Coral Bleaching: Predicting reef recovery

The impact of climate change on the earth’s oceans has been studied extensively over the past two decades. With an increasing amount of evidence detailing it’s negative effects on the marine environment, scientists are striving to identify ways to repair the damage that has already been done and to find solutions to prevent future damage. One of the primary impacts of climate change on our oceans, coral bleaching,  has been shown to have devastating effects on whole reef ecosystems which can be long lasting and often permanent. Bleaching occurs due to either the expulsion of the coral’s endosymbiotic algae Zooxanthellae (responsible for giving the corals their coloration) or the loss of the zooxanthellae’s photosynthetic pigments, when exposed to various forms of stress associated with climate change (e.g. increased water temperatures, acidification, oxygen starvation) leaving the corals faded and colourless.

A study published in Nature this week by an international team of researchers from Australia, the United Kingdom and France investigated the impact of climate change over 20 years on reefs in the Seychelles, from which they were able to determine the factors that dictate the recovery of bleached reefs. Through determining these factors, predicting whether future recovery of already bleached reefs could be possible.

Lead author, Dr Nicholas Graham from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia says “Water depth, the physical structure of the reef before disturbance, nutrient levels, the amount of grazing by fish and survival of juvenile corals could help predict reef recovery. Remarkably, the two most easily measured variables, water depth and the physical structure of the reef before disturbance, predicted recovery with 98% confidence”.

The data was collected both before and after a mass bleaching event that occurred in the Seychelles during the summer of 1998 as a result of excessively high sea surface temperatures, where from 50% to over 90% coral mortality was observed at different sites. Of the reefs affected by the episode, twelve recovered while nine did not, with the event having a significant impact on the biodiversity of local fish populations (this changed substantially when reefs did not recover). Similar bleaching events have occurred across the tropics. The Great Barrier Reef experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, and 2006, with coral mortality reaching 90% in some areas. Bleaching events in the Maldives, Sri Lanka, Kenya and Tanzania have also resulted in up to 90% loss of coral cover (Middleton, 2004).

Co-author Dr Aaron MacNeil from the Australian Institute of Marine Science says the insights can be applied to studies and management aimed at improving the outlook of coral reefs around the world. “By carefully managing reefs with conditions that are more likely to recover from climate-induced bleaching, we give them the best possible chance of surviving over the long term, while reduction of local pressures that damage corals and diminish water quality will help to increase the proportion of reefs that can bounce back.”


What actually is coral bleaching?

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Scleractinian corals exist as polyps that build skeletons of calcium carbonate and belong to the phylum Cnidaria (along with jellyfish, anemones etc). These coral polyps receive their nutrient and energy resources through either capturing tiny planktonic organisms with their nematocyst capped tentacles or having an obligate symbiotic relationship with a single cell algae known as zooxanthellae.

Zooxanthellae live symbiotically within the tissue of the coral polyp and assist the coral in the production of nutrients through its photosynthetic processes. These processes provide the coral polyp with fixed carbon compounds for energy, enhance calcification and mediate elemental nutrient flux. The host coral polyp in return provides its zooxanthellae with a protected environment to live within and a steady supply of carbon dioxide  to fuel its photosynthetic activities.

The coral polyp itself is translucent and acquires its coloration from pigments within it’s endosymbiotic zooxanthellae. Bleaching is a common stress response exhibited by corals in response to a variety of disturbances, whereby the zooxanthellae is expelled from the tissue of the polyp. If the disturbance is significant enough, zooxanthellae density within the polyp will reach such low levels that all coloration of the coral will be lost leading to a bleached white appearance due to the cnidarian’s calcareous skeleton showing through it’s translucent tissues. 

Image source: Great Barrier Reef Marine Park Authority

Image source: Great Barrier Reef Marine Park Authority

Such disturbances that can instigate the expulsion of zooxanthellae leading to bleaching include anomalously low and high sea temperatures, solar irradiance (exposure to sunlight), sedimentation and acidification, all of which can result from the effects of climate change. It is theorised that if global warming trends continue on shallow tropical and subtropical seas, we may expect an increase in the frequency, severity and scale of coral reef bleaching.

– JK


Journal reference:

  1. Nicholas A. J. Graham, Simon Jennings, M. Aaron MacNeil, David Mouillot, Shaun K. Wilson (2015) Predicting climate-driven regime shifts versus rebound potential in coral reefs. Nature, 2015; DOI: 10.1038/nature14140

References used in article:

  1. Middleton, N (2004) Managing the Great Barrier Reef (Geography Review, January 2004)

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