Corrosive waters in Southern Ocean destroying pteropod shells
By Bob Berwyn
FRISCO — Numerous lab experiments have already shown that some shell-forming ocean species will likely suffer as the ocean absorbs atmospheric carbon dioxide and becomes increasingly acidic.
Now, a new study based on 2008 research in the Scotia Sea shows that the shells of tiny marine snails called pteropods are already being dissolved by ocean acidification where atmospheric CO2 being absorbed by the sea is exacerbating acidic conditions resulting from upwelling of cold water from deep below the surface.
The tiny animals are a valuable food source for fish and birds and play an important role in the oceanic carbon cycle. Pteropods are open-ocean animals, moving about by using water wings that evolved from their snail feet.
“They’ve been found in the stomachs of fish and seabirds, and even whales … they are sometimes found in high concentrations and are quite a key part of the food web,” said British Antarctic Survey scientist Dr. Geraint Tarling. The snails feed on phytoplankton or general ocean detritus that they capture by casting out large mucus webs, Tarling explained.
During the 2008 science cruise, the BAS researchers teamed up with scientists from the Woods Hole Oceanographic Institution and the National Oceanic and Atmospheric Administration to discover severe dissolution of the shells of living pteropods — a sure sign that ocean acidification is going to have significant impacts on marine life.
“What you find in the literature at the moment, is a general prediction that by 2050, increasing carbon dioxide will make surface waters more acidic,” Tarling said. The upwelling of cold water from the deep already creates pockets of acidified water near the surface, and those pockets are getting larger and larger, driven by increasing winds, Tarling explained. By mid-century, the acidification will be widespread in surface waters, with no areas free from the corrosive conditions that directly impact shell-forming species and potentially other marine organisms.
If CO2 levels keep increasing at today’s rate, the average surface ocean pH will fall from 8.1 to around 7.8 by the end of this century. It’s not exactly clear how that changing chemistry will affect marine organisms and ecosystems, or the complex carbon cycle that includes the formation and subsequent deposition of carbon-containing shells. The Sea Surface Consortium has been formed to answer some of those questions, primarily by studying the response of real-world plankton to acidification, as opposed to lab experiments.
The 2008 research 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.
“We know that the seawater becomes more corrosive to aragonite shells below … the saturation horizon, which occurs at around 1000 meters depth,” said lead author. Dr. Nina Bednaršek. “However, at one of our sampling sites, we discovered that this point was reached at 200 meters depth, through a combination of natural upwelling and ocean acidification,” said Bednaršek, formerly with the BAS and now with NOAA.
“Marine snails … 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,” she added.
“Although the upwelling sites are natural phenomena that occur throughout the Southern Ocean, instances where they bring the ‘saturation horizon’ above 200m will become more frequent as ocean acidification intensifies in the coming years,” Tarling said.
“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,” he said.
“Climate models project a continued intensification in Southern Ocean winds throughout the 21st century if atmospheric carbon dioxide continues to increase,” said co-author Dr. Dorothee Bakker from the University of East Anglia. “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 concluded.
The Summit Voice: Extensive dissolution of live pteropods in the Southern Ocean. Nature Geoscience (2012).