SOUTH PORTLAND, Maine — They’re called dead muds.

Rising levels of carbon dioxide in the atmosphere combined with unregulated nitrogen pollution are having a deadly effect on Maine’s shellfish, some researchers say.

Scientists are starting to measure the impact of increasingly acidic waters on coastal organisms, and what they’ve found is alarming. Formerly fertile shellfish flats are becoming uninhabitable wastelands of dreck.

The phenomenon is another threat to Maine’s shellfish industry, estimated to be worth $60 million annually.

“They call them dead muds,” said Mark Green, an oyster grower and marine science professor at St. Joseph’s College in Standish. “The darker muds and sulfur-rich muds don’t have any clams, and those are the flats that have lower pH levels. Places where historically there have been great harvests that supported clammers for decades, you now see water quality changes that are reflected in the mud.” The more acidic the water, the lower the pH.

In these places, researchers aren’t finding dead or unhealthy shellfish. They’re finding nothing at all. It is a complete eradication.

“If you put a larval shellfish in a mud flat that has a pH level of 6.8 or 6.9, you won’t find it 24 hours later — it’ll totally dissolve,” Green said. “It’s well documented now that we see pH levels that are causing larval shellfish to die, and in relatively large numbers. And the pH projections in the future are [much more acidic than] what’s been seen in laboratories to cause massive die-offs.”

Dean Doyle is a longtime shellfish harvester based out of Phippsburg. He said clammers are quick to promote practices that sustain their industry, such as seeding clams in the mud flats to cultivate future populations for digging. But Doyle, who said he has turned to foresting for additional income in the absence of reliable shellfish harvests, said muds like those described by Green are hopeless. And they’re growing.

“We’ve lost huge amounts of area,” Doyle said. “We can’t even keep areas that we seed. They just disappear. They just dissolve away. Areas where I used to dig 2 or 3 bushels, I can’t find a single clam now. [Clamming] is not even a full-time job anymore in my town. It’s not like it was 10 or 12 years ago.”

Denis-Marc Nault, a biologist with the Department of Marine Resources, said he’s aware of the concern over high acidity in clam flats, but with a staff of just two people to watch over nearly 150,000 acres of intertidal flats, the state hasn’t been able to map out acidic trouble areas or conduct enough studies to be sure acidity is causing the die-offs in places where clammers are seeing them.

“We are looking at it,” Nault told the Bangor Daily News. “We’re interested in it. I do have areas where I can set clams and they grow just fine, but I have other areas where the clams struggle with shell erosion or a loss of larva. Is it acidification? Is it a drop in the pH in the mud itself? I don’t know.”

Nault said changes in salinity can be deadly for young shellfish as well, and said he and the one other department biologist assigned to monitor Maine’s flats are stretched too thin to do the comprehensive research necessary to determine how pervasive the dead muds are.

“It’s something we think about,” he said. “It’s definitely a concern.”

Acid oceans

Conversationally, the notion that Earth’s oceans are becoming more acidic is often lumped into a larger discussion about climate change, said Joseph Salisbury, a researcher in the University of New Hampshire‘s marine program and a South Portland resident. But unlike the politically volatile subject of global warming and its potential causes, Salisbury said, there’s no such debate about ocean acidification.

“We’ve seen carbon dioxide in the atmosphere go from 200 parts per million up to 392,” Salisbury said. “It’s almost doubled. Some people may argue that this does not affect global warming, but there’s no question there’s an increase in carbon dioxide in the atmosphere — and it has caused an increase in the ocean’s acidity.”

Adding carbon dioxide to the ocean adds to the water’s concentration of hydrogen ions, increasing its acidity. Green said the Earth’s oceans absorb more than 21 million metric tons of carbon dioxide each day.

“There may be controversy surrounding global warming, but there’s no debate about the fact that the ocean is becoming more acidic,” said Paul Dobbins, former oyster farmer and co-founder of the Portland-based kelp grower Ocean Approved LLC. “It doesn’t receive all that much coverage, because it’s not controversial, but [the ocean’s acidity is] what has all of us who are working with ocean organisms and shellfish worried.”

Casco Baykeeper Joe Payne said the pH of coastal ocean waters has decreased by 0.02 over the past few decades.

“I’ve had people say to me, ‘Well, that’s just .02, that’s tiny,’” Payne said. “Your blood and seawater are 98 percent the same. If your blood pH changed by .02, you’d be comatose. That’s what it does to the ocean.”

What Payne described as a “major problem” in the form of worldwide ocean acidification, though, isn’t the only problem for Maine’s shellfish flats.

The double whammy

There are no limits now on how much nitrogen can be discharged into Maine waterways.

“If you have a very productive ecosystem, which the Gulf of Maine is, and you add a little nitrogen, it gets even more productive,” said Salisbury.

The environmental advocacy group Friends of Casco Bay, to which Payne belongs, has launched a campaign urging Mainers to keep an eye out for “green slime” — thick carpets of algae covering shallow coastal waters. The green slime is a sign of excess nitrogen, Friends of Casco Bay warns, and excess nitrogen carries a host of consequences for an ecosystem aside from bad aesthetics.

The same nitrogen that causes a freakish bloom in algae causes an explosion in the microscopic phytoplankton populations. Those massive numbers of phytoplankton then die, fall to the mud at the base of the water and decompose, releasing more concentrated carbon dioxide to the fragile mud ecosystem and driving the pH further into acidic territory.

“Only now are we considering the effects on shelled organisms,” Salisbury said. “This is a big concern, and it’s only recently been studied. People have always known about the chemistry, but the effects on the organisms [are just being researched].

“There may be adaptations that some organisms can make, but we do know that things are changing probably faster than most organisms can keep up with,” he continued. “It could be that these factors are conspiring to make survival difficult for commercially valuable shellfish, but really the jury is out.”

Controlling nitrogen

A 2008 resolution of the Legislature, promoted by Friends of Casco Bay, directed the Maine Department of Environmental Protection to develop limits for the amount of nitrogen allowable in Maine waters.

Angela DuBois, who works in the department’s Bureau of Land and Water Quality, is in charge of drafting those numeric limits in time to meet a July 2012 deadline, after which the Legislature will review and consider implementing them.

DuBois said setting those limits is “the first step” in a longer process to control nitrogen pollution. When the numbers are set, the DEP will try to maintain them by regulating the amount of nitrogen discharged by identifiable sources, such as sewage treatment plants.

The department also has programs set up to educate the public about controlling “nonpoint sources” of nitrogen pollution, such as residential and farm fertilizers and road runoff. But DuBois acknowledged the nonpoint sources will be “more difficult” to control.

That could leave dischargers such as factories or sewage treatment plants shouldering much of the burden of curbing nitrogen levels.

“Plants our size are looking at [$500,000 to $750,000] to build tertiary treatment chains,” said Christopher Higgins, head of the Boothbay Harbor Sewer District. “That’s going to make an impact on sewer fees, especially when [the cost is spread out] over 1,200 users.”

For his part, Higgins is partnering with Dobbins to experiment using kelp to control nitrogen. The saltwater vegetation absorbs nitrogen from the water, and Higgins has a Department of Marine Resources permit for two kelp beds, one near the the district’s primary treatment plant and another near Bayville.

“A nice benefit of this is that kelp eats nitrogen, and there are places on the Maine coast where there’s excess nitrogen because of runoff and other pollution,” Dobbins said of the sea plant, which is popular in many parts of the world as a food and fuel source. “He’s looking to use nature to take excess nitrogen out of the water — it’s called ‘bioremediation,’ using nature to help clean itself.”

Bill Mook, head of the Walpole-based Mook Sea Farm, said his Damariscotta River oyster farming operation has taken note of dropping pH in the water and chooses to refill its nursery tanks during times of day when the waters are tested to be less acidic.

“We did see the incoming pH levels were oftentimes a little lower than we would have expected, running down below 8 consistently, and even down as low as 7.7 and 7.8,” he said recently. “We also found that if we’re able to maintain pHs a little higher, we’re able to maintain higher larval populations.”

Finally, Green’s research suggests that one of the best controls for pH in mud is the return of clamshells. While harvested clams are typically taken upland, shucked and eaten and the shells tossed in the garbage, Green advocates for the shells to be saved and spread back over the flats from which they were harvested.

The shells provide doses of calcium carbonate — acting like “Tums” in the mud, he said — to combat the lowering pH.

“Putting clamshells back seems to, at least in a couple of places where I’ve done it, create a three- to fivefold increase in the number of juvenile clams,” Green said. “It doesn’t tackle the root of the issue, but it does help ameliorate the problem.”

Green said the various localized attempts at slowing the creep of acidity into coastal marine life will help for only so long.

“The ocean has plenty of buffering capacity for carbon dioxide, but we’re injecting it too fast for the ocean to keep up,” he said. “The only way to stop it is to curb carbon dioxide emissions. This is really, in my mind, still a looming catastrophe.”

Seth Koenig

Seth has nearly a decade of professional journalism experience and writes about the greater Portland region.