The rising acidity of the world’s oceans could devastate coastal communities around the United States over the next century, according to a new analysis. And because ocean acidification is exacerbated by other water quality problems such as agriculture and urban runoff, regions along the Gulf Coast and Atlantic seaboard, which were formerly considered less vulnerable, now look to be among the most at risk.
To date, most ocean acidification research has focused on better understanding which organisms will be the most vulnerable to rising acid levels, without considering the economic role those organisms play. The new work makes this link explicit, at least with shellfish, an industry that generates nearly $1 billion in the United States annually. The new work is “outstanding,” says Jeremy Mathis, director of the Ocean Environment Research Division at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle, Washington, who was not involved with the study. “This is exactly what we have been missing.”
The acidity of the world’s oceans has increased by roughly 30% over the past several decades, driven primarily by the burning of fossil fuels. As carbon dioxide levels rise in the atmosphere, about one-quarter of it dissolves into the top layer of the ocean. There it reacts with water to form carbonic acid, which in turn lowers the pH of the water. By doing so, it lowers the availability of carbonate ions, which oysters and other shellfish use to build their shells. Even more insidious, the drop in carbonate ions reduces a measure called the aragonite saturation state, which refers to the level of a mineral form of calcium carbonate called aragonite that oyster larvae need to form their growing shells. Laboratory studies on many shellfish have shown that if the aragonite saturation state falls below about 1.5, shellfish larvae are not able to build their shells and die before ever getting a toehold on life.
Numerous ocean chemistry and modeling surveys have shown that ecosystems along the West Coast of the United States are already vulnerable to high acidity levels. Part of this is natural: Cold water from the deep ocean upwells along the West Coast, and cold water is able to dissolve more carbon dioxide. Add in the rising atmospheric carbon dioxide levels, and the combination is already wreaking havoc among oyster fisheries in the Pacific Northwest.
Conventional wisdom in recent years was that shellfish farmers in the Pacific Northwest and Alaska, which is naturally also surrounded by cold water, have the greatest risk of losing their livelihood due to ocean acidification, says Julia Ekstrom, a social scientist who studies coastal communities at the University of California (UC), Davis. But that conclusion centered solely on acidity levels rising due to increasing atmospheric CO2 levels.
Ekstrom and her colleagues at the environmental group Natural Resources Defense Council, UC Davis, and 10 other institutions decided to look more broadly. They added in other factors that can impact acidity levels on a regional basis, including the discharge of water from large rivers and eutrophication in estuaries. Eutrophication occurs when agricultural and urban runoff boosts the levels of nitrogen and phosphorus that triggers algal blooms. When the algae die, they are eaten by bacteria that release carbon dioxide into the water as they respire. Large rivers, meanwhile, can either reduce acidification if they carry large amounts of minerals that buffer the acidity, or increase it if they hold few such minerals.
In addition to evaluating this broader set of physical factors, Ekstrom and her colleagues also analyzed the economic dependence of coastal communities on shellfish farming and their likely ability to adapt if ocean acidification threatens their current livelihood. Together those measures made up what they refer to as a social vulnerability score. The researchers then combined the physical and social risks to rank U.S. coastal communities to the threat of rising acidification, though only to shellfish farming.
The map they produced was very different from that considering the risks to ocean chemistry alone. As they report today in Nature Climate Change, they found that 16 of the 23 coastal bioregions around the United States are highly vulnerable to ocean acidification. Topping the list wasn’t the Pacific Northwest, but Massachusetts, due to the relatively high percentage of income that’s at risk due to shellfish farming and the moderately high acid levels expected in the region because of its latitude. Also at high risk were the Pacific Northwest, mid-Atlantic, and several regions along the Gulf Coast. For many of the regions along the Eastern Seaboard and Gulf Coast, the problem wasn’t the immediate threat of acidification due to atmospheric CO2 per se, but rather the added effects of nutrient runoff and rivers that raise regional acidity levels.
In a somewhat backhanded way that’s potentially good news, says Chris Gobler, a marine biologist at Stony Brook University in New York, because unlike rising atmospheric CO2 levels that require a global response to fix, those issues can be addressed by local and regional actions. That could give policymakers identifiable ways to reduce, or at least delay, the impacts of ocean acidification. “That makes this a great advance in the policy arena,” Gobler says, who wasn’t involved with the work.
But making those local and regional changes won’t be easy. The annual occurrence of an extensive dead zone in the Gulf of Mexico, due to agricultural runoff throughout the Midwest, has already been linked to modern farming practices. That hasn’t changed farming practices yet. Now the question is whether adding the risk of contributing to ocean acidification will make any difference. If it doesn’t, oysters, clams, and other shellfish could disappear from wide swaths of U.S. waters over the next century and with them the communities that depend on them for jobs and a way of life.