Scientists have found evidence that shellfish are being harmed by the effects of global warming. Already endangered by overfishing and disease, the authors of a new study say oysters are becoming smaller and less robust as greenhouse gases alter the acidity of water in estuaries and ecosystems where they live.
Researchers with the Smithsonian Environmental Research Center in Edgewater, Maryland, studied the impact of rising carbon dioxide levels in the atmosphere - one byproduct of carbon-rich industrial emissions - on the larvae of two closely-related oyster species - Eastern oysters, which are indigenous to the Atlantic coast of the United States, and Suminoe oysters, which are native to Asia.
Adult oysters are bottom feeders, but as newborn- larvae they float freely to the water's surface for two to three weeks before settling to the bottom of an estuary and attaching to a hard surface.
Scientists wanted to see how these developing larvae were being affected by the increased acidity of the ocean water caused by excess carbon dioxide in the atmosphere. Whitman Miller - an environmental scientist and ecologist with the Smithsonian research center - says oceans normally absorb about half of the carbon dioxide that's produced by living creatures.
But when excess CO2 is pumped into the atmosphere and dissolves into seawater, that contributes to an acidification, or a lowering of the pH of the oceans.
A byproduct of that process - carbonic acid - rapidly converts to carbonate and bicarbonate ions, which Miller says are corrosive to the calcium carbonate shells of oysters and other marine species.
"I was interested in seeing how elevated CO2 and reduced pH would affect their growth and calcification rate. And my hypothesis was that larval oysters would grow more slowly and calcify less under elevated CO2 than under ambient or pre-industrial conditions," said Mr. Miller.
Researchers cultured Eastern and Suminoe oysters in water that was held to four separate CO2 concentrations, reflecting atmospheric conditions during pre-industrial times, acidity levels today and those predicted for 50 and 100 years from now.
The researchers measured the surface area of the shells to assess their growth and did a chemical analysis to see how much calcium carbonate was in the shells. Calcium carbonate is the building block of oyster shells. "What we found was that the native Eastern oysters from the Chesapeake Bay and the east coast United States were very sensitive to lowered pH that is brought about by elevated CO2, and found their growth was significantly slowed and that their calcification was significantly reduced at these elevated CO2's," he said.
The scientists found the shell area of the experimental, pre-industrial oysters was 16 percent greater and their calcium content 42 percent higher compared with those exposed to CO2 levels predicted for the end of this century.
However, Miller says the Suminoe oysters were not affected by the higher acidity levels. Miller thinks the Suminoes may already have adapted to chemical changes in the ocean but that increasing levels of acidity could eventually harm them as well.
Miller says with less calcium in their shells, baby oysters take longer to mature, floating longer on the water's surface and making them easy prey to other animals.
And now Alan Duckworth - chief scientist with the marine preservation group Blue Ocean Institute - says ocean acidification is threatening to wipe out what remains of oyster beds, which have been decimated by coastal pollution and overfishing. "Shellfish alone are worth about $10 billion globally. So, it's a massive industry involved with shellfish agriculture. And the problem is that as the oceans become more acidic, it's possible that these shells will become weaker or in fact as time goes on will even dissolve," he said.
The findings of the study on ocean acidification and its impact on oysters is published in the open access journal PLoS One.