Mussels don't stick around in acidic ocean water

http://www.dailyclimate.org/tdc-newsroom/2014/09/acidification-mussels

By Miguel Llanos
Sept. 9, 3014

•••••

Her job: testing how mussels in this idyllic bay, home to the nation’s largest harvester of mussels, are affected by changing ocean conditions, especially warmer and more acidic waters. It’s a question critical to the future of mussel farmers in the region. More important, it's key to understanding whether climate change threatens mussels around the world, as well as to the food chains mussels support and protect in the wild.

"Along the West Coast, mussels are well-known ecosystem engineers," said Bruce Menge, an Oregon State University researcher who studies how climate impacts coastal ecosystems. "They provide habitat for dozens of species, they provide food for many predators and occupy a large amount of space, so are truly a 'dominant' species."

Carbon from greenhouse gas emissions has steadily turned seawater more acidic, disrupting organisms accustomed to the slightly alkaline waters of the past 20 million years.

In the case of mussels, an earlier University of Washington lab study found that increased carbon dioxide weakens the sticky fibers, called byssus, that mussels use to survive by clinging to objects like shorelines or the ropes used by commercial harvesters.

"If byssal thread weakening does eventually become important," Menge added, "the consequences would be major if not catastrophic."

Newcomb's goal now is to apply in the real world what was learned in the lab. "Instead of spending a lot of time tightly controlling the temperature and pH conditions mussels grow in, I use the natural seasonal variation to try to answer the same questions," Newcomb said.

•••••

The University of Washington marine biologist is there courtesy of Penn Cove Shellfish, which is also the oldest and best known mussel operation in the United States. If you’re a mussel fan, you've probably had a few – they're sold at Costco as well as upscale restaurants across the country.

•••••

Prior to the Industrial Revolution and the explosion of manmade CO2, ocean pH averaged 8.2. Today it's 8.1, a 30 percent increase in acidity on the logarithmic scale. Computer models peg ocean acidity at 7.8 to 7.7 by the end of the century at the current rate of greenhouse gas emissions.

The surprising lab discovery was that mussel byssus weakened by 40 percent when exposed to a pH of 7.5.

At Penn Cove, low pH levels are not uncommon – Newcomb has even seen 7.4 in the year that she's been sampling.

"We're worried they're going to see it more frequently," said Emily Carrington, Newcomb's graduate adviser and leader of the University of Washington team that published the earlier lab results.
Trouble forming shells

The Penn Cove work follows well-publicized research that shows oyster larvae in hatcheries along Washington's coast are having trouble forming shells because of the acidic waters.

"Mussels are the new oysters," Carrington said. But there's a twist: Mussels confirm that ocean acidification's impact extends in mollusks beyond an inability to form a shell.

The lab finding is "part of a growing body of evidence that ocean acidification alone and combined with other stressors will have effects beyond shell mineralization," said Terrie Klinger, a University of Washington marine sciences professor studying the impacts of acidification.

Other research, she said, shows more acidic seas spur "changes in fish behavior and toxin production in harmful algae."

Penn Cove Shellfish and the UW researchers joined forces after company harvesters saw empty patches on some cultivation ropes that should have had mussels. Was that due to weakened byssus?

"We had a significant amount two years ago, and little bit less last year," said Ian Jefferds, general manager at Penn Cove Shellfish. "The question arose, 'Was there something related to that?'"

•••••

It's a global issue. Among major shellfish producers, New Zealand is creating an acidification monitoring network while researchers in Spain are planning a study similar to that in Penn Cove.

And across the Mediterranean, mussel farmers have reported problems. "A great majority of producers experienced important difficulties in past years as a consequence of summer heat waves," said Luis Rodrigues, a researcher with a European Commission panel studying acidification.

Those issues include slough-off in Italy and Montenegro, as well as thinning of shells and even die-offs in waters that topped 82.4 degrees F.

A related lab study showed "total mortality" of mussels when exposed to waters above 82.4 degrees, said researcher Frederic Gazeau.

That team also "observed a very clear impact of acidification on the capacity of mussels to produce their byssal thread," noted Gazeau. "At low pH, it was very easy to pick up mussels individually; they could not attach themselves anymore."

Of course, it's not just loss on the farms that worries scientists. In the wild, mussels form vast beds along coastlines, and some areas have seen slough-off as well.

"It's quite common in California in recent years," said Herb Waite, a University of California, Santa Barbara, biochemist who studies mussels. "A favorite collecting spot will suddenly disappear."

•••••

"It does matter where in the water mussels are," Newcomb added, noting that even preliminary results suggest growing them closer to the surface tends to mean less acidity. However, a trade-off there can be warmer temperatures.

The research will need two more years of data gathering before any definitive conclusions, but Carrington said the earlier lab tests underscore the need for field data.

"Would you be worried about being weakened by 40 percent?" she asked. "I would be."