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Global warming's evil twin threatens West Coast fishing grounds

Within the next few decades, ocean acidification – an effect of global warming – could leave sea creatures along the West Coast unable to maintain their protective shells, according to a new study.

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The team selected the location because of its high biodiversity and its economic value as a source of seafood. But they also selected it because the waters are naturally more acidic than waters in many other parts of the Pacific.

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The reason: Water rises from the deep ocean during natural upwelling events, and it's naturally more acidic than water near the surface. The upwelling brings nutrients that have led to the area's high biological productivity. But the water's relatively high acidity means shell-builders start their process in already stressful conditions. This is particularly true of juveniles.

Over time, organisms have adapted to this "background" acidity. From a shell-builder's perspective, as long as the seawater is overstocked with calcium, magnesium, and dissolved carbonates, it can process these to build and maintain its shell and do so while spending a minimum amount of energy on the job. From a biogeochemist's perspective, water is supersaturated with the necessary building blocks.

Ocean acidification's effect

But the ocean also is absorbing roughly 26 percent of the carbon dioxide humans are releasing to the atmosphere through burning fossil fuel and through land-use changes. This CO2 from above dissolves in the seawater, forming a very weak carbonic acid.

As seawater becomes increasingly acidic, it eats into the water's inventory of calcium and magnesium. If the seawater becomes undersaturated in these minerals, shells dissolve because their inhabitants can't maintain them in the face of insufficient raw materials.

Generally, the water itself isn't lethal, Gruber says. Instead, it retards the organism's growth, making it harder to survive.

Even with relatively low growth in human-triggered CO2 emissions, by 2050 the saturation levels of key minerals drops quickly. Within the next 30 years, the top 200 feet of water near shore is likely to become undersaturated throughout the summer. By 2050, more than half the waters in the study area become undersaturated all year. But sea-floor habitats could see year-long undersaturation within the next two to three decades, the study projects.

A double whammy

The results come against a backdrop of increasing acidification in waters throughout the Pacific Basin. In a study accepted for publication in the journal Biogeochemical Cycles, Dr. Feely and colleagues show that on average, what he terms the "corrosive layer" of ocean water has risen through the upper 2,000 feet of water at a pace of about three to six feet per year between 1991 and 2005.

"But there are locations where it rises much faster than that," he continues. "The Washington-Oregon-California coast is particularly vulnerable to these kinds of processes because of the combined impacts of anthropogenic CO2 and upwelling. This draws highly corrosive waters onto the [continental] shelf."

Although the California Current system was the focus the new study from Gruber and his colleagues, the problem occurs elsewhere. Other key locations include a vast region of upwelling stretching north off the coast of Peru, as well off Portugal and in two locations along Africa's west coast. In the Atlantic, however, the effects of acidification are less pronounced because it harbors a larger inventory of calcium, magnesium, and carbonates than does the Pacific, Gruber says.

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