Warmer seas will stress coral


By Michele Gierck

While the need to protect vulnerable people around the globe is widely recognised, people are less aware of the need to protect the vulnerable areas of the earth itself, including its rivers and oceans.

Climate change is perhaps the greatest challenge confronting the international community. The study of oceans and reefs offers us insight into the consequences of not taking immediate action to combat this challenge.

On a recent trip to Heron Island, I visited the Heron Island Research Station (HIRS) and met with Dr Selina Ward, a scientist at the Centre for Marine Studies (CMS) at the University of Queensland, which operates the station.

Research from the CMS and the Australian Research Council Centre for Excellence for Coral Reef Studies (CoECRS) reveals that there are two main threats facing reefs around the globe: rising sea temperatures, which leads to coral bleaching, and increasing ocean acidification.

The Great Barrier Reef is estimated to contribute $5.4 billion and over 60,000 jobs to the Australian economy. Worldwide, reefs support up to 200 million people.

Dr Ward explained that the level of symbiosis between different inhabitants of the Great Barrier Reef is incredible. There is a high level of mutualism between the fish and the coral, and the HIRS has undertaken substantial research on symbiosis and climate change. Dr Ward explained:

‘The most important symbiosis that we work on as a laboratory is the symbiosis between corals and single celled organism called zooxanthellae’. The zooxanthellae, which lives inside the coral is vital to the coral because the zooxanthellae photosynthesise, that is, they convert the sun’s light into energy, and then give the corals 95 percent of that energy.

Dr Ward adds:

‘They also provide glycerol to make fats and corals can’t get by without fats, and they assist with calcification that lays down the coral skeleton. And for the zooxanthellae, they get somewhere safe to live, they are protected within the cells of the coral, and they also get the nitrogen and phosphorus from the coral excretion. It’s wonderful tight mutualism that goes on.’

However, warming sea levels, which cause coral bleaching, are threatening this relationship.

Dr Ward explains:

‘When the corals bleach [it] is due to zooxanthellae loss. If a coral undergoes some kind of stress, the photosynthetic apparatus of the zooxanthellae breaks downs and the zooxanthellae disappear from the coral tissue…Even when a coral is bleached completely white it will generally have at least 10 percent of the zooxanthellae population still there. If the stress is for a short time they can recover.’

But if the stress lasts a long time, the coral dies.

Coral bleaching has become increasingly common since the 1980s. The worst mass bleaching event was in 1998 when 16 per cent of the world’s corals died-off.

Professor Hoegh-Guldberg, director of the CMS wrote recently:

‘Warming waters temperatures are now climbing so high in warmer than normal years that corals exceed their tolerance for temperature and the symbiosis breaks down. This relationship between temperature and mass coral bleaching is so strong that satellites can predict bleaching…by simply using measurements of anomalously high sea temperatures.’

This monitoring in times of fine weather is very accurate, less so if cloud cover and storms occur, blocking UV light reaching the reef.

Professor Hoegh-Guldberg continued: ‘Coral reefs are acting for the Earth like the canaries that coalminers used to take with them down mines. When the bird fell off its perch, the miners knew that the air quality was deteriorating and they needed to leave the mine. Coral reefs are telling us that climate change is beginning to have large consequences for natural ecosystems, including rainforests.’

Oceans have absorbed over one-third of all the CO2 emitted by human activity into the atmosphere. As a result, oceans have become more acidic, and there has been a reduction in the concentration of carbonate ions which corals use to build the calcium carbonate skeletons of coral reefs.

Professor Hoegh-Guldberg, who recently provided advice to the Intergovernmental Panel on Climate Change (IPCC), says that ocean acidification is a great concern.

At the recent Australian Marine Science Association Conference held in Melbourne, Dr Ken Anthony from the CMS and CoECRS made a presentation entitled ‘Future Reefs: How Will They Survive the Climate Change Challenge’. The research he presented, which was based on the effects of thermal stress and increased atmospheric concentrations of CO2 on coral reefs, predicted that the Great Barrier Reef will undergo dramatic change in the next fifty to one hundred years, with corals, many of them now prolific, no longer able to grow, reproduce or survive.

So the key question is this; can corals be resilient enough, or evolve quickly enough to cope with such rapid change? Research at the CMS focuses on this question.

If substantial coral is lost, even in pockets of a reef, then many other reef organisms will also be threatened. The consequences for those who live in the vicinity of reefs around the world could be dire.

There is a lot at stake, and it’s not going to be confined to the Great Barrier Reef.

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