoM. Gut content analysis and underwater assessment were carried out between March and September 2019. Known spongivores such as Apolemichthys xanthurus, Pomacanthus annularis and P. imperator had about 75, 50 and 80% of their diet as sponge. Other fishes such as Sargocentron rubrum (30%), Myripristis murdjan (30%) and Scarus ghobban (10%) were also found to feed on sponges which are not regular sponge feeders. It is presumed that increased availability of sponges as a result of climate change has made these fishes opportunistically feed on sponges. A total of 58 reef fish species including obligate and opportunistic sponge feeders were observed at Pattinamaruthoor patch reef. Protection of spongivore fishes from overexploitation is critical as it could allow corals to recover from climate change implications.

Terpios hoshinota is an encrusting cyanobacteriosponge which grows aggressively over live coral colonies and has been reported to undergo outbreaks which kill corals. In an underwater survey conducted on the reefs of Gulf of Mannar, an outbreak of this coral-invading sponge was witnessed for the first time. It was found invading approximately 5% of the Montipora divaricata colonies (n = 383) at 1 m depth in Vaan Island. The affected site had a high coral cover (85.13%) dominated by montiporids (79.97%). T. hoshinota was confirmed via underwater morphology and microscopic observation of distinctive lobed tylostylespicules. This sponge is reported to have caused significant damage to many reefs around the world and hence immediate steps are needed to protect the reefs of Gulf of Mannar. Further studies are needed to assess the extent of T. hoshinota invasion in Gulf of Mannar and progression rate over live coral colonies.

Global climate change has aggravated the severity of space competition put up by marine sponges in the tropical coral reef ecosystems. We report here an outbreak of coral-killing sponge Clathria (Microciona) aceratoobtusa (Carter, 1887) over live coral colonies of the genus Turbinaria in an unprotected reef (mainland patch reef) in the Gulf of Mannar in India. An outbreak of this orange-reddish sponge was observed during an explorative dive in April 2020. Live coral cover in the reef was 62.06% (SD±3.36), which was dominated by the genus Turbinaria with 82.31%. Among the total of 549 Turbinaria colonies counted in five transects, 21.86% (n = 120) of colonies were found infested by the sponge. As the mainland patch reef lies outside the boundary of marine protected area, it is exposed to human-induced threats, apart from climate change implications. This report would serve as an evidence to indicate the intensity of the space competition waged by sponges in a disturbed reef ecosystem.

Intensive underwater surveys have been conducted to assess the extent of coral bleaching and subsequent mortality in the Gulf of Mannar between March and October 2016. The extent of bleaching was 23.92% ± 10.55% during the period between March and June 2016, and the live coral cover was drastically reduced to 22.69 ± 9.07% during October 2016 with a mortality of 16.17 ± 8.46%. Fast-growing coral forms, including the genera Acropora, Montipora and Pocillopora were most affected, not only by bleaching but also by severe mortality. Boulders, including the genera Porites, Favia and Favites were found to be resistant to bleaching. During the bleaching period, water temperature was between 31.2°C and 32.6°C. The current bleaching is in alignment with the third global coral bleaching event which occurred between 2014 and 2017. Management interventions, including protection and rehabilitation using the native resistant coral species will not only help in the recovery process, but also increase the live coral cover.

We used a 12-yr data set of benthic cover (2005–2017), spanning two bleaching events, to assess changes in benthic cover and coral community composition along 21 islands within Gulf of Mannar (GoM), southeast India. Overall, between 2005 and 2017 reefs had a simultaneous decrease in relative coral cover (avg. =  − 36%) and increase in algal cover (avg. =  + 45%). Changes in benthic cover were not consistent among islands, ranging from − 34 to + 5% for coral cover and from − 0.3 to + 50% for algae. There was a spatial gradient in coral mortality, which increased among islands from west to east. However, there was a disconnect between coral loss and subsequent increases in algae. Algal cover increased more on islands in west GoM where coral loss was minimal. Environmental co-factors (coral cover, percent bleaching, degree heating weeks, fish densities, Chl-a, pollution) explained > 50% of the benthic cover responses to successive bleaching. Coral survival was favored on islands with higher fish densities and chlorophyll-a levels, and increases in algal cover were associated with higher measures of pollution from terrestrial runoff. Coral morphotypes differed in their response following successive bleaching resulting in changes in the relative abundance of different coral morphotypes. Existing climate projections (RCP8.5) indicate a 22-yr gap in the onset of annual severe bleaching (ASB) for reefs in the east versus west GoM, and ASB was ameliorated for all reefs under the RCP4.5 projections. There is limited knowledge of the resilience of GoM reefs, and this study identifies coral morphotypes and reefs that are most likely to recover or decline from successive bleaching, in the context of forecasts of the frequency of future bleaching events in GoM.

Fishing pressure on marine ecosystems has increased drastically all over the world, especially in developing countries and particularly in India. Shore seine, a traditional fishing method, involves bottom trawling which makes huge impact on the ecosystems and consequently on fishing yield. It was observed that important habitats such as seagrasses and coral reefs are affected severely by this shore seine operation. Most of the catch was observed to be juvenile in nature or very small in size. Immediate measures are needed to check this operation by creating awareness among the fishermen coupled with providing alternative livelihood options.

Malvan Marine Sanctuary (MMS), Maharashtra, India is rich in coral reefs and the associated resources, which provide livelihood for the people involved in fishing and tourism. The elevated sea-surface temperature triggered by climate change had caused the coral reefs around the world to undergo severe bleaching during 2014–2016. Scientists have declared this as the third global coral bleaching event. Two underwater surveys during December 2015 and May 2016 were conducted in MMS to assess the intensity and trend of coral bleaching. A high prevalence of coral bleaching, i.e. 70.93% (SD = 4.53) was recorded inside MMS during December 2015, with a mortality of about 8.38% (SD = 0.91). After a lapse of six months, corals were found to recover. This is borne out by the reduction in the bleaching prevalence to 6.77 ± 0.12% during May 2016. Climate change being a global issue, reduction in the local stressors such as fishing and tourism is highly recommended in order to allow the corals to recover and enable sustainable utilization of coral reef resources around MMS.

A study was undertaken to assess the morphological variations during the period 1969–2018 in the twenty-one islands in Gulf of Mannar, southeast India, using Survey of India Toposheet, Google Earth images and field survey techniques. The results show that two of the islands have eroded and submerged, fifteen islands area cover has reduced and four have their area expanded during the last 49 years. Overall, the Tuticorin group of islands has experienced the highest percentage of land cover reduction (78.55%), followed by Keelakarai (43.49%), Vembar (36.21%) and Mandapam (21.84%) groups. The exceptions are the four islands of the Mandapam group viz. Hare, Manoli, Krusadai and Shingle, whose area covers have increased (16.44%). A small change in the area of an island induces a greater change in its morphology. From the Island Change Ratio plot, it is observed that islands with an area cover of 10–30 ha have suffered loss of more area cover and undergone mobility from the core area. The constant changes undergone and the mobility exhibited by these islands clearly indicate that they are dynamic. The main factors that are responsible for the current changes in their extent and position are coral mining and sea level rise. Thus, it is clear that the low-lying reef islands of Gulf of Mannar are very dynamic, and that their expansion, loss in area cover and mobility are mainly controlled by coral mining and climate change. Immediate conservation action is necessary to prevent the islands from submergence.