Global warming's evil twin threatens West Coast fishing grounds

Global warming's evil twin threatens West Coast fishing grounds

 

Over the next few decades, coastal waters off of California, Oregon, and Washington are in danger of becoming acidic enough to harm the rich fisheries and diverse marine ecosystems there, according to a new study. Blame it on global warming's evil twin.

The process changing the seas' chemistry has been dubbed "ocean acidification." It refers to the impact that rising carbon dioxide levels in the atmosphere are having on seawater. CO2 levels are increasing as humans burn fossil fuel and change land-use patterns. The oceans absorb up to 26 percent of those emissions - a number that is expected to go up as the Arctic Ocean loses more of its summer sea-ice cover.

By 2050, the team conducting the study estimates, more than half the near-shore waters governed by the California Current system are likely to become so acidic throughout the year that many shell-building organisms will be unable to maintain their armor . That point could come within the next 20 to 30 years for some sea-floor habitats on the continental shelf, the researchers estimate.

While the team anticipated it would see marine conditions deteriorate with rising atmospheric CO2 levels, "I was really surprised to see how quickly some of these changes will be occurring," says Nicolas Gruber, a biogeochemist at the Swiss Institute of Technology in Zurich who led the team.

The team "points out fairly clearly that if it wasn't for anthropogenic carbon, we wouldn't be passing that tipping point" from encroaching, acidic water, says Richard Feely, a senior scientist at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in in Seattle. "That's a very important part of that paper."

Although the study doesn't directly address the question of which creatures get hit hardest first, the team does suggest that other studies indicate oysters could be vulnerable, especially as juveniles. Still, the team acknowledges that some organisms are hurt by even small changes in acidity, while others can tolerate larger changes, at least for relatively short periods of time.

The results were posted Thursday on ScienceExpress, the online outlet for research journal Science. Science will publish the results in paper form later.

A delicate environment

To some, the phrase "ocean acidification" may trigger visions of house keys melting in the surf. While the changes are more subtle than that, at least on a human scale, they can harmful to many forms of marine life.

As CO2 dissolves in the ocean, seawater gradually acidifies. Shell-building marine creatures - ranging from tiny plankton to headliners for bouillabaisse and bisque - have a far more difficult time building and maintaining their protective shells. The tiny creatures that build coral reefs also have a harder time drawing on the chemical construction materials once available to them.

In this new research, Dr. Gruber's team conducted modeling studies of the effect that rising CO2 levels are likely to have on ocean chemistry along a stretch of coastline influenced by the California Current system, which runs from that runs long the West Coast from British Columbia through the southern end of Baja California. The area the team focused on stretches from Point Conception, near Santa Barbara, northward to the California-Oregon border.

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.

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.