Southern Ocean: Snail shells dissolve due to ocean acidification

December 06, 2015

Antarctica (Bernd F. Laeschke – 26.11.2012): Antarctica’s marine snails are one of the first known victims of ocean acidification. The shells of the tiny marine animals known as pteropods are being dissolved, researchers from British Antarctic Survey (BAS) and the University of East Anglia (UEA), in collaboration with the Woods Hole Oceanographic Institution and the National Oceanic and Atmospheric Administration (NOAA), discovered. The snails living in the Southern Ocean waters are a valuable food source for fish and birds and play an important role in the oceanic carbon cycle.

“We know that the seawater becomes more corrosive to aragonite shells below a certain depth - called the ‘saturation horizon’ - which occurs at around 1000 meter depth,” says Dr. Nina Bednaršek, a NOAA scientist and lead author of the study. “However, at one of our sampling sites, we discovered that this point was reached at 200 meter depth, through a combination of natural upwelling and ocean acidification. Marine snails - pteropods - live in this top layer of the ocean. 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.“

The team examined an area of upwelling where winds cause cold water to be pushed upwards from the deep to the surface of the ocean. Upwelled water is usually more corrosive to a particular type of calcium carbonate (aragonite) that pteropods use to build their shells. The team found that as a result of the additional influence of ocean acidification, this corrosive water severely dissolved the shells of pteropods.

Ocean acidification is caused by the uptake of carbon dioxide from the atmosphere emitted as a result of fossil fuel burning. 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.

“Although the upwelling sites are natural phenomena that occur throughout the Southern Ocean, instances where they bring the ‘saturation horizon’ above 200 meter will become more frequent as ocean acidification intensifies in the coming years,” says Dr. Geraint Tarling from BAS, co-author and science cruise leader. “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. 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.”

Polar Regions are where ocean acidification effects are most likely to be seen first, since their colder temperatures mean a large susceptibility of the aragonite saturation to the absorption of anthropogenic carbon dioxide.

“Climate models project a continued intensification in Southern Ocean winds throughout the 21st century if atmospheric carbon dioxide continues to increase,” according to Dr. Dorothee Bakker, a scientists from the University of East Anglia and co-author of the study, says. “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.”