Updated by: Arabinda Naik
Coral bleaching means decoulration and deaths of coral polyps (coral animal) due to various factors. Coral polyps live in symbiotic relation with zoozanthallae algae. These zoozanthallae provide the different food and oxygen to coral polyps. These zoozanthallae also give different colours to coral polyps till they remain in the body of corals. But when the zoozanthallae are removed or expelled from coral bodies, coral polyps become colourless and turn white. This process is called coral bleaching.
Thus, coral bleaching is a process which causes loss of different colours from coral organism and turns them white. The bleached coral organism loses their food supplies due to removal or expulsion of their symbiotic partners zoozanthallae algae from their bodies and ultimately, they die of starvation.
Earlier The Great Barrier Reef has experienced two major bleaching events in recent decades, in the summers of 1998 and 2002 when, respectively, 42% and 54% of reefs were affected by bleaching.
According to report published in Down to Earth (August 15, 1999) 30 percent of corals are in critical condition and a further 30 per cent are under severe environmental stress. According to the report of the United Nations’ Inter-Government Panel on Climate Change (IPCC) ‘If the projected levels of climate change are not stopped, the doom may be just 30 years away’.
In 1998 the temperature in the Indian Ocean was recorded 2°C higher than the normal temperature. El Nino phenomenon has also been associated with coral bleaching (coral death). El Nino was the strongest on record in 1997-98 and hence caused large-scale bleaching of corals.
Causes of Coral Bleaching:
Coral reef bleaching has become more widespread, frequent, and lethal in the last two decades. Therefore, coral reef ecosystems are severely threatened due to a variety of causes. Some of these causes are natural, such as diseases, predators, and storms.
While other threats are caused by people, including pollution, sedimentation, unsustainable fishing practices, and climate change, which is raising ocean temperatures and causing ocean acidification. Many of these threats can stress corals, leading to coral bleaching and possible death, while others cause physical damage to these delicate ecosystems. Important causes of coral bleaching have been discussed below:
Climate change: Corals cannot survive if the water temperature is too high. Climate change has already led to sharply increased rates of coral bleaching – killing vast areas of reef - and this is predicted to increase in frequency and severity in the coming decades. Scientists estimate that at current rates of ocean warming and reef decline, most of the world’s coral reefs could be lost in the next few decades.
Overfishing: This affects the ecological balance of coral reef communities, warping the food chain and causing effects far beyond the directly overfished population.
Unsustainable coastal development: Tourist resorts and other coastal infrastructure have been built directly on top of reefs or close enough to them to cause significant damage. The impacts of coastal development can vary widely, and cumulatively can put coral reef systems under considerable additional pressure. Some resorts and coastal developments empty their sewage or other wastes directly into water surrounding coral reefs.
Pollution: Urban and industrial waste, plastics, sewage, agrochemicals, and oil pollution are poisoning reefs. These toxins are dumped directly into the ocean or carried by river systems from sources upstream. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae.
Sedimentation: Erosion caused by construction (both along coasts and inland), mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can 'smother' corals by depriving them of the light needed to survive. The destruction of mangrove forests, which normally trap large amounts of sediment, is exacerbating the problem.
Careless tourism: Careless boating, diving, snorkelling, and fishing happens around the world, with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs.
Coral mining: Live coral is removed from reefs for use as bricks, road-fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters who don't know or don't care about the longer term damage done, and harvested for the live rock trade.