Ocean acidification dissolving shell of planktonic Southern Ocean snails

November 25th, 2012 by British Antarctic Survey

A paper “Extensive dissolution of live pteropods in the Southern Ocean” published online in Nature Geoscience suggests that the effects of ocean acidification on marine ecosystems are beginning to emerge.

Dr Geraint Tarling, of the British Antarctic Survey, and colleagues, examined shells collected from a pteropodLimacina helicina antarctica, (a planktonic marine snail) in the surface waters of the Southern Ocean, which circles Antarctica.

This species mainly inhabits the top 200 metres of oceanic waters where they graze on phytoplankton and detritus.

Shells of these molluscs, which are a valuable food source for fish and birds and play an important role in the oceanic carbon cycle, showed significant signs of dissolution in the more acidic regions of the ocean.

The researchers attribute the dissolution to the mixing of deep carbon-dioxide-rich waters with surface waters affected by atmospheric carbon dioxide, which has been increasing in the atmosphere as a result of the burning of fossil fuels.

A number of laboratory experiments have demonstrated the potential effect of ocean acidification on marine organisms.

However, to date, there has been little evidence of such impacts occurring to live specimens in their natural environment.

The finding supports predictions that the impact of ocean acidification on marine ecosystems and food webs may be significant.

Dr Nina Bednaršek, of the National Oceanic and Atmospheric Administration (NOAA), said “we know that the seawater becomes more corrosive to aragonite shells below a certain depth — called the ‘saturation horizon’ — which occurs at around 1000 metres depth.”

“However, at one of our sampling sites, we discovered that this point was reached at 200 metres depth, through a combination of natural deepwater upwelling and ocean acidification.”

This combination reduced concentrations of aragonite in Southern Ocean surface waters where pteropods live to below saturation levels. At this point, the aragonite shells start to dissolve.

“The corrosive properties of the water caused shells of live animals to be severely dissolved and this demonstrates how vulnerable pteropods are. Ocean acidification, resulting from the addition of human-induced carbon dioxide, contributed to this dissolution,” Dr Bednaršek said.

Dr Geraint Tarling said “although the upwelling sites are natural phenomena that occur throughout the Southern Ocean, instances where they bring the ‘saturation horizon’ above 200 metres will become more frequent as ocean acidification intensifies in the coming years.”

“As one of only a few oceanic creatures that build their shells out of aragonite in the polar regions, pteropods are an important food source for fish and birds as well as a good indicator of ecosystem health,” Dr Tarling said.

“The tiny 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.

Dr Dorothee Bakker from the University of East Anglia, said “climate models project a continued intensification in Southern Ocean winds throughout the 21st century if atmospheric carbon dioxide continues to increase.”

“In turn, this will increase wind-driven upwelling and potentially make instances of deep water — which is under-saturated in aragonite – penetrating into the upper ocean more frequent.”

“Current predictions are for the ‘saturation horizon’ for aragonite to reach the upper surface layers of the Southern Ocean by 2050 in winter and by 2100 year round,” she said.

This research was funded by the UK’s Natural Environment Research Council (NERC) and the European Union Marie Curie Early Stage Training Network.

In an accompanying journal article “A sea butterfly flaps its wings,” Dr Justin Ries suggests that the corrosion documented “may be a harbinger of what is in store for surface waters throughout much of the Southern Ocean”.


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