CO2 + N = Double Trouble for Casco Bay

Posted on Nov 20, 2011

Carbonic acid, the same compound that produces the bubbles in carbonated beverages, is gradually dissolving many creatures that make up the marine food web. The next time you take a sip of sparkling water, try to imagine it as a mini-ocean, populated with swimming, floating, and crawling sea creatures. Now, imagine them dissolving like ice cubes.

All around the world, scientists are finding changes in the ocean’s chemistry that are weakening or even dissolving the shells of clams, mussels, corals, and tiny creatures at the base of the marine food web.

Where is the excess carbonic acid in the ocean coming from? Scientists estimate that the ocean absorbs nearly a third of all the carbon dioxide (CO2) released into the atmosphere by burning fossil fuels. Roughly one million tons of CO2 are being absorbed by the ocean every hour. The ocean food web is showing the effects of more acidic conditions.

This “other CO2 problem,” called ocean acidification, rivals other global impacts of excess carbon dioxide, such as climate change, sea level rise, and extreme weather. Increasingly acidic water impairs the ability of sea creatures, from fish and squid to sea urchins and copepods, to grow, reproduce, and fight off disease. According to researchers at Woods Hole Oceanographic Institution, ocean acidification will affect 99% of the commercially-important species in the world’s oceans. That could cost the United States $3.8 billion in lost fisheries revenue annually.

For the past 20 years, Friends of Casco Bay’s scientific staff and volunteer water quality monitors have been measuring pH as one indicator of ocean health. The pH scale measures whether sea water is acidic (like orange juice) or basic/alkaline (like ammonia). Ocean water is normally slightly basic.

This past spring, 72 water quality monitors for Friends of Casco Bay were trained to calibrate, use, and maintain digital pH meters. Until then, our citizen stewards had used a color comparator to match their water samples against standardized solutions representing varying levels of ocean pH. “The new pH meters make our measurements even more precise, allowing us to see subtle changes over time,” says Friends of Casco Bay Research Associate Mike Doan.

Our data shows that the average level of pH in Casco Bay has decreased by .01 on a 14-point, logarithmic scale. “It may sound like a small change, but it is a significant decrease. The fact that our water is becoming more acidic is alarming. The damage could extend all the way up the marine food chain,” says Casco BAYKEEPER® Joe Payne.

Coastal waters like Casco Bay are being assaulted from two sources: • Ocean acidification: Surface waters assimilate carbon dioxide released into the atmosphere from burning coal, oil, and gas. • Coastal acidification: Carbon dioxide is released into the water through processes triggered by nitrogen pollution from the land. When algae blooms die, decomposing bacteria release carbon dioxide into the water and nearshore sediments.

A Dual Threat to Coastal Waters

Runoff from fertilized lawns and city streets, emissions from smokestacks and tail pipes, and sewage discharges all flush excess nitrogen into coastal waters. This nitrogen bonanza triggers an explosion of green algae and microscopic phytoplankton. Soon, these algal blooms die and decay. Their decomposition releases CO into the water.

Dr. Mark Green of Saint Joseph’s College here in Maine is a nationally- recognized researcher who is studying the effects of nitrogen pollution on larval clams. Using mud taken from Casco Bay, he has found that clams that live in sediments with low pH have a hard time building their shells; larval clams, called spat, may actually dissolve. Dr. Green calls this “death by dissolution.” He says that clams may be “the canaries in the coal mine,” presaging the effects of ocean acidification on the rest of the world’s marine life.

What’s a clam bake without steamers?

Maine clammers harvesting soft-shelled clams (Mya arenaria) raked in $11.05 million in 2010 (preliminary DMR figure). Most of these shellfish are taken when they are 3 to 4 years old, after they reach the legal size of 2 inches in diameter.

Casco Baykeeper Joe Payne worries, “What would happen to the clam fishery if an entire year’s worth of clam spat were to dissolve? It wouldn’t take more than a couple of years before the industry and consumers would feel the impact.”

Inspired by Dr. Green’s lab results, Friends of Casco Bay decided to undertake a survey of clamflats to determine if there is a link between pH and clam survival. After consulting with Dr. Green and pH meter manufacturers, Mike Doan and Liz Thompson, a student intern from Southern Maine Community College, developed a procedure to measure pH in sediments. Clammers, shellfish wardens, and marine biologists helped them identify sites from Falmouth to Phippsburg so they could compare the pH of productive clam flats to those where harvesting is no longer profitable. Preliminary results from surveying 14 sites indicate that pH is significantly lower on clamflats that are no longer productive than on those that are actively harvested. Could acidic mud be contributing to the decline in clam populations?

What can we do about it?

Friends of Casco Bay staff and volunteers have been collecting data on nitrogen pollution for over ten years. This data helped convince the State Legislature to require the Maine Department of Environmental Protection to set limits on nitrogen discharges into coastal waters. We are continuing to monitor the State’s efforts on developing these standards. At the local level, Friends of Casco Bay works with neighborhood associations and other community groups to discourage homeowners’ use of nitrogen-rich fertilizers.

Clams themselves could help improve conditions on the clamflats. Recycled shells help to buffer the sediments, like gardeners adding lime to their lawn. Dr. Green is currently conducting an experiment in Broad Cove in Cumberland, peppering the mud with calcium carbonate, the mineral that builds clam shells. (Calcium carbonate, the main ingredient in Tums and Rolaids, also buffers acidic stomachs.)

So far, the results are promising. Could clam harvesters be recruited to measure the pH of clam flats and add calcium carbonate where acidity is a problem? Many questions need to be answered, such as, how much calcium carbonate is enough? How much will it cost? How often does it need to be applied? And not least, how do you keep it from being washed away by tides and storms?

As goes Casco Bay…

Initially, altering the pH of the mud on a Casco Bay clamflat may be only a temporary, localized solution, but if this technique works, it could be expanded to other coastal regions.