The world's oceans at risk from rising acidity
source : Steve Connor The Independent
A significant increase in the acidity of the Pacific Ocean has been detected by scientists, who believe it could upset the delicate balance of marine ecosystems and lead to their collapse.
Rising ocean acidity is one of the results of increasing levels of carbon dioxide in the atmosphere and scientists are concerned that the phenomenon could make it impossible for key species in the marine food chain to make their protective shells.
The scientists, led by Richard Feely of the US National Oceanic and Atmospheric Administration (NOAA), surveyed 13 different locations off the west coast of North America, from Canada to Mexico, and found higher-than-expected levels of acidityin seawater samples taken near the surface.
"Our results show for the first time that a large section of the North American continental shelf is impacted by ocean acidification," the scientists said in their study, published online in the journal Science.
The survey took place in May and June last year using a research ship that collected seawater samples as it sailed across the North American continental shelf from Queen Charlotte Sound in Canada to San Gregorio Baja California Sur in Mexico. The scientists said that other regions of the world may also be affected to a similar extent, because rising levels of man-made carbon dioxide in the air cause ocean acidity to increase wherever the gas dissolves in seawater to form carbonic acid.
Over the past 250 years, since the start of the industrial revolution, the amount of CO2 in the atmosphere has increased by about 100 parts per million and about a third of this extra carbon dioxide in the air is estimated to have dissolved in the oceans, where it has helped to raise acidity levels.
"The atmospheric concentration of CO2 is now higher than it has been for at least the past 650,000 years, and is expected to continue to rise at an increasing rate, leading to significant changes in our climate by the end of the century," the scientists said.
Each day the oceans absorb about 30 million tonnes of carbon dioxide from the atmosphere. This increasing concentration of carbonic acid has changed the chemistry and the biology of the oceans to an extent that can now be detected by scientists.
"This phenomenon, which is commonly called 'ocean acidification', could affect some of the most fundamental biological and geochemical processes of the sea in the coming decades, and could seriously alter the fundamental structure of pelagic [free- swimming] and benthic [seafloor] ecosystems," the scientists said.
Dr Feely said many species of fish that are not directly affected by rising acidity levels would nevertheless still be indirectly affected by the inability of key marine organisms to make their protective shells. "The best example, of course, is coral reefs. They support 25 per cent of the entire biodiversity of the world's oceans. If they should be lost, then many of the fish species that depend on them would also be lost," Dr Feely said.
"Coral reefs also provide food resources for 500 million people throughout the world. Deep- water corals have been found along the US coast, from California to Alaska," he said.
When carbon dioxide dissolves in seawater the increased acidity reduces the availability of dissolved substances called carbonate ions, which are necessary for the formation of the calcium carbonate skeletons and shells of marine organisms.
In addition to corals and shellfish, microscopic marine plankton also form shells that eventually sink to the sea floor to become beds of limestone. Some of these organisms, called coccolithophores, have been found to increase their shell-making capacity in response to rising levels of carbon dioxide, but Dr Feely said that this may also alter the balance of the marine habitat.
"With some species of coccolithophores increasing in abundance at the expense of others, the ocean ecosystem might shift towards species that carry more carbon away from the surface, causing greater uptake of carbon into the oceans," Dr Feely said.
How greenhouse gas emissions are killing the oceans
Many of the most important marine organisms, from tiny plankton called coccolithophores to coral reefs and the more familiar shellfish species, can only exist by building protective shells of calcium carbonate. This process of 'marine calcification' cannot occur if the oceans become too acidic. The sea is naturally alkaline, but rising levels of carbon dioxide in the air are causing the oceans to become more acidic. The chemistry behind the process of shell-making relies on a complex series of chemical equations kept in a state of equilibrium. More acidity means that the delicate chemical balance becomes unbalanced. Scientists have estimated that some 118 billion tonnes of carbon released into the air as carbon dioxide between 1800 and 1994 have been taken up by the oceans. Indeed, about a third of the carbon dioxide produced by human activities since the start of the Industrial Revolution has been absorbed by the seas. So without the capacity of the oceans to act as a natural carbon sink, the concentration of carbon dioxide in the air today – about 380 parts per million – would be significantly higher. Marine calcification produces carbon dioxide in the short term, but in the long term it takes carbon out of the atmosphere.