Coral Reef Bleaching

By: Anil Rao

Coral reef bleaching is caused by anthropogenic and natural variations in reef environment including warm sea surface temperature, solar irradiance, sedimentation, xenobiotics, subaerial exposure, inorganic nutrients, freshwater dilution, and epizootics. Coral bleaching events have been increasing in both frequency and extent worldwide in the past 20 years. Global climate change plays a role in the increase in coral bleaching events, and could cause destruction of major reef tracts and extinction of many coral species.

Until 1980’s, the only coral bleaching event recorded was due to flooding from Hurricane Flora that resulted in a large drop in salinity – bleaching and killing corals in Jamaica. Mass coral bleaching was first recognised on the Pacific coast of Panama following the 1982-83 El Nino, which caused the death of over 99 percent of corals and the complete loss of reef structures in the Galapagos Islands and the death of over 50 percent of corals in Panama. The sensitivity of corals to minute sea temperature changes then became a major concern of researchers as predictions of global warming and stronger, farther-reaching El Nino events came to light. The 1997-98 El Nino is the strongest on record, resulting in unprecedented coral bleaching and death across the globe.


Causes of Bleaching

As coral reef bleaching is a general response to stress, it can be induced by a variety of factors, alone or in combination. It is therefore difficult to unequivocally identify the causes for bleaching but a few stressors may be implicated in coral reef bleaching events.



Coral species live within a relatively narrow temperature margin. Anomalously low and high sea temperatures can induce bleaching. Bleaching can occur during sudden temperature drops accompanying intense upwelling episodes, and seasonal cold air outbreaks.

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Solar Irradiance

Bleaching during the summer months, during seasonal temperature and irradiance maxima often occurs disproportionately in shallow living corals and on the exposed summits of colonies. Both photosynthetically active radiation and ultraviolet radiation have been implicated in bleaching.


Subaerial Exposure

Sudden exposure of reef flat corals to atmosphere during events such as extreme low tides, ENSO-related sea level drops or tectonic uplift can potentially induce bleaching. The consequent exposure to high or low temperatures, increased solar radiation, desiccation, and sea water dilution by heavy rains could all play a role in zooxanthellae loss, leading to coral death.

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Relatively few instances of coral bleaching have been linked solely to sediment. It is possible, but has not been demonstrated, that sediment loading could make zooxanthellae species more likely to bleach.


Fresh Water Dilution

Rapid dilution of reef waters from storm generated precipitation and runoff has can cause coral reef bleaching. Generally, such bleaching events are rare and confined to relatively small, near shore areas.


Inorganic Nutrients

Rather than causing coral reef bleaching, an increase in ambient elemental nutrient concentrations (e.g. ammonia and nitrate) actually increases zooxanthellae densities 2 to 3 times. Although eutrophication is not directly involved in algae loss, it could cause secondary adverse effects such as lowering of coral resistance and greater susceptibility to diseases.



Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants, such as copper, herbicides and oil. Because high concentrations of xenobiotics are required to induce algal loss, bleaching from such sources is usually localised and transitory.

Spatial and temporal range of coral reef bleaching

Mass coral mortalities in coral reef ecosystems have been reported in all major reef provinces since the 1870s. The frequency and scale of bleaching disturbances has increased dramatically since the late 1970’s. This is possibly due to more observers and a greater interest in reporting in recent years. More than 60 coral reef bleaching events out of 105 mass coral mortalities were reported between 1979 and 1990, compared with only 3 bleaching events among 63 mass coral mortalities recorded during the preceding 103 years.

Nearly all of the world’s major coral reef regions – Caribbean/western Atlantic, eastern Pacific, central and western Pacific, Indian Ocean, Arabian Gulf, Red Sea, experienced some degree of coral bleaching and mortality during 1980s.


Programmes of Ministry of Earth Sciences

The Ministry of Earth Sciences, formerly known as the Department of Ocean Development, has multi pronged studies that cover various aspects of ocean science. Development of human activities clustered along the narrow strip of coastal waters, creates conflicts between activities and the goal of conservation of the nature. The natural coastal systems and the areas in which human activities involve the use of coastal resources extend well beyond the limit of territorial waters. This leads to generation of different kinds of waste, which are discharged into the coastal marine environment either directly or indirectly through the rivers, canals and creeks. This results in degradation of coastal water quality, which has become a major cause for concern. These known problems are likely to be compounded in future as a result of the general trend in environmental and socio-economic conditions.

Protection and preservation of marine environment is one of the priority areas of Government of India. As per the Allocation of Business Rules, 1961, Ministry of Earth Sciences is responsible for preservation and conservation of marine environment. Thus the study of coral habitats is included in many related programmes, briefly outlined below.

Under the programme for Marine Research & Technology Development, an assessment of marine living resources is undertaken. The Centre for Marine Living Resources and Ecology (CMLRE) the erstwhile Sagar Sampada Cell in Kochi has the mandate of evaluating existing information on marine living resources, correlating it with the influence of physico-chemical parameters on the biota, identifying gaps and formulating future strategies for sustainable exploitation of marine living resources. It also encourages optimal utilisation and effective conservation of marine living resources among other things. The development programme also undertakes investigations on toxic algal blooms in the Indian EEZ, aimed at identifying the harmful and toxic algal bloom forming species, dynamics of bloom formation and culture aspects. During 2002-2003, toxic algal studies were carried out in seven FORV Sagar Sampada cruises. Samples were collected from 150 stations for toxic algal studies. 234 species of microflora consisting of 118 species of dinoflagellates, 112 species of diatoms, and 4 species of blue green algae were recorded during this period from the EEZ. The ocean living resources provide ample scope for extraction of drugs and chemicals for medical and other purposes. About 500 marine organisms, were collected and organisms possessing anti diabetic, anti-diarrheal, anti-fungal, anti-cholesterol, anti-bacterial and spermicidal activities were identified. The developmental programme also undertakes Coastal Ocean Monitoring and Prediction System (COMAPS) where a wide range of developmental activities viz., industry, tourism, fishing, aquaculture, etc., and large scale human inhabitation take place in the coastal areas. The COMAPS programme has been in operation at 82 locations for systematic collection of data and analysis with respect to 25 parameters relating to physical, chemical and biological including microbiological characteristics of water and sediments. The study also envisages concentration of heavy metals and pesticide residues. Besides, models are being developed for the evaluation of assimilative capacities of coastal marine areas for contaminant introduction and to enable the Pollution Control Boards to evolve a mechanism for mitigating the adverse impact of pollution on marine environment.

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