The Impact of Coastal Acidification—It Shucks to Be a Clam

In the summer of 2014, Friends of Casco Bay placed hatchery-reared baby clams in the mud at Recompence clam flat in Freeport, Maine, where we measured very low pH levels. Image A shows a clam prior to deployment in the mud. Image B shows a clam after just one week in the mud, where it became heavily pitted due to the high acidity of the mud. Image C is a close-up of the same clam. All of the deployed clams exhibited obvious signs of pitting.

As our coastal waters become more acidic (as the pH decreases), clams, mussels, and other shellfish are having a harder time building and maintaining their shells. Juvenile clams may dissolve outright. Our research has found a disturbing link between acidic mud and clam flats where it is no longer profitable for clammers to harvest shellfish.

In 2011, Friends of Casco Bay began to publicize and investigate coastal acidification. We developed a scientific procedure for sampling the acidity of mud on clam flats. We wanted to compare the pH of clam flats that are actively being harvested by clammers to those that are no longer productive.

This groundbreaking work assesses how acidified sediments threaten the survival of baby clams in Casco Bay. Three years of data show that areas with the highest acidity (lowest pH) are the same flats where clams are now scarce.

We found a strong correlation between high levels of nitrogen and carbon in the mud—indicating organic matter— and lower pH. In other words, a lot of dead, decaying stuff makes matters worse.

Many people are interested in the results of our cutting- edge coastal acidification research, including the 1,700 registered Maine diggers who support a $16.8 million-a-year industry harvesting soft-shell clams. Those of us who define summer as a delicious plate of steamers have a gastronomic interest, too!