Coral bleaching: Lost at sea?

Coral reefs support a quarter of the world's marine life, but rising ocean temperatures are killing them. The impact of their decline could be huge, says marine biologist Olivia Durkin
The Independent 29 Jun 10;

A a result of rising sea temperatures, we are seeing the degradation and eventual destruction of one of the most beautiful ecosystems on Earth. Corals around the world are succumbing to yet another mass "bleaching event"; reefs that were once a rich mosaic of colours are now shockingly white as corals fade and die.

Corals are in fact a combination of animal, algae and "rock". Colonies are made up of many identical individuals called polyps that secrete a stony skeleton. Polyps contain microscopic algae called zooxanthellae living within the coral animal tissue; the relationship is mutually beneficial, or symbiotic. Zooxanthellae use sunlight to provide energy and nutrients for the coral through photosynthesis, in return they are provided with shelter. In a reef, each colony acts as a building block, pieced together to form intricate structures that provide habitats for an abundance of reef fish and many other creatures.

Bleaching is the ultimate stress reaction, when environmental conditions decline to a point where they cannot sustain the coral-algae relationship. Zooxanthellae, which are responsible for the magnificent colour of corals, are expelled from the coral, leaving the transparent tissue on a stark white skeleton. Although still alive, bleached corals must work harder without zooxanthellae to obtain the energy they need for growth and survival. If periods of stress are prolonged, the corals will die. Other reef residents such as giant clams, sea anemones and soft corals, which also contain zooxanthellae, are sharing the same fate.

Widespread bleaching tends to be attributed to abnormally high sea temperatures in addition to high levels of light. This happens in times of prolonged calm weather and crystal-clear water. While this may sound like the picture-postcard perception of a typical coral reef environment, it is in fact turbulent waters that keep coral healthy and scientists worry that the current event has the potential to be the worst ever.

Sea temperatures are at an all-time high. Major bleaching incidents are increasingly prevalent. The US National Oceanographic and Atmospheric Administration (NOAA) revealed that 2010 has been the hottest year in recorded history. Prior to this, 1998 was the hottest for 130 years, leading to unprecedented intense bleaching and coral mortality worldwide which wiped out more than 90 per cent of shallow water corals in the Indian Ocean. NOAA's Coral Reef Watch monitors and predicts bleaching events using HotSpots, a measure which highlights areas where sea surface temperatures rise above levels that can lead to bleaching.

As predicted by NOAA, bleaching began this February in Mauritius and it has progressed throughout the Indian Ocean and South East Asia, including reefs off Thailand, Cambodia, Malaysia and Philippines. Florida and the Caribbean are next, with strong warming of the surrounding sea and severe bleaching expected for the coming months.

Even the world's most southerly coral reef at Lord Howe Island has suffered its worst bleaching event. Lying off eastern Australia, the island is well known for its pristine and beautiful environment. For true coral reef formation, relatively stable conditions are required – and Lord Howe Island is at the threshold of coral reef tolerance. Thermal-induced bleaching at the southern limits of coral reef formation is a cause for considerable concern; evidence that dramatic sea warming has spread to the subtropical regions as well as the tropics. The distribution of coral reefs is expected to shift dramatically with increasing sea temperatures. Elsewhere in Australia, the Great Barrier Reef has escaped severe bleaching. Cyclone Ului, which hit the Queensland coast in March, may have been its saving grace. Maldivian reefs were also spared severe bleaching with the arrival of monsoon weather in early May, quickly dissipating the warmer-than-average conditions. Unfortunately, other reefs were not so lucky and delayed monsoon winds and rain led to a prolonged period of calm weather and elevated sea temperatures. Chad Scott, from the Save Koh Tao conservation group, recalls the situation in the Gulf of Thailand this year: "We experienced some very hot weather and there was no rain for three months. As a consequence, water temperatures peaked at 34oC." Intense bleaching events like these can ruin reefs. Dr Clive Wilkinson, editor of the Status of Coral Reefs of the World reports, states that "approximately 16 per cent of the world's reefs were effectively destroyed in 1998".

This year, there are fears of equivalent losses. "In Thailand, the bleaching is reported to be the most severe in 20 years of monitoring. In 1998, the reefs of the Andaman Sea were essentially unaffected, this year these once outstanding reefs are up to 95 per cent bleached," says Dr James True from Prince of Songkhla University, Thailand, who witnessed the 1998 and 2010 bleaching events.

The situation in Cambodia is equally serious. In Cambodia, a UK based conservation organisation is monitoring the impacts of the bleaching event on local reefs. Coral Cay Conservation (CCC) uses teams of volunteers to collect crucial information about the status of coral reefs around the world. This information is used to develop management recommendations for the host governments. Jan-Willem van Bochove, head marine scientist for CCC, has growing concerns for the future of coral reefs.

The bleaching we are currently witnessing is severe and widespread; van Bochove says, "not many people in the UK are aware of the coral reef crisis currently unfolding in South East Asia. CCC provides the opportunity for volunteers to be involved in reef conservation, and helps create awareness of the importance of coral reefs for a healthy planet."

Although bleached coral colonies can recover, it can take a while. It could be many weeks or months before severely bleached corals regain their colour, through the uptake of new zooxanthellae. More importantly, it could be years before recovering corals reproduce again. So, it may be more than a decade, if at all, before a severely bleached reef recovers.

In fact, many reefs in the Indian Ocean and Western Pacific impacted by the 1998 bleaching had shown promising signs of recovery, with the presence of young colonies and successful reproduction. However, recovering reefs have been set back by repeated bouts of coral bleaching.

With sea temperatures increasing in the future, bleaching events are predicted to become more frequent. Continued events will lead to a dramatic change in reef appearance; small, unattractive, boulder-like corals will replace the wide expanses of branching "sensitive" corals that dominate the typical perception of a beautiful reef. Coral reefs are the rainforests of the sea. Despite the fact that reefs only occupy around 1 per cent of the world ocean surface – an area half the size of France or 30 million football pitches – they support over a quarter of the world's marine life. Reefs dominated by slow growing "resistant" species provide a poor habitat for marine life, leading to fewer fish, a decline in biodiversity and extinction of many species.

Not only that. Coral reefs provide food, coastal protection and support fishing and tourism industries. Losing these charismatic ecosystems, and the goods they provide will have worrying consequences for at least 500 million people worldwide, especially the 150 million people who live within the Coral Triangle and, as outlined by several researchers, the 30 million people worldwide directly dependent on coral reefs for food and livelihood.

Over the next 30 years, coral reefs are expected to become highly susceptible to frequent bleaching events, and we will lose the functionality of our reefs. Recurrent bleaching, along with ocean acidification, is likely to be the greatest threat.

So whether the current bleaching turns out to be the worst on record or not, it is certainly a wake-up call. The fact is the sea is getting warmer and frequent bleaching events are a grave consequence of this. Humankind must do all it can to limit continuing warming of the sea or damage to the reefs themselves. We must reduce our carbon emissions significantly, improve water quality, promote the effective coral reef management and enhance reef recovery through reef restoration projects. The repercussions of repeated mass bleaching are dire. Not only for marine life but ultimately for us all.

Olivia Durkin is currently co-ordinating the Coral@CBiPT bleaching project at the Centre of Biodiversity of peninsular Thailand. Find out more about bleaching at Coralreefwatch.noaa.gov