Oceans turning acidic decades earlier

Greenhouse gases are turning the oceans acidic enough to dissolve the shells of sea creatures decades earlier than scientists had expected, with potentially catastrophic consequences for marine life.

Distribution of the observed surface seawater temperture during our cruise along the Pacific continental margin in May-June 2007.

Many marine organisms produce calcium carbonate (chalk) shells but, when the acidity of the water is increased, a point is reached at which that calcium carbonate starts to dissolve.

Today an American team publishes evidence that this acidic “tipping point” has been reached on the continental shelf along the west coast of North America, where many delicate organisms live.

The work underlines rising concerns that man made emissions will affect the world’s oceans, through acidification, in a much more direct way than climate change.

“This is potentially very bad news,” commented Mr Paul Halloran of Oxford University, a British expert. “The impact on tourism and fisheries may have huge economic consequences.”

He was commenting on the discovery by a team led by Dr Richard Feely of the US Government’s National Oceanic and Atmospheric Administration in Seattle reports that waters which a century ago were not corrosive have now become able to dissolve shells.
Marine scientists have known for years that water below a certain depth corrodes shells as a result of acid produced by ‘rotting’ organic matter that floats from the surface to deep waters.

But scientists are alarmed to see that the minimum depth at which water becomes corrosive is now shallow enough to be washed up onto the continental shelf – where many vulnerable organisms live.

The rise in acidity could cut the availability of the constituents (carbonate ions) which coral reefs, clams, molluscs and some plankton use to produce their hard skeletons of calcium carbonate.

Increased acidity may also directly affect the growth and reproduction rates of fish, as well as affecting the plankton populations which they rely on for food, and have potentially disastrous consequences for marine food webs.

In the journal Science, Dr Feely’s team from America, Canada and Mexico reports how they collected data from 13 survey lines made from a ship at depths from 0-3500 m along routes stretching from the waters of central Canada to northern Mexico.

They found greater acidity – as shown by lower pH levels – in seawater closer to the surface than researchers had predicted. The pH ranged from 8.1 to as low as 7.6, when they expected the figures to be no lower than 8.0.

In particular, they note that waters are more acidic at greater depths and that these acidic waters are being moved up to the surface by seasonal “upwelling” currents.

“Our results show for the first time that a large section of the North American continental shelf is impacted by ocean acidification. Other continental shelf regions may also be impacted where anthropogenic (man-made) carbon dioxide-enriched water is being upwelled onto the shelf,” says Dr Feely.

The team says that, as a matter of urgency, more information is now required to assess the scope of potential damage to fragile marine ecosystems. Scientists fear that some sea life will be unlikely to be able to adapt as the acidity of the sea has not changed so rapidly in recent history.

The atmospheric concentration of carbon dioxide is now higher than it has been for at least the last 650,000 years, and is expected to continue to rise at an increasing rate. By one estimate, the release of carbon dioxide during the next several centuries will increase ocean acidity more rapidly than during the past 20 million years.

Estimates based on the Intergovernmental Panel on Climate Change emission scenarios indicate that atmospheric carbon dioxide levels could exceed 500 parts per million (ppm) by the middle of this century, and 800 ppm near the end of the century.

This increase would result in a surface water pH decrease of approximately 0.4 pH units, enough to start dissolving chalk. This would be an increase of more than 200% of the hydrogen ion concentration (responsible for acidity, and represented by pH) compared with the pre-industrial.

There are also fears that acidification could amplify global warming. The ability of plankton to absorb carbon from the sea and take it to the bottom when they die may be reduced.

Another vitally important carbon “sink” could be turned off, and the rate of global warming will increase. However, the effects of carbon dioxide on marine life will be complex.

One recent study by Dr Halloran, with Dr Debora Iglesias-Rodriguez of the University of Southampton and colleagues concluded that increased carbon dioxide is good news for microscopic ocean plants (coccolithophores) which produce greater amounts of calcium carbonate, not less as many had predicted.

But, Mr Halloran adds: “This can occur because many biological processes benefit from the increased carbon availability, but this is of no use if, once the organisms have produced their shells, the water surrounding them causes that shell to dissolve.”

“Although we think we understand the chemistry of the oceans relatively well, until we understand how that chemistry is moved about by currents within the ocean, there could be many unexpected consequences,” says Mr Halloran.

“It is important that scientists discover which areas of the ocean are most sensitive to the input of man-made carbon dioxide, and then focus on how the organisms living in those locations will be effected.”

Dr Iglesias-Rodriguez adds: “The results by Feely et al highlight the urgent need to understand the effect that these ‘corrosive’ undersaturated waters will have on marine animals and plants, particularly those whose survival depends upon the making of shells and skeletons of calcium carbonate.

“This may be particularly important not only in corals, and marine microscopic animals and plants, but also on larvae of organisms that produce skeletons at early stages of development.

“Direct observations in these ‘corrosive’ waters will provide key information as to how calcifying marine organisms respond to ocean acidification.”

Telegraph.co.uk, 22 May 2008. Article.

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