Ocean Acidification Makes Shellfish Suffer, Too
Changes in the chemical makeup of our oceans pose a serious threat to coral reefs. But the damage that ocean acidification causes extends far beyond the tropics. Cold-water ecosystems are being hurt as well — as are industries that rely on those ecosystems.
Here’s one example: In the mid-2000s, oyster larvae began dying en masse in production facilities on the West Coast.
“The mortality was complete – 100 percent – at some of our hatcheries,” says Terry Sawyer, co-founder of Hog Island Oyster Company, which rears, harvests and serves the succulent bivalves on the shore of Tomales Bay in Marin County.
At the time, Hog Island was buying its juvenile oysters from four hatcheries in California and Oregon. Sawyer would place orders and pay in advance, but for several years, business became unpredictable. Inventory die-offs swept through the hatcheries in waves. The facilities could not reliably meet the demand, and while oyster farms, like Hog Island, struggled to keep their own production up to speed, some hatcheries nearly folded.
It didn’t take long for scientists working with the industry to observe a key correlation: Just prior to the oyster die-offs at the hatcheries, the seawater being pumped into the facilities registered a sharp dip in pH levels – the telltale symptom of ocean acidification.
“This was the first time we’d seen an economic impact of ocean acidification in the States,” says Chris Langdon, a biologist and professor of fisheries at Oregon State University who, since 1986, has been studying how West Coast shellfish respond to changing water chemistry.
Few people had been tracking the ocean’s pH before last decade’s oyster crisis began, but it is safe to guess, says Langdon, that average pH levels dipped below a critical threshold around 2007.
“It was like a switch – 2004, 2005, 2006 were all right, and then all of a sudden things fell apart,” Langdon says.
After several years of high mortality and lagging production, scientists designed and developed – and helped the facilities install – equipment capable of both detecting unfavorable water chemistry and adjusting it as needed before the water was pumped into the hatchery tanks. Today, hatchery production is more or less back to normal, and the industry as a whole is, at least for the time, stable.
But acidification itself has not gone away. It’s getting worse, and is on a trajectory to seriously upset ocean chemistry in the decades ahead. That poses new problems for aquaculture and wild shellfish populations alike.
“The effects are increasing – we’re seeing it happen,” Sawyer says. “These are early signals. Our industry is an indicator of real global changes happening now. We’re at the front lines.”
Acidification is a direct result of carbon dioxide emitted into the atmosphere, which not only creates a warming greenhouse effect but also diffuses into the ocean, where it dissolves into seawater and creates carbonic acid. Carbonic acid occurs naturally in the ocean but becomes more problematic for some organisms at greater-than-usual concentrations.
The ocean, thankfully, is not anywhere near acidic. Chemically, it remains alkaline. However, accumulation of carbonic acid is nonetheless lowering the ocean’s pH. This makes it more difficult for many invertebrates to build shells and exoskeletons. Eventually, the impacts of ocean acidification could be monumental. Though some creatures may benefit, acidification will destroy coral reefs and seriously harm populations of animals like krill and pteropods.
For oysters, acidified water poses the greatest challenges very early in their lives – especially during a four- to six-hour period during which the larval oysters begin forming shells. To build this first layer of protection around their bodies, the larvae secrete aragonite, a form of calcium carbonate. The aragonite serves as body armor for several weeks as the larvae float freely in the water column. Eventually, they attach to substrate and grow into the familiar twin-shelled shape of adult oysters.
But the larvae have difficulty producing aragonite once pH levels drop past a certain level. This is precisely the fact of chemistry that surfaced about a decade ago at the West Coast’s oyster hatcheries.
In fact, the problem literally surfaced. Langdon notes that the hatcheries’ larval die-offs often came immediately after strong upwelling events along the coast. Upwelling occurs when wind-driven surface turbulence draws cold, deep bottom water, laden with nutrients, toward the surface. Upwelling is the principal engine behind the productivity of the West Coast’s chilly ocean waters.
However, since phytoplankton that eats CO2 near the surface eventually dies and sinks, that CO2 accumulates at the bottom of the ocean, where it forms carbonic acid. So while upwelling provides nourishment for the nearshore, shallow-water ecosystem, it can also deliver acidified seawater.
Now, Langdon is collaborating with oyster farmers to develop plans of action to combat – or, if necessary, adapt to – acidification. Fortunately, there has been some progress. He says scientists have observed that individual oysters of the commercially important species Crassostrea gigas (the Pacific oyster) have shown genetic variability in how they respond physiologically to acidified water. This, Langdon explains, means there could be potential to select and breed oysters that can tolerate lower seawater pH.
“But we don’t know if this is a heritable trait [that can] be selected in breeding programs,” he says.
Even if breeding for resilience to acidification is a possibility, this solution may work only for a time. Acidification, he explains, may eventually outdo species’ capacity to adapt and evolve.
“There will be a point where biology can’t compensate,” Langdon says. “There are chemistry boundaries that biology won’t be able to break.”
Acidified water negatively affects adult oysters and mussels, too.
“They’re smaller, and their shells are thinner, weaker and more vulnerable to predators,” says Tessa Hill, a UC Davis associate professor in Earth and Planetary Sciences. Hill has closely studied invertebrate responses to acidified water at the Bodega Marine Laboratory and has also been collaborating with Hog Island Oyster Company.
Currently, she and colleagues are running experiments on red abalone, the large edible sea snails highly valued by both a recreational dive-based fishery and as a product of aquaculture facilities. Through one tank of the mollusks the biologists are pumping seawater with a pH level adjusted to match acidity levels of about a decade ago. The other group of abalone is living in highly acidified water approximating what scientists predict to be the chemistry of seawater about a century from now. Hill says she and her team are still waiting to see how the abalone respond.
A similar sort of experiment was conducted on the Great Barrier Reef by scientists from Stanford who pumped de-acidified water over a parcel of coral. The coral thrived compared to adjacent coral immersed in natural ocean conditions. That, the authors concluded in a paper published last year in the journal Nature, was because higher seawater pH levels – like those of the past – allowed more rapid production of aragonite. That study conclusively showed that coral reefs are already suffering the consequences of acidification.
So are pteropods, small pelagic sea snails, and krill, both foundational members of the marine food web and both of particular importance to West Coast salmon.
The impacts of acidification will probably linger far into the future, even if all carbon dioxide emissions stopped tomorrow. That’s because the atmospheric and oceanic changes that have been set in motion would be carried forward by a powerful momentum.
“There is a lag time,” Hill says.
That is, there is a delay between greenhouse gas emissions and observed atmospheric and oceanic effects. That means the planet is almost certainly committed to decades or centuries more of both temperature increases and increasingly acidified water. Hill warns that every belch of CO2 humans put into the atmosphere can be considered “a long-term commitment.”
Fortunately, there is at least a little wiggle room to make use of with mitigation measures. Hill, in collaboration with Sawyer at Hog Island, is currently studying how kelp and eel grass beds affect water chemistry. In much the same way in which terrestrial plants pull CO2 from the air, both kelp and eel grass draw carbonic acid out of the water. Planted strategically around shellfish farms, they could potentially boost the pH of the water back to more invertebrate-friendly levels.
That would be a Band-Aid measure that doesn’t address the root of the problem – specifically, humanity’s persistent dependence on incinerating fossil fuels. Still, Hill is hopeful that societies will successfully transform and adopt more sustainable ways.
“We as individuals and as government have great power to change the current trajectory,” she says.
Sawyer, as an oyster grower and as a collaborator on some of Hill’s and Langdon’s research, has every reason to be as hopeful. His business partner, he says, was recently in Washington D.C. discussing with lawmakers the need for energy reform as a tool for stopping acidification. However, Sawyer is uncertain the driving sources of acidification can be thwarted quickly enough to stave off serious ecological and economic impacts.
“The sail and the rudder have been set on that ship,” he says. “As the pH [of seawater] keeps dropping, you’re eventually going to start seeing effects on adult oysters. We’re in deep doo-doo at that point.”
