Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation. The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like unicellular flagellate protozoa that are photosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Under stress, corals may expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached".
Bleaching occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained. Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion. This process is a "downward spiral", whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae.
Physiologically the lipid composition of the symbiont thylakoid membrane affects their structural integrity when there is a change in temperature, which combined with increased nitric acid results in damage to Photosystem 2. As a result of accumulated oxidative stress and the damage to the thylakoid of chloroplasts there is an increase in degradation of the symbiosis and the symbionts will eventually abandon their host. Not only does the change in temperature in the water increase the chances of bleaching, but there are other factors that play a role. Other factors include an increase in solar radiation (UV and visible light), regional weather conditions, and for intertidal corals, exposure to cold winds.
Coral bleaching is a generalized stress response of corals and can be caused by a number of biotic and abiotic factors, including:
- increased (most commonly), or reduced water temperatures
- oxygen starvation caused by an increase in zooplankton levels as a result of overfishing
- changes in water chemistry
- increased sedimentation
- bacterial infections
- changes in salinity
- herbicides
- low tide and exposure
- elevated sea levels due to global warming (Watson)
- mineral dust from African dust storms caused by drought
four common sunscreen ingredients, that are nonbiodegradable, and can wash off from tourists swimming, snorkeling, or diving.
While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures. The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability.
Mass bleaching events
Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1°C higher than the summer maximum) and the incidence of coral bleaching.
Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence bleaching.
Great Barrier Reef
The Great Barrier Reef along the coast of Australia experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, and 2006. Some locations suffered severe damage, with up to 90% mortality. The most widespread and intense events occurred in the summers of 1998 and 2002, with 42% and 54% respectively of reefs bleached to some extent, and 18% strongly bleached. However coral losses on the reef between 1995 and 2009 were largely offset by growth of new corals. An overall analysis of coral loss found that coral populations on the Great Barrier Reef had declined by 50.7% from 1985 to 2012, but with only about 10% of that decline attributable to bleaching, and the remaining 90% caused about equally by tropical cyclones and by predation by crown-of-thorns starfishes.
Other areas
Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.
Evidence from extensive research in the 1970s of thermal tolerance in Hawaiian corals and of oceanic warming led researchers in 1990 to predict mass occurrences of coral bleaching throughout Hawaii. Major bleaching occurred in 1996 and in 2002.
Coral in the south Red Sea does not bleach despite summer water temperatures up to 34°C.
Healthy Coral
Bleached Corals
Economic and political impact
According to Brian Skoloff of The Christian Science Monitor, "If the reefs vanished, experts say, hunger, poverty and political instability could ensue." Since countless sea life depends on the reefs for shelter and protection from predators, the extinction of the reefs would ultimately create a domino effect that would trickle down to the many human societies that depend on those fish for food and livelihood. There has been a 44% decline over the last 20 years in the Florida Keys, and up to 80% in the Caribbean alone.
