Four hundred corded nylon ropes hang from the mussel raft that floats near Bangs Island. Hundreds of thousands of flinty-blue mussels hang suspended in the dark, cold waters of Casco Bay. Once harvested, the mussels will grace the tables and displays of high-end restaurants and seafood stores up and down the East Coast — part of a growing industry that prizes quality over quantity. It's an industry that holds great promise for Maine but one that faces a significant challenge in the form of ocean acidification.
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The chemistry of our oceans is changing, according to scientists who monitor the world's coastal waters. Seawater is becoming more acidic thanks to too much carbon dioxide in our atmosphere. The shift isn't noticeable to humans and seawater remains essentially basic in terms of its pH level, but the change is robbing seawater of its carbonate ions — molecules made up of one carbon atom and three oxygen atoms that are critical building blocks for calcification and shell development. At risk is anything that builds a shell, including mussels, clams, oysters and even lobsters. Also at risk are the fishermen and small, independently owned companies that are the lifeblood of Maine's coastal economy.
Maine's bivalve aquaculture industry, primarily mussels and oysters, isn't a big industry, but it is helping Maine solidify its reputation as a source of high-quality seafood. With fisheries revenues declining due to dwindling stocks (see chart, below) and a challenging economic model, many communities and companies in the coastal economy are focusing on value-added products and charging a premium for quality instead of making money through volume — a paradigm bivalves fit into nicely.
But many in the industry aren't aware of the threat posed by rising levels of carbon dioxide in the atmosphere. "We didn't know much about ocean acidification at first," says Paul Dobbins, co-owner of Ocean Approved, the small-scale aquaculture operation that owns the rafts near Bangs Island. "But the more we learned, the more we knew that this was a big problem."
Scientists have long known that seawater acts like a massive sponge for carbon dioxide, pulling it out of the atmosphere and converting it into carbonic acid. The natural process has been going on for billions of years. But the amount of carbon dioxide in our atmosphere has pushed the reaction into overdrive, resulting in the highest acidity levels in hundreds of thousands of years. "[Acidity] has been going up since the late 1800s and there is really no question about it being from the burning of fossil fuels," says Joe Salisbury, a research scientist at the Ocean Processes Analysis Laboratory at the University of New Hampshire. "We have excellent inventories of the carbon dioxide produced from fossil fuels and the type of carbon dioxide showing up in the oceans. The numbers match perfectly."
Carbon dioxide in our atmosphere is driving most of the changing ocean acidity, but heavy rains can push areas of Maine's coastal waters into dangerous territory. Freshwater discharge from coastal rivers is acidic relative to seawater. Furthermore, nutrient-rich runoff, which can generate algal blooms like the ones that wreaked havoc on this summer's shellfish harvest, can increase the acidity of the seafloor. "The more organic matter you put on the bottom, the more carbon dioxide it will produce and the more acidic the substrate becomes," says Salisbury. "And when the freshwater discharge goes up and there's more mixing with the saltwater at the surface, it will stress the mussel and oyster larvae."
When acidity levels creep upward, juvenile bivalves, known as "spat," are the first to suffer. Shell growth is critical in these early stages, and pH levels that adult mussels and oysters are able to shrug off can dissolve spat looking for a place to settle. Scientists expect the overall acidity in the Gulf of Maine to reach levels that would affect mussel and oyster larval recruitment within the next 50 years. But periods of high rainfall could temporarily push acidity levels even higher.
Even adult bivalves are at some risk from elevated levels of acidity, since the metabolic cost of growing shell rises with increasing acidity. With the amount of carbon dioxide in our atmosphere, we're already locked in to enough acidification to negatively affect our fisheries. Energy otherwise intended for growth and reproduction will be diverted to shell growth, leading to what researchers estimate could be up to a 25% reduction in shellfish harvest by 2060 in Maine.
Researchers at the Woods Hole Oceanographic Institute report that ocean acidification could jeopardize the $3.8 billion (in 2007 dollars) in dock prices fishermen earn nationwide, called ex-vessel revenue. Of that, $748 million, or 19%, directly involves the harvests of mollusks — a group of marine organisms including bivalves, snails and squid — which are particularly susceptible to low levels of carbonate ions.
For New England, researchers estimate ex-vessel losses totaling between $300 million and $1.2 billion by the year 2060. This represents a small fraction of the overall New England economy, but the impacts are likely to be focused primarily in coastal regions. That's a problem, because many of the coastal areas are already under significant financial strain. "Many Maine fishermen are just breaking even as it is," explains Dick Clime from Coastal Enterprises Inc. "With mortgages on their homes to pay for their boats and their fuel and gear, most of them don't have the deep pockets necessary to survive much of a drop in harvests."
In many instances, when an industry shuts down, the resources and capital shift elsewhere and the net loss to the economy is small. In Maine's case, however, the coastal economy has few remaining options. "The relative impact is going to be higher for coastal economies," explains Dan Holland, . "It's not clear that you would have other industries stepping in to take the place of fisheries, so you're likely to have those regional impacts. And they are likely to persist over time."
The loss of mussel and oyster spat not only threatens wild populations of bivalves and the nearly $18 million annual harvest, it also jeopardizes mussel and oyster aquaculture operations. Many shellfish aquaculture operations source their larvae from the wild because of the high cost of growing larvae in tanks. This leaves them extremely vulnerable to fluctuations in wild populations — a vulnerability familiar to oyster farmers in the Pacific Northwest. Washington oyster farms experienced massive die-offs of their oyster spat in each of the last four years. The likely cause? Cold seawater upwelling from the deep Pacific has become so acidic it is now corrosive to oyster spat, according to researchers in the Pacific Northwest.
With aquaculture and wild-catch fisheries both in potential trouble, the economic reliance on Maine's lobster industry is heightened. After all, it was lobstering that helped coastal communities recover from the collapse of groundfish in the 1990s. But lobsters are at risk from ocean acidification as well.
Lobstering accounts for two-thirds of Maine's commercial fisheries harvest, by value, and drives the state's coastal tourism industry thanks to its pastoral setting. The $244 million lobster harvest in 2008, down from $285 million in 2007, dwarfs the harvest of any other marine organism in Maine, and lobsters are one of the most recognizable of Maine's products. The threat ocean acidification poses to lobsters lies less in dissolving the shells of juveniles and more in the cumulative impact of other stresses.
Lobsters actively pull carbonate ions from the surrounding seawater and accumulate them in their tissues before building their shells. Bivalves grow their shells using whatever concentrations of carbonate ions are present in the surrounding water. This means lobsters can handle the lower carbonate ion concentrations in an acidic ocean better than oysters or mussels. But fewer carbonate ions means more work for lobsters looking to build their shells, and therefore higher physiological stress.
Stressed organisms are more susceptible to diseases present in their environment. "Many people have commented about the increased number of pathogens that have struck reef corals, like white band disease," says Bob Steneck, a professor in the University of Maine School of Marine Sciences. "But these pathogens haven't evolved in the last decades; it's just that the cost of doing business in terms of calcification has gone up. It's the same thing with lobsters, and physiological stress is one of the prime reasons for the shell disease outbreak in southern New England."
In 1996, lobsters in the waters south of Cape Cod were afflicted by a rare shell disease. The outbreak crippled the Rhode Island lobster industry and has cost the coastal economies hundreds of millions of dollars. The trigger? A recent study points to the cumulative effect of a number of different factors, including increasing water temperature and lower dissolved oxygen. According to the study, these factors didn't cause the disease, they merely created conditions where a disease could spread like wildfire, infecting up to 45% of lobsters in some areas.
The lobster population in the Gulf of Maine is remarkably healthy, experiencing record high numbers, according to the Department of Marine Resources. But while high densities are good for harvests, they can facilitate disease transmission. Given the dependence of our coastal economies on lobsters, an outbreak on the scale of what happened in Rhode Island could be devastating.
Gone are the days when Maine's economy rose and fell on the back of its fishing industry. But for small, coastal communities with few options, wild-catch fisheries and aquaculture remain mainstays of economic development. Back on the mussel rafts, Dobbins explains that Maine mussels in general (and Ocean Approved's mussels in particular) are "some of the best tasting in the world." He worries that the long-term prospects of mussel farming in Maine are bleaker, thanks to too much carbon dioxide in the air.
"We've already had a few years of poor recruitment (low numbers of juvenile mussels) at one of our sites, and we don't know why that's happening," says Dobbins. "If it's acid related, and things get worse, we could end up getting no recruitment one year and we'd lose our whole year's crop. That would be a challenge."
Peter McDougall, a writer and marine biologist based in Freeport, can be reached at firstname.lastname@example.org.