What is Coral bleaching?
Coral bleaching – When a reef loses its colourful algae (zooxanthellae) and other producers, exposing the colourless coral animals as well as the underlying white skeleton of calcium carbonate.
Mechanism and Impact
The zooxanthellae in the corals work like an internal symbiotic vegetable garden, carrying out photosynthesis and providing nutrients, which help reef-building corals, create reef structures. When a coral bleaches, it loses its zooxanthellae, and will die within a matter of weeks unless the zooxanthellae can be replaced. The term bleaching is used because the dazzling colors of living corals are due to the colours of zooxanthellae in coral tissue, and when zooxanthellae are lost, corals appear white, or “bleached.” In some species, tissue growth is halted, skeletal accretion is stopped.
Once the zooxanthellae death occurs, the coral’s soft tissue becomes a food source for scavengers, making the increasingly bare skeleton a feasible site of attachment for rapidly growing seaweeds and other opportunistic organisms.
As long as the structure of coral reefs is maintained, the corals offer protection to a wide variety of marine life. If favourable conditions return and are continued and sustained, coral larvae may settle on the reef structure, renewing the reef building process.
Once the reef is reduced to rubble, fish and other marine organisms are no longer supported. Local human populations are thus placed at risk; over time fisheries stocks are greatly diminished, and shoreline erosion increases.
Mass coral bleaching has affected by and large every reef region in the world. The spatial extent and severity of impacts of coral bleaching have been increasing throughout the world over the last few decades. A particularly severe, worldwide bleaching event occurred in 1998, which may have destroyed 16 per cent of the world’s reefs. The Great Barrier Reef was affected by this global bleaching event and by another event in 2002. More localised bleaching occurred in the southern Great Barrier Reef in 2006.
The initial onset of mass coral bleaching can range from gradual and patchy to rapid and uniform, and can occur with varying synchrony over hundreds or thousands of square kilometres. Detecting the early signs of a mass-bleaching event requires a wide network of observers providing regular reports of conditions throughout the region.
Causes of Coral Bleaching
If corals are subjected to intense solar radiation (especially ultraviolet wavelengths), reduction of marine salinity, exposure to the air (by low tides or low sea level), sedimentation, or xenobiotics (chemical contaminants, such as, copper, herbicides, and oil) then stress-related bleaching can be induced. Often, these conditions are at least an indirect consequence of extremes in weather (such as tropical cyclones), which may be proceeded by or occur concurrently with elevated sea surface temperatures. As a consequence, multiple factors may act in concert to cause bleaching. High solar irradiance (particularly ultraviolet wavelengths) is thought to be especially stressful to corals when coupled with elevated sea surface temperatures.
- Rise in Temperature (and light)
Coral bleaching is generally associated with a rise in sea surface temperatures. Like all living things, corals and zooxanthellae survive and grow well within a preferred range of abiotic environmental conditions. Temperatures outside their operating range are detrimental to them. Unlike fish that can move to more suitable environmental conditions, migrate from one to another place, and escape if the environmental conditions are unfavourable, corals cannot escape areas of high temperature because they are attached to the reef structure. If temperatures are high for too long, the symbiotic relationship between the coral animal and its tenants, the zooxanthellae, collapses.
When corals are stressed by high temperatures and light, some of the chemical reactions in photosynthesis break down. The solar radiation, which is normally used to produce food for the algae and coral, is diverted into the production of oxygen radicals. These oxygen radicals, similar to the chemicals implicated in human aging, are highly corrosive. They damage the parts of the zooxanthellae – the chloroplasts – where photosynthesis takes place.
The coral polyps can also be damaged as a result of heat stress. Rising water temperatures block the photosynthetic reaction that converts carbon dioxide into sugar. This results in a build-up of products that poison the zooxanthellae. This causes a large number of damaged zooxanthellae to leave the coral. To save itself, the coral spits out the zooxanthellae and some of its own tissue. As the zooxanthellae leave corals, the corals are bleached white. The bleached coral can recover, but only if cooler water temperatures return and the algae are able to grow again. Without the zooxanthellae, the coral slowly starves to death.
If high temperatures are relatively short-lived, the zooxanthellae that remain within the coral tissue divide rapidly and the coral gradually regains its colour and survives. If the high temperature conditions are prolonged or particularly severe, the zooxanthellae won’t reproduce, and many corals will die. Different coral species show variations in their bleaching and even within a particular coral colony, there will be variations in the susceptibility to bleaching. There are two non-exclusive mechanisms for a rise in temperature and its sustenance.
- Doldrum conditions. Weather patterns typified by clear skies and slight or still wind or waves result in little or no movement and mixing of warm and cold waters. Under these conditions, solar radiation warms surface and shallow waters (particularly during the summer when the sun is overhead).
- Current transport. Ocean currents transport “pools” of warm water, sometimes delivering them to reef regions. These warm waters can linger for months before moving on or dissipating.
- Increased exposure to ultraviolet (UV) radiation;
- Large amounts of storm water from heavy rains flooding the reef;
- The exposure of coral to certain chemicals or diseases, such as,
- Introduciton of new pathogens and thus new diseases, e.g., white band disease, black band disease, coral plague sediments such as sand or dirt covering the coral, called as euzootics; and
- Chemical contaminants, such as, copper, herbicides, and oil, called as Xenobiotics.
- Excess nutrients, such as, ammonia and nitrate from fertilisers and household products entering the reef ecosystem. (The nutrients might increase the number of zooxanthellae in the coral, but it is possible that the nutrient overload increases the susceptibility of coral to diseases).
- Sedimentation by rivers, which causes opaqueness in corals, clogs their mouth and kills them.