Mussels that enrich a bowl of bouillabaisse or clams that anchor a cup of chowder face a growing threat from oceans soured by carbon dioxide emissions from burning fossil fuels, researchers have noted.
But opportunities exist to forestall ocean acidification's effects on US shellfish and on the communities that harvest them, at least in the near term, according to a new analysis.
The opportunities arise from an increasingly mature understanding of the factors that contribute to ocean acidification in coastal areas and how shellfish respond. They also arise from a growing public awareness of the problem, at least in some regions, accompanied by actions to adapt to the changes, according to the team conducting the analysis.
Acidification represents the ocean's chemistry adjusting itself to the rising amounts of carbon dioxide that humans are pumping into the air, mainly from the fossil fuels they burn. The oceans take up about 25 percent of this CO2, which combines with seawater to form a weak carbonic acid.
The acid is a virtual magnet for calcium ions in the water. When they combine, the process reduces the inventory of calcium available to marine organisms to build their protective calcium-carbonate shells.
For clams, oysters, mussels, scallops, and other types of bivalve mollusks, their 48 hours as larvae are the most critical period for shell formation, notes George Waldbusser, a marine ecologist at Oregon State University in Corvallis and a member of the team publishing its analysis Monday in the journal Nature Climate Change.
The analysis represents the first national ocean-acidification vulnerability study that drills down to the county level to identify areas where livelihoods are most at risk, in addition to looking at the risks to the shellfish themselves, says Linwood Pendleton, an economist and senior scholar at Duke University's Ocean and Coastal Policy Program, and another member of the research team. The study also looks at physical, social, and political factors that contribute to the risk – which point toward potential adaptation approaches.
From a shellfish perspective, humanity's CO2 emissions are not the only source of acidic waters. Winds and currents along a coast can encourage naturally acidic water from the deep ocean to reach the surface. This coastal upwelling is driven by changes to winds and currents, whose intensity, duration, and frequency can be affected by global warming. Acidity also increases where rivers discharge vast amounts of fresh water into a relatively less acidic ocean. And it can increase as a byproduct of excessive amounts of nutrients flowing into estuaries, which can serve as shellfish nurseries.
For shellfish, 16 out of 23 biologically based regions along the US coast face general ocean acidification or one of the other contributors. Ten regions face threats from more than one source of acidic water.
The stretches of coast first reaching a critical point where general ocean acidification begins to disrupt shell-building run along the Pacific Northwest and southern Alaska, the analysis shows. Indeed, that threshold may already have been surpassed along many stretches of those coasts, the study suggests.
After more-localized sources of acidic water are taken into account, other stretches of coast reach the threshold decades sooner than they would if general ocean acidification was the only source of acidic water.
On the human side, vulnerability comes in various guises.
"It's incomes, it's jobs, and it's also the alternatives" available to shell-fishing as a job, Dr. Pendleton says.
Political recognition of the acidification problem, as well as the scientific resources available to monitor conditions and advise local resource managers, also vary by region and can affect a region's ability to adapt, the researchers note.
The overall capacity to adapt is highest along the West Coast, the Eastern Seaboard from Virginia north, and the southern and central Florida coasts. The areas with the least capacity generally fall along the Gulf Coast and a stretch from the Carolinas through northern Florida.
The ability to gauge the combined marine and social risks from ocean acidification at a county level provides a means "to break down what's happening" in ways that reveal "bite-sized ways to make a difference," says Sarah Cooley, science outreach manager for the Ocean Conservancy in Washington and one of the team members conducting the analysis.
To be sure, work already is under way to adapt shell-fishing, especially farmed shellfish, to a changing ocean chemistry.
In the Pacific Northwest, where engagement on the issue is high, shellfish growers are installing instruments to monitor water chemistry. They either close off water circulation from outside the pens for a period or add lime to the water to reduce its acidity.
For natural colonies, researchers are exploring ways to use crushed shells spread among beds of wild shellfish as a source of minerals to reduce acidity around the beds.
Ironically, New England is a region crawling with marine scientists, but compared with other climate change-related issues, such as sea-level rise, the impact of acidification on fisheries hasn't received a comparable amount of exchanges between scientists and the shell-fishing community, Dr. Cooley notes.
But that's changing. The Northeast Coastal Acidification Network opened for business two years ago to bring together scientists, resource managers, fishing interests, and government agencies to focus on the issue along a stretch of coast from Long Island, N.Y., to Maine.
"We're seeing these partnerships develop as we speak," Cooley says.