A worldwide bleaching event in 1998, associated with El Nino, harmed 16 per cent of the world’s coral reefs, with possibly half of them damaged irreversibly. Another 32 per cent are thought to be threatened over the next two-three decades while over 10 per cent have already been lost. Over the past century, the mean global surface temperature has increased by 0.4 to 0.8oC resulting in the coral reefs being damaged by bleaching associated with high sea temperatures. Empirical evidence suggests that corals possess a wide range of adaptive and accumulative mechanisms, and that most of these mechanisms appear relatively robust and/or resilient. However, the huge variety and magnitudes of anthropogenic threats to reef environments are likely to undermine this resilience, particularly when coupled with the effects of climate change.
The Lakshadweep islands, 120- 200 nautical miles off the south west coast of India consist of 12 atolls and several submerged coralline banks. Most of the atolls have low-lying islands on the east, a reef on the west and a lagoon in between. The largest island is Minicoy with a length of about 9 km, and an area of 4.37 sq km.
The bleaching episode in the summer of 1998, when the sea surface temperature in the tropics rose by 1 to 2ºC above the seasonal maximum, reportedly resulted in the severest coral mortality in the Lakshadweep group of islands – nearly 80 per cent of the coral area was affected (R Arthur, 2000, Coral bleaching and mortality in three Indian reef regions during an El Niño southern oscillation event, Current Science.) Scientists pointed out that large-scale reef erosion could be disastrous for these tiny atolls significantly changing beach dynamics – leaving them unprotected from storms and cyclones.
Mass Coral Bleaching: A mass coral bleaching event at the Lakshadweep during May-June 2010 was recorded by the Centre of Advanced Study in Marine Biology, Annamalai University. Underwater documentations were undertaken in Agatti where the bleaching phenomenon was conspicuous and perceived throughout the lagoon at the rate of around 70 to 80 per cent – similar to that which occurred in 1998. It was also observed that the reef associated organisms like giant clams and sea anemones were affected by nearly 83 to 92 per cent with the daily mean sea surface temperature (SST) reaching a maximum of 34oC and remaining well above the crucial optimal limit of 31oC throughout the study period.
Annual mean SST trend shows that the reef areas of Lakshadweep have warmed from 28.5oC in 1985 to 28.9oC in 2005, at a rate of 0.2oC per decade as pointed out in a study by E Vivekanandan et. al in 2008, titled ‘Thermal thresholds for coral bleaching in the Indian seas’ and published in the Journal of the Marine Biological Association of India. The annual average maximum SST did not increase, but the annual average minimum SST increased from 27.2oC to 27.8oC, at a rate of 0.3oC per decade in the Arabian Sea. The effect of El Niño on SST was evident in 1997-1998 when the SST reached 31oC (ibid), as compared to the maximum SST of 34oC recorded during May-June 2010 with daily mean values of above 31oC, resulting in bleaching of corals and depletion of other reef associated organisms. As outlined in another project ‘Studies on the coral reef and fish resources of the selected islands at Lakshadweep’ by R Vinoth in 2012 (Annamalai University) during the same period, a decrease in population of reef-associated fishes such as coral groupers butterfly and parrot fishes, etc. was also observed here. Similarly, during March-May 2010, an increase in daily average SST of 1oC was observed in the Andaman and Nicobar Islands where a maximum of 32.5oC was recorded compared to 31.8oC of the previous year (P M Mohan et. al., 2010, Coral Bleaching in and around Port Blair, Andaman and Nicobar Islands, Seshaiyana). However, the influence of this rise on the coral reefs of Andaman and Nicobar Islands are comparatively less than that of the Lakshadweep.
The recent coral reef declines seem to be related mostly to anthropogenic impacts like over exploitation, overfishing, increased sedimentation and nutrient overloading. Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low temperature extremes, El Nino Southern Oscillation (ENSO) events, sub-aerial exposures, predatory outbreaks and epizootics. Reef bleaching is a generalised stress response and evidence indicates that elevated temperature causes mass bleaching of corals leading to a rapid loss of pigmentation of coral and whitening of the colony. The process entails stony corals (hard skeleton) expelling the unicellular algae (zooxanthellae) that share a symbiotic relationship with the coral polyp translocating products of photosynthesis to its host in return of inorganic nutrients. Thus, the photosynthetic activity of the coral polyps is severely reduced.
A proliferation of correlative studies for different parts of the world have shown a close association between bleaching and higher sea temperatures. The mass coral bleaching events of Indian seas in 1998 has added further credence to the argument that elevated temperature is the primary variable triggering the coral bleaching.
An increase in the frequency of bleaching events could have drastic consequences with irreparable loss of biodiversity and loss of livelihood. Arguments that corals will acclimate to temperature changes are unsubstantiated and evidence suggests that the genetic ability of corals is already being exceeded. The destruction of coral reefs can result in changes in the fish populations – corals die, become overgrown by algae, corallivorous fish replaced by algal grazers, etc. If the reef degenerates further, reef dwelling species of fish may disappear entirely, being replaced by pelagic species. Ultimately, biodiversity in terms of corals, fish, shellfish and other reef inhabitants plummets leading to loss of food security and an increase in poverty.
Endnote: According to a report ‘Coral bleaching, coral mortality, and global climate change’ presented by R Pomerance to the US Coral Reef Task Force in 1999 – around 8 percent (0.5 billion people) of the world’s population live within 100 km of coral reef ecosystems thus proving that coral reefs and livelihoods are closely linked. For example, as pointed out in the paper ‘Reefs at risk: a map-based indicator of threats to the world’s coral reefs’ by D Bryant et.al. in 1998, (World Resources Institute: Washington) 25 per cent of the fish catch in developing countries is provided by coral reef associated fisheries. Further, coral reefs provide sustainable livelihood options through tourism development and help in providing new drugs and biochemical substances.
Scientific evidence strongly suggests that global climate change is already affecting a broad range of marine species and ecosystems. Although it is not known with certainty as to which of these mechanisms is causing the observed changes, it is evident that these changes are occurring across the oceans and across a range of types of marine life. The observed changes are consistent with those predicted for climate change, occurring during the warmest years and giving strong indications that long-term global warming is beginning to alter marine ecosystems. In addition to increased sea temperatures, there are two other mechanisms by which climate change can impact coral reefs: through higher levels of dissolved carbon dioxide (CO2), and by increased frequency and intensity of extreme weather events. The oceans constantly absorb CO2 from the atmosphere. As humans continue to emit high levels of CO2, it is likely that oceans will absorb a higher amount too. Reef structures are formed by coral secretions of calcium carbonate. High levels of atmospheric CO2 alter water chemistry, and reduce the rate of calcification, ultimately reducing the density of coral skeletons, and weakening them. This, combined with increased storm activity and mass mortality from bleaching, can greatly accelerate reef erosion.
Recent years have seen a series of particularly intense coral bleaching events, resulting in a massive die-off of reefs in the Indian Ocean. The fact that corals, estimated to be several hundred years old, were killed in this event, indicated that such large scale mortalities are unique and that the forcing mechanism in this case – elevated SST due to global warming is moving outside the range of natural expectation. SSTs in tropical regions have increased by almost 1oC over the past 100 years and are currently increasing at the rate of 1o to 2oC per century. If this current trend continues, and if climate change predictions become a reality the reefs might disappear entirely within the next 20-50 years as projected by O Hoegh-Guldberg in 1999 in the paper titled ‘Climate change, coral bleaching and the future of the world’s coral reefs’ published in Marine and Freshwater Research. Also, continued anthropogenic threats are further exacerbating the strain on the corals turning the vibrant reefs barren. In this context, it is imperative that continuous monitoring programmes should be undertaken in the Lakshadweep coral areas to quantify changes, raise the public awareness and put in place an effective management system for conservation and protection of the coral reef ecosystem.