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Why are the coral reefs we see mostly white?

Coral reefs around the world are turning white in record numbers, a phenomenon scientists call coral bleaching,and researchers warn it is becoming more frequent, more widespread, and more severe as ocean temperatures rise.

The change is pretty prominent as reefs that once displayed brilliant oranges, purples, and blues are now being reduced to pale, skeletal versions of themselves. According to the Australian Institute of Marine Science (AIMS), such shift is driven largely by warming seas linked to climate change, and it now threatens reef ecosystems on a scale not seen in the historical record.

Zooxanthellae

The cause traces back to a relationship most people never see: a partnership between coral animals and microscopic algae called zooxanthellae that live inside their tissue. The algae receive shelter and sunlight from the coral. As a response to that they produce food through photosynthesis, supplying more than 90% of a coral's nutritional needs and giving reefs their signature color.

That partnership breaks down under stress, as when water temperatures rise too high for too long, the algae become harmful to their coral hosts, and the coral expels them. Once the algae are gone, the coral's tissue turns transparent, exposing the white skeleton beneath which is the process known as bleaching.

Bleached coral is not necessarily dead coral, according to AIMS. It is, however,  in a very weakened and vulnerable state. Whether it survives depends on how long the stressful conditions could last.

Why it's getting worse

Researches point to heat stress as the leading cause of bleaching. Corals live within a narrow temperautre range, and a rise of just 1 to 2 degrees Celsius above the normal summer maximum, sustained for several weeks would be enough for trigggering a bleaching response to the corals. Other factors such as including pollution, reduced salnitity from storm runoff, and unucual light exposure could also contribute even when warming ocean temperatures would remain as the dominant cause of large scale events.

Mass bleaching , which coral dies off across wide regions simultaneously is a relatively recent development as the first documented case ever occured in 1983. Prior to the late 1990s bleaching was largely a localized evemt. Since then, the incidents have grown more and more common. The Great Barrier Reef alone recorded four separate mass bleaching event between 2016 and 2022 (AIMS).

Impacts

Coral bleaching is a big problem for the ecosystem as fish populations tend to decline after bleaching events, as the reefs provide structural habitat that supports much of the Ocean's marine life for not only feeding but shelter.

The harm does not just finish within the reef, as the impacts reach coastal communities as well. Reefs support fish industries, that would impact them.