PROVIDENCE — Lobsters are already slowly moving out of southern New England as waters warm, but the iconic crustacean faces another future threat as the climate changes.

As oceans absorb more carbon and become increasingly acidic, juvenile lobsters likely will have a harder time growing and forming strong shells to protect them from predators, according to a recent University of Rhode Island study.

“I’m not sure yet what the mechanism is that is affecting their growth,” URI doctoral student Erin McLean, who led the research, said. “But it takes energy for them to regulate the increased acidity, which is energy they cannot then put toward growth.”

And it’s not just lobsters that could be harmed by the oceans’ changing pH levels. Shellfish populations in Rhode Island and Massachusetts in general are among the most vulnerable in the United States to ocean acidification, according to a recent study in the journal Nature Climate Change.

The study identified the two states among 15 at-risk areas in the nation because colder, northern waters, such as Narragansett Bay, Buzzards Bay and Nantucket Sound, are absorbing more carbon dioxide from the atmosphere and acidifying faster than warmer waters.

Other more localized factors are also playing a part, including nutrient pollution from fertilizers and sewage systems that can add more carbon to the water and the flow of fresh water from poorly buffered rivers, such as the Blackstone in Rhode Island or the many waterways that drain into the Gulf of Maine, which lack minerals to mitigate the effects of acid. Shallow coastal waters also are more susceptible to changes in the ocean’s chemistry.

About a quarter of all carbon emissions from power plants, cars and other sources are absorbed by the oceans, making them more acidic and reducing carbonate levels. Shellfish use carbonates to make their shells, and when fewer of the compounds are available, organisms must expend more energy to build shells and less on eating and survival, researchers say.

The stakes are high in Rhode Island and Massachusetts because of the value of shellfish as an environmental, cultural and economic resource. Southern Massachusetts makes more money from shelled mollusks than any other region in the United States with annual harvests in Bristol and Plymouth counties, Cape Cod and the islands totaling more than $300 million in revenues annually in recent years. And tiny Rhode Island ranks seventh in the nation in economic dependence on shellfishing, with annual harvests bringing in an average of $14 million over the past decade.

While Rhode Island’s wild quahog beds have recovered over the past several years after decades of decline, the state also has seen a steady increase in aquaculture. There are 52 farms in the state, and the total value of their products, mainly oysters, has more than doubled in the past five years to $4.2 million, according to the state Coastal Resources Management Council. In Massachusetts, the value of farmed oysters and other shellfish was estimated at $25.4 million in 2013, according to the state Executive Office of Energy and Environmental Affairs.

The Nature Climate Change study, which was led by researchers at the environmental groups the Natural Resources Defense Council and the Ocean Conservancy, set out to go beyond global models by identifying local risk factors.

“They weren’t previously factored into the conversation,” Lisa Suatoni, senior scientist at the NRDC and a co-author of the report, said. “There are a lot more places at risk than conventional wisdom tells us.”

Those places include New England and the Gulf of Mexico, as well as the Pacific Northwest, where the effects of acidification have already caused serious problems. In 2008 and 2009, oyster harvests plummeted in Oregon and Washington as deep-lying acidic waters welled up to the surface. Hatcheries recovered only after a warning system was developed to give growers time to protect their oyster beds before currents brought in more acidic waters.

So-called upwelling may not be an issue in southern New England, but Mark Gibson, deputy chief of marine fisheries at the Rhode Island Department of Environmental Management, described acidification as a “significant threat” to local fisheries.

“I think it can have potentially more negative effects than warming,” he said. He added that while cold-water fish are moving out of Narragansett Bay and being replaced by species more accustomed to warmer water, for shellfish, “it’s not so easy to get away from acidified water.”

Even though the chemistry of acidification is well understood, not as much is known about the effects on marine life in Rhode Island and nearby waters, Gibson said. It may be affecting quahog growth rates, which appear to be going down in Narragansett Bay. There’s also speculation that it plays a part in lobster shell disease, he said.

In the experiment at URI, McLean divided 24 juvenile lobsters between three tanks: one with dissolved carbon concentrations equivalent to present-day levels; one with higher levels that are expected in 2100; and the last with even higher levels predicted for 2200. She weighed and measured the lobsters every day for four months and also recorded the number of molts for each.

The lobsters in the first tank mirroring current conditions went through four molts and on average each experienced a 23 percent increase in length between molts and an 86 percent increase in weight. Most of the lobsters in the second and third tanks went through only three molts. In the tank simulating 2100 conditions the lobsters grew 19 percent in length on average and 71 percent in weight between molts. Those in the tank with 2200 conditions averaged growth of only 17 percent in length and 54 percent in weight.

A year ago, Rhode Island released its first Shellfish Management Plan, a 300-plus page document that sets out ways to ensure the long-term sustainability of Rhode Island’s populations of oysters, quahogs, soft-shell clams and other shellfish. One of the challenges identified in the plan is ocean acidification and it recommends more local research on the issue.

The General Assembly has also created a commission to more aggressively study the effects of acidification in Rhode Island. The group held its first meeting in September.

The Nature Climate Change study recommends taking steps to address acidification. At the top of the list is reining in carbon emissions. But there are others. Reducing water pollution is one, as is looking for strains of mollusks that are more resistant to acidic waters.

By highlighting the roles played by local factors in affecting acidification, the study also points to the importance of local policies to address the problem, Suatoni said. Rhode Island has worked hard to reduce chemical runoff, and Suatoni said the state should “double-down” on those efforts. The state — as well as Massachusetts — also has nationally recognized marine researchers who are already looking into the threats posed to local bays and inlets, she said.

“There are vulnerable communities in the U.S.,” Suatoni said. “All of these communities can use the assets they have to try and address the deficits.”

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