Rising temperatures are causing mass coral bleaching concerns

Photo credit: NOAA

Photo credit: NOAA

Abnormally warm ocean temperatures are creating conditions that threaten to kill coral across the equatorial Pacific, north Pacific and western Atlantic by causing a phenomenon called bleaching.

Corals have a symbiotic relationship with unicellular algae called zooxanthellae, which are responsible for the colour seen in corals. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis (carbon dioxide and water), which are the by-products of cellular respiration. In response, the zooxanthellae produce organic products, predominately carbohydrates and act as a major energy source. These compounds are used by the coral to create their calcium carbonate skeletons.

Coral bleaching occurs when corals are stressed due to changes in environmental conditions including temperature, light and nutrients. When stressed, the coral polyps expel the zooxanthellae, leaving behind the white calcareous skeleton of the coral.

Since algae are a coral’s major food source, most corals struggle to survive without their zooxanthellae and so begin to starve after bleaching commences, which in the long run causes decreased coral growth, reproduction and eventually may even result in death. A reduction in coral reef area would not only mean there would be less shoreline protection, but also fewer habitats for the thousands of fish that depend on the reefs. The total area of the world’s coral reefs is less than 1% of the entire marine environment yet they are home to 25% of known marine species so any destruction would be catastrophic.

The good news is that corals are capable of recovering from mild bleaching events. If conditions return to normal relatively fast, corals can regain their zooxanthellae and survive. However, under extreme warming, the corals will die and so new colonies will have to grow. The problem is that the recovery scale is more in line with decades rather than a few years.

As ocean temperatures continue to rise due to climate change, bleaching events are expected to become more common. The National Oceanic and Atmospheric Administration (NOAA) has predicted a mass bleaching of corals throughout October, specifically in the north Pacific, equatorial Pacific and western Atlantic oceans, because of the rising temperatures.

The massive bleaching event could well continue into 2016. A total of 6% of the world’s remaining coral reefs are forecasted to be lost, covering a combined area of 15,000 square kilometres. However, it is important to note that reefs are also vulnerable to increasing acidity as well as temperature changes and many other threats so the overall a great area may be effected.

Global bleaching events have only been recorded twice, once in 1998 and once in 2010. In 1998 a huge El Niño resulted in 16-19% of the world’s coral reefs dying. The recent anticipated mass bleaching event does have the El Niño to partly blame, but it is largely driven by global warming as well.

Last year, Hawaii experienced widespread coral bleaching for the first time since 1996. Scientists are worried that bleaching will expand to the western Atlantic and again into Hawaii, which could be tragic for the corals that have not yet recovered from last year’s stress. The Florida Keys also saw a devastating blow to coral nurseries in 2014 and so scientists are equally worried that there won’t be enough time for them to recover before bleaching strikes again. Likewise, the Caribbean, which has endured repeated coral bleaching over the past 30 years, may also face consecutive bleaching events.

The NOAA’s bleaching prediction supports a paper published last week in the journal Science that examined how ecosystems react to warming from climate change under two different carbon dioxide emission pathways. The paper states that if carbon dioxide emissions are substantially reduced to limit the Earth’s warming to 2 degrees Celsius, it will be still be inadequate. Coral reefs and many other ecosystems will still suffer. A rapid curbing of carbon emissions is required to stop the temperatures rising more than 2 degrees if the resilience of coral reefs have any hope.

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Lucy Grable

Lucy Grable

MSc Species Identification and Survey Skills student at Reading Uni | BSc Marine Zoology | Website Editor MARINElife | Zanzibar humpback whale researcher|Marine wildlife enthusiast

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