Antarctic marine wildlife is under threat, study finds

The Southern Ocean The research took place in the Southern Ocean

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Marine snails in seas around Antarctica are being affected by ocean acidification, scientists have found.

An international team of researchers found that the snails' shells are being corroded.

Experts says the findings are significant for predicting the future impact of ocean acidification on marine life.

The results of the study are published in the journal Nature Geoscience.

The marine snails, called "pteropods", are an important link in the oceanic food chain as well as a good indicator of ecosystem health.

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"They are a major grazer of phytoplankton and... a key prey item of a number of higher predators - larger plankton, fish, seabirds, whales," said Dr Geraint Tarling, Head of Ocean Ecosystems at the British Antarctic Survey (BAS) and co-author of the report.

The study was a combined project involving researchers from the BAS, the National Oceanic and Atmospheric Administration (NOAA), the US Woods Hole Oceanographic Institution and the University of East Anglia's school of Environmental Sciences.

Ocean acidification is a result of burning fossil fuels: some of the additional carbon dioxide in the atmosphere is absorbed into oceans.

This process alters the chemistry of the water, making it more acidic.

Marine snails Pteropods are an important food source for fish and birds

During a research cruise in the Southern Ocean in 2008, scientists assessed the corrosive effects of upwelled water on pteropod shells.

Upwelling occurs when winds push cold layers of deeper seawater from around 1,000m towards the surface layers.

Seawater from these depths is more corrosive to aragonite, the type of calcium carbonate that forms pteropod shells. The point at which this occurs is known as the "saturation horizon".

"Carbonates in shells dissolve more when temperatures are cold and pressure is high, which are the characteristic properties of the deep ocean," Dr Tarling explained.

Scientists found that the combined effect of increased ocean acidity and natural upwelling meant that in some areas of the Southern Ocean the saturation horizon was around just 200m - the upper layer of the ocean where pteropods live.

Dr Tarling explained the significance of these findings: "The snails do not necessarily die as a result of their shells dissolving, however it may increase their vulnerability to predation and infection, consequently having an impact to other parts of the food web."

He said that although upwelling sites are a natural phenomenon in the Southern Ocean, "instances where they bring the saturation horizon above 200m will become more frequent as ocean acidification intensifies in the coming years".

Marine snail shell dissolution A pteropod (Limacina helicina antarctica) showing acute levels of shell dissolution
Interpreting the results

Dr Tarling said the study is "very much... a pilot study" and that it has provided an important body of work regarding "how pteropods will respond to future oceanic conditions".

To date there have been a number of laboratory studies predicting the effects of ocean acidification on marine organisms, but none assessing the impacts on live specimens in their natural environment.

"It took us several years even to develop a technique sensitive enough to look at the exterior of the shells under high-power scanning electron microscopes, since the shells are very thin and the dissolution pattern, subtle," commented Dr Tarling.

He went on: "We are now undertaking a much more comprehensive programme completely focussed on the effects of ocean acidification, not just on pteropods but to a wider range of organisms."

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