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What is Coral Bleaching?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

Coral bleaching is a phenomenon in which corals lose their distinctive color because they expel the one-celled organisms which live with them in a symbiotic relationship. If a coral is fully bleached, it will turn entirely white, and it will die unless the stress on the coral is reduced, allowing the symbiotic organisms to return to the coral. Worldwide, coral bleaching began to increase at a rate which some scientists viewed as alarming in the 1990s, with researchers fearing that massive losses in the world's coral population could be highly likely.

Research on coral bleaching has shown that it is caused by stress to coral. Coral is in fact a very delicate organism, adapted to survive in nutrient-poor waters, but it does not tolerate environmental changes well. A healthy coral reef can sustain a wide assortment of organisms, promoting biodiversity, and if the corals die off, the creatures which surround the reef may die off as well. Therefore, coral bleaching isn't just a threat to coral, but to marine health in general in some regions of the world.

Healthy coral should display a bright array of colors.
Healthy coral should display a bright array of colors.

A number of things can stress corals. Rising water temperatures and increased ultraviolet radiation are suspected to be two major stressors for corals. In addition, coral can be disturbed by nutrient runoff from the land, heavy sedimentation, pollution, increasing salinity, acidification of the surrounding water, and infection with various organisms. If the coral bleaching is recognized early and people act quickly, it can sometimes be arrested, allowing the coral to recover, but in other cases biologists are powerless to help the coral.

Climate change is putting pressure on the Great Barrier Reef, causing coral bleaching.
Climate change is putting pressure on the Great Barrier Reef, causing coral bleaching.

During a period of bleaching, coral is still alive. However, as it loses its one-celled buddies, it can become unhealthy. Corals rely on zooxanthellae algae to photosynthesize, providing energy from the sun which can be utilized by the coral. When these algae are expelled for a prolonged period of time, a coral can start to die. It is common for bleached reefs to start to break apart, which can lead to problems in the future, even if the bleaching is stopped and the reef is coaxed into recovering. The more damage there is, the longer it will take for the reef to be restored.

Some researchers believe that coral bleaching may actually be an adaptation on the part of the coral to cope with environmental changes. By expelling the zooxanthellae algae, the coral can make room for algae which might be better adapted to the changed environment, potentially allowing the coral to survive. Historical evidence from the geological record seems to support this idea, but biologists have sounded a note of caution; while coral may be able to cope with rising temperatures on its own, it cannot handle pollution and changes to the chemical composition of the water.

Frequently Asked Questions

What is coral bleaching and why does it occur?

Coral bleaching is a phenomenon where corals lose their vibrant colors, turning white or very pale. This occurs when corals are stressed, typically due to elevated water temperatures, which cause them to expel the symbiotic algae (zooxanthellae) living in their tissues. These algae provide corals with food through photosynthesis and are responsible for their coloration. Without them, the coral's white skeleton becomes visible.

How does climate change affect coral bleaching?

Climate change is a major driver of coral bleaching events. As greenhouse gas emissions raise global temperatures, sea temperatures also increase. According to the National Oceanic and Atmospheric Administration (NOAA), the frequency of mass bleaching events has increased, with severe bleaching now occurring about every six years, compared to every 25-30 years in the 1980s. Warmer waters stress corals, leading to more frequent and severe bleaching.

Can bleached corals recover, and if so, how?

Bleached corals can recover if the stressor, particularly high water temperatures, is removed in time and if they are otherwise healthy. The corals can re-establish their symbiotic relationship with algae. However, recovery can take from several years to decades, and repeated bleaching events can weaken corals, making recovery more difficult and leading to increased mortality rates.

What are the consequences of coral bleaching for marine ecosystems?

Coral bleaching has far-reaching consequences for marine ecosystems. Corals are foundational species in reef ecosystems, providing habitat and food for a diverse range of marine life. Bleaching can lead to reduced biodiversity, as fish and other organisms lose their homes and food sources. This can disrupt fisheries and local economies, impacting food security and livelihoods for millions of people worldwide.

What can be done to prevent coral bleaching?

Preventing coral bleaching involves addressing climate change by reducing greenhouse gas emissions globally. Locally, efforts include reducing pollution, managing fisheries, and protecting reefs from physical damage. Marine protected areas can also help by reducing local stressors, giving corals a better chance to withstand the stress of rising sea temperatures and recover from bleaching events.

Are there any coral species more resistant to bleaching?

Some coral species are more resistant to bleaching than others. For instance, massive corals like boulder corals tend to be more resilient than branching corals like staghorn corals. Scientists are researching the genetic and environmental factors that contribute to this resilience, with the hope of supporting reef conservation efforts by identifying and protecting these more robust species.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

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    • Healthy coral should display a bright array of colors.
      By: pirosa
      Healthy coral should display a bright array of colors.
    • Climate change is putting pressure on the Great Barrier Reef, causing coral bleaching.
      By: StrangerView
      Climate change is putting pressure on the Great Barrier Reef, causing coral bleaching.