Dr. Mark Green spends his summers in the mud of Maine’s coastal areas, researching the fate of larval bivalves, also called spat.

What he’s found isn’t encouraging. The mud in some places along Maine’s coast is so acidic that spat risk dissolving if they try to settle. While that’s bad news for bivalves, it provides valuable insight into what scientists can expect from ocean acidification.

Scientists have reported a decline in the pH of the world’s oceans from 8.2 during pre-industrial times to 8.1 today-a slight drop that represents a dramatic increase in acidity. Carbon dioxide lies at the root of this change. “Atmospheric CO2 is a main cause of ocean acidification, but the coastal mud is acidifying as a result of other human activities too,” said Green, a marine science professor at St. Joseph’s College in Maine. “It’s also happening through increased nutrient loads in runoff, which adds carbon dioxide to the mud, and the removal of buffers that the clams need to survive.”

Sewage, fertilizer and soil erosion contribute to higher acidity in coastal waters by promoting algal blooms. When the blooms die off, the plankton sink to the bottom and decompose, producing carbon dioxide in the process. When carbon dioxide from the atmosphere or from decomposing organic matter mixes with seawater, it reacts to form carbonic acid-a weak acid that reacts with other molecules found in the water.  One such molecule is the carbonate ion, which is critical to shell-building marine organisms, and is the reason ocean acidification poses such a threat to bivalves and other marine life.

Green’s research sites now have pH levels below what’s projected for ocean waters 50 years from now, making them excellent case studies. The sites are in inter-tidal mud flats in West Bath and South Portland.

“Acidification in the ocean is going to be a slow and steady progression,” says Green. “In major estuaries like Casco Bay and the Chesapeake, however, the changes will occur on a much faster time scale. That makes them the best naturally-occurring conditions to study the effects of ocean acidification.”

Scientists believe that mollusks, which include bivalves, lobsters and squid, are canaries in the coal mine for the effects of ocean acidification. The effects vary by species but include everything from stunted growth in adults to dissolving shells and death in juveniles. Researchers predict that ocean pH could drop to 7.8 over the next half-century. That could cost the United States $3.8 billion in lost fisheries revenues, according to a recent report from the Woods Hole Oceanographic Institute.

Spat looking to settle on mud with a pH of 7.0 to 7.5 (the range seen across Green’s sites) risk death by dissolution as the carbonic acid reacts with their existing shell faster than the spat can add layers. Faced with almost certain death, most spat choose to stay afloat in order to find a more suitable spot. That leads to further risks of being eaten by predators or being pushed out to sea by a strong current. “Clams, like other bivalves, follow a pattern of very high mortality in the early life stages,” says Green. “This death by dissolution represents a real threat to them, and I believe that pH explains at least some of the tremendous [spatial] heterogeneity found in clam beds.”

Clams, mussels, oysters and even scallops face similar challenges as juveniles, especially when it comes to an acidifying ocean. Adults have the capacity to divert energy from growth and reproduction to make up for the lower availability of carbonate ions in the surrounding waters, whereas juveniles do not. The same is true for finfish, and research on both freshwater and marine fish suggest that in the larval stages, even finfish may suffer negative impacts from lower pH levels.

Based on the amount of carbon dioxide already present in the atmosphere, researchers believe that the Gulf of Maine and the rest of the world’s oceans are locked into further increases in acidity. And the Gulf of Maine ecosystem, unbalanced by the loss of top predators (cod) and an overabundance of a single species (lobsters), is already in a stressed state.

Green’s research does provide some good news, however. His studies have shown that adding crushed clam shells to the mud at his sites has greatly increases the likelihood that spat will settle. The shells increase the local availability of carbonate ions-something that no longer happens as shells are removed during fisheries harvests. “Buffering may prove to be an applied science management solution to ocean acidification,” says Green. “When you’re losing 99 percent of the set of spat in the first couple of days, you don’t have to change the mortality much to generate a huge increase in the population.”

Nevertheless, ocean acidification risks causing problems long before organisms start dissolving. For instance, unusually warm water and low oxygen levels were two factors identified as potential causes of the devastating outbreak of shell disease in southern New England lobsters in the late 1990s. While lobster shells are different than bivalve shells and so are much less dependent on carbonate ions, scientists still expect higher acidity to add to the cost of shell-growth.

It’s very difficult to know what will happen to the ecosystem as a whole if the pH continues to decline. Disease outbreaks in lobsters or population losses of bivalves or even finfish may or may not occur. But most scientists agree that something will eventually give. “Ecosystems are so complex that no one understands all of the different interactions and complexities involved,” says Green. “It’s almost impossible to know what will happen if you start removing or adding things to the system, until you’ve already removed or added them. And then it’s generally too late.”

Peter McDougall is a marine biologist and freelance writer who lives in Freeport with his wife and their dog.