Coral: life's a bleach... and then you die

Despite what you may read or see in the mainstream media, out in the real world, massive and rapid changes are taking place in many ecological systems as a result of global warming. The Earth seems to be already convinced of global warming and is responding quickly.

Perhaps the most significant, and likely most enduring, are the shifts taking place in the Earth's oceans. Whilst many readers may have read or heard about Ocean Acidification, there are numerous other changes taking place in the oceans which should be equally as concerning. One such phenomena to appear in the last few decades is mass coral bleaching, a consequence of the continued warming of the oceans. Once vast stretches of colourful reefs teeming with marine life are being reduced to lifeless rubble covered in seaweed or slime. Many areas are not recovering, and the scale and frequency of bleaching worldwide is getting worse. In fact, early reports suggest 2010 may have witnessed the largest single bleaching event ever recorded.

The lowdown on coral bleaching
Reef-coral are actually a symbiosis (a mutually beneficial relationship) between the coral polyp, an anemone-like creature, and tiny algae called zooxanthellae. The coral provide shelter and nutrients for the algae , and in exchange the algae provide carbohydrates (food) to the polyp, using energy from the sun (photosynthesis) and the nutrients provided by the coral. These algae live in the skin tissue of the polyp and produce the coloured pigments which make coral reefs so visually spectacular. When this partnership breaks down the polyps expel the algae, which leads to the "bleached" effect. Although the polyp does feed using its tentacles to snare food, the bulk of its nutrition (90%+) comes from the algae, and they are a critical component of coral skeleton formation and therefore reef maintenance and growth. Without symbiotic algae, the coral can die from starvation, or become so weakened by a lack of food, that it succumbs to harmful bacteria (Mao-Jones 2010), and/or seaweeds which can poison and kill coral on contact.

Because reef-coral have adapted tolerance to a narrow band of environmental conditions, bleaching can occur for a number of reasons, such as ocean acidification, pollution, excess nutrients from run-off, high UV radiation levels, exposure at extremely low tides and cooling or warming of the waters in which the coral reside. Typically these events are very localized in scale and if bleaching is mild, the coral can survive long enough to re-acquire new algal partners. So bleaching in itself is not something new, but mass coral bleaching on the huge scale being observed certainly appears to be, and represents a whole new level of coral reef decline.

Ocean warming is driving mass coral bleaching

As coral reefs operate very near to their upper limit of heat tolerance (Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long. The intensity of bleaching corresponds with how high, and how long temperatures are elevated and, as one might expect, the intensity of bleaching affects the rate of survival. Small rises of 1 -2 degree C, for weeks at a time, usually induce bleaching.

This episodic ocean warming has been most pronounced worldwide during El-Nino events, when the Pacific Ocean exchanges heat to the atmosphere and surface waters. In recent years though, severe mass bleaching is happening outside of El-Nino because of the "background" ocean warming. The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this (Eakin 2010) . Evidence connecting warm surface waters and mass coral bleaching has strengthened to the extent that the National Oceanic and Atmospheric Administration (NOAA) has a coral bleaching alert system in place. This alert system accurately forecasts mass coral bleaching based on satellite data of sea surface temperatures.

Hot water + Coral = Dead coral
So how does hot water kill coral?. It requires both high water temperatures and sunlight. Oxygen is released as waste during photosynthesis and like all chemical processes this is affected by temperature, speeding up as more energy (warmth) is applied. When water temperatures rise too high the protective mechanisms to prevent heat damage, employed by the coral and the algae, are overwhelmed. The zooxanthellae algae produce high levels of oxygen waste which begin to poison the coral polyp. In acts of self-preservation the coral kick out the algae, and in doing so become susceptible to starvation, opportunistic diseases, competitive seaweeds and macroalgae (slime to you and me) . Coral can succumb to the effects of bleaching years later, and for those coral that survive, growth effectively ceases and full recovery can take anything up to a decade.

Coral resilience is futile

On a world scale coral reefs are in decline, and it makes for rather depressing reading for an avid diver like myself. Over the last 30-40 years 80% of coral in the Caribbean have been destroyed (Gardner 2003) and 50% in Indonesia and the Pacific (Bruno & Selig 2007). Bleaching associated with the 1982 -1983 El-Nino killed over 95% of coral in the Galapagos Islands (Glynn 1990), and the 1997-1998 El-Nino alone wiped out 16% of all coral on the planet. Globally about 1% of coral is dying out each year. Not all of this continual decline is solely down to bleaching of course, pollution and other human activities are also contributing, but bleaching is speeding up the loss of coral.

cont.