Protecting Clams and Maine’s Coastal Heritage

Posted on May 20, 2012

All around the world, scientists are detecting changes in the ocean’s chemistry. Our oceans are becoming more acidic. This change is weakening and dissolving clam shells, coral skeletons, and other sea creatures. The ocean absorbs nearly a third of all carbon dioxide pumped into the atmosphere by burning fossil fuels. In addition, carbon dioxide is released into coastal waters when algal blooms, triggered by nitrogen runoff from land, die and decay. When carbon dioxide mixes with water, it creates carbonic acid, the culprit that is increasing ocean acidity, lowering pH, and threatening marine food webs.

pH is one measure we use to assess the health of our coastal waters. The pH scale ranges from 0 for a strong acid (such as battery acid) to 14 for a strong base (such as oven cleaner). Pure water is 7; Casco Bay, on average, is 7.9. Our 20 years of data show that Casco Bay, while still healthy overall, is becoming increasingly acidic. This is not good news for baby clams and other juvenile shellfish.

While we have been sampling our waters, Dr. Mark Green of St. Joseph’s College has been testing clam flats along our coast. With regard to acidification, clams are the “canaries in the coal mine.” Dr. Green has determined that mud with a pH above 7.8 provides healthy habitat for developing clams. If the pH is between 7.8 and 6.8, clam growth is compromised; below 6.8, clam spat and baby clams simply dissolve. Dr. Green has also researched rehabilitating flats with low pH by spreading ground clamshell hash over them to buffer the acidic conditions and make the flats places that baby clams could once again call home.

Last year, we decided to build on Dr. Green’s lab findings by sampling 14 clam flats around Casco Bay. Friends of Casco Bay researchers developed a protocol to measure the pH of the mud on both those flats that are actively being harvested for clams and those that are no longer productive. We found that the pH on nonproductive clam flats averaged 7.6 pH, compared to 8.2 on those flats where there were enough adult clams to harvest.

Our goal for this summer is to expand our research to more than 25 clam flats around the Bay, and to collect additional parameters such as sediment grain size and organic carbon from decaying matter. Our information may help clammers and resource managers identify which clam flats may be good candidates for pH rehabilitation.