PORTLAND, Maine — A study released Tuesday finds a link between high levels of ocean water acidity and declining soft-shell clam populations in Casco Bay.

The finding adds another piece to the puzzle of shellfish decline in the region, and suggests people can help limit the damage without even going near the water.

Landings of soft-shell clams have been steadily declining statewide since the late 1970s, from almost 40 million pounds in 1977 to just about 10 million pounds in 2014, according to data from the Maine Department of Marine Resources.

The new report, “A Changing Casco Bay,” suggests a contributing factor to the drop is the bay’s increasing acidity.

The study is the result of more than a decade of water quality sampling by volunteers and staff from the nonprofit Friends of Casco Bay. According to the group, their data collection methods have been repeatedly verified by the U.S. Environmental Protection Agency.

FOCB tested the acidity of 30 clam flats identified as “productive” or “no longer productive.” According to researcher Mike Doan, the group identified the flats after extensive talks with clammers, resource managers and shellfish wardens.

“We would ask, can you give us an example of a really healthy clam flat, and an extreme example of a really unproductive flat, of what they call ‘dead mud,’” he said.

The researchers found a statistically significant difference in pH, the unit that describes the acidity of a solution, between the two groups.

“Three years of data show that areas with the highest acidity … are the same flats where clams are now scarce,” the report found.

A growing body of research has displayed how increasingly acidic seawater, fueled by worldwide increases in atmospheric carbon dioxide, hinders the ability of young shellfish to build their protective shells.

More carbon dioxide in the air means that more dissolves into the ocean, where it reacts with water to form carbonic acid. This process lowers the pH of the water, making it more acidic.

Larval shellfish need the chemical compound calcium carbonate to build their shells. Young soft-shell clams, for example, pull calcium carbonate out of the seawater to build up their hard shells.

Generally speaking, the concentration of calcium carbonate decreases as oceans become more acidic, due to the influx of carbonic acid. More acid means less raw building material for young oysters, clams and mussels.

FOCB’s data shows this interaction happening at a local level, Doan said.

“We’re seeing pH levels in the mud that are below the threshold [i.e., more acidic] of what’s been shown in the lab to be detrimental to the smallest clams,” he said.

‘Double whammy’

So what’s causing the bay to become more acidic?

FOCB describes the factors contributing to the lower pH levels in Casco Bay as a “double whammy.”

The first driver of acidification is global climate change, the authors found. Over the past 200 years, the world’s oceans have become 30 percent more acidic, according to the Smithsonian Institution. This is faster than any known change in ocean chemistry in the last 50 million years.

But the authors also point to a more local cause of acidification in the bay.

At sites that the group has monitored for 13 years, FOCB found a statistically significant downward trend in pH, with ocean water becoming more acidic by 0.014 pH units per year.

“This is a serious and disturbing trend,” the report found.

The authors suggest that a major reason for this increased acidity is nitrogen runoff from sewage, storm-water runoff and air pollution.

The researchers came to this conclusion after noticing a trend of higher nitrogen levels closer to shore and decreased levels further away. “This is evidence that much of the excess nitrogen found in Casco Bay is coming from land-based activities,” they said.

Nitrogen makes its way to the ocean through wastewater treatment discharges, fertilizer runoff from farms and city streets, and atmospheric fallout of vehicle and industrial emissions, the report said.

Nitrogen pollution causes acidification by spurring massive blooms of algae. Just as the nitrogen in fertilizer helps a lawn grow, the nitrogen “bonanza” running off into the bay “can stimulate the growth of large blooms of algae, beyond what animals in the ecosystem can consume,” the report said.

Much of that algae ends up dying and settling in the mud. As that organic matter decomposes, the bacteria breaking down the dead algae consume oxygen and release carbon dioxide. That carbon dioxide makes the seawater more acidic, and juvenile clams have a harder time building their shells.

Maine landings of the clams brought in almost $20 million in 2014, according to the Department of Marine Resources. Trying to slow the rate of acidification will be vital to the health of the industry.

“We have a lot of work to do,” the report concluded.

‘The worst’ years

In an interview Monday, John Lemont, a commercial harvester in Brunswick, said “the last couple of years have been the worst.”

He said he suspects warmer water and invasive green crabs as some of the biggest contributors to the decline, but he sees the effects of acidic water on young clams’ shells as well.

“[Acid] tends to eat the shell … as they start developing,” he said.

Some towns are trying to counter the detrimental effects of acidic seawater by “buffering” clam flats with pounds of crushed clam shells. The hope is that adding more calcium carbonate to the ecosystem will reduce the impact of the acidic water in the mud flats.

Experiments with “shell hash” in Cumberland, Freeport and Brunswick have shown varying degrees of success. Brunswick Harbormaster Dan Devereaux said that strategy has helped stabilize clam beds about half of the time.

FOCB offered suggestions for what Mainers can do to help slow the rate of acidification in the bay. The report said limiting fertilizer use on lawns and in gardens and pumping out boat sewage at designated facilities, can help reduce nitrogen runoff.

“Fortunately, unlike some environmental problems,” the study found, “there are many things each of us can do to tackle threats to the health of Casco Bay.”