SEATTLE — The world’s oceans constitute an enormous heat sink, absorbing heat from the atmosphere. They also absorb carbon dioxide,and in the process they are becoming increasingly acidic.

Richard Feely, senior fellow at the NOAA Pacific Marine Environmental Laboratory in Seattle, described the chemistry during a climate change seminar presented by the Metcalf Institute of the University of Rhode Island.

The use of fossil fuels has released more than 2 trillion tons of additional carbon dioxide into the atmosphere in the past 200 years, and the oceans have absorbed about 25 percent of that. The carbon dioxide reacts with the water to create carbonic acid, increasing the acidity of seawater. Since the pre-industrial age, seawater has undergone a 0.1 pH change – an acidity increase of 30 percent.

The term “ocean acidification,” Feely said, refers to the process of lowering pH, not the end state. “This is the other side of the story of climate change.”

Evidence

Evidence of the changing chemistry shows up in sensitive marine creatures affected by increased photosynthesis in plants, decreased calcification of shells and changes in physiology.

At the event earlier this month, Feely described shifts in the diversity of phytoplankton, free-floating microscopic plants that produce chemical energy from life. These plants form the basis of the worldwide food web, which stretches from minuscule life forms to crustaceans to fish to humans.

At the top of the food chain, about 3 billion people worldwide depend on seafood for almost 20 percent of their animal protein, according to United Nations statistics cited by Eric Swenson, outreach director of the Global Ocean Health Program.

The value of seafood worldwide totals $217.5 billion, and in the U.S. it accounts for more than 1 million jobs.

Shallin Busch, research ecologist at NOAA’s Northwest Fisheries Science Center, said the acidified environment near Italy’s Mount Vesuvius is a window into how different communities react to the chemical changes.

Creatures like shellfish, urchins and coral draw calcium carbonate from the surrounding water to form their crystalline structures. As carbon dioxide levels increase, it becomes harder for the animals to grow the crystals. The more delicate the shell, the more sensitive it is to changes in chemistry, she said.

Responses in the food web will be complex as some species are degraded and predator-prey interactions change, she said. “Not all species have to be sensitive to affect the food web.”

Seeing the effects

Aquaculture along Pacific Northwest shorelines is seeing the effects of acidification. Bill Dewey, from Taylor Shellfish Farms in Shelton, Wash., said shellfish are “the canary in the coal mine,” providing a warning about what is happening in ocean ecosystems.

In the first two days of life, oyster larvae precipitate 90 percent of the body weight as calcium carbonate shell, using energy derived from the egg. If they expend too much energy building their shell, they cannot mature.

Operators at the oyster farm noticed some of the shellfish were showing resistance to the growing acidity, so they have begun breeding oysters than can adapt.

The farm is also increasing its research and monitoring at hatcheries and expanding the larval capacity at its hatchery in Kona, Hawaii, whose warmer waters are a refuge of sorts from the acidification effects in the Northwest. 

Besides building resilience, Swenson said, the shellfish industry might protect vulnerable larvae from “bad water” and protect important habitats by limiting localized effluents.

Feely said that as the oceans’ pH has changed over the past 550 million years, ecosystems had time to adjust because dissolving sediments rebalanced the acidification. Fossil records show that major life die-offs correlate to short-term acidification events. 

“The current rate of acidification is nearly 10 times faster than any period over the past 50 million years,” he said.

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