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New Study Links More Acidic Ocean to Oyster Die-Offs

By Lauren Morello, E&E reporter
April 22, 2012; 10:56 AM

A new study links a shift in ocean chemistry caused by carbon dioxide emissions to massive oyster die-offs at a commercial hatchery in the Pacific Northwest.

Scientists have suspected that more acidic ocean water was to blame for the past several years, but the new study is the first to draw a direct connection between CO2-rich seawater and widespread oyster larvae deaths that began roughly five years ago.

"There are other people who have looked at oysters and seen they have sensitivity to CO2," said co-author Burke Hales, a biogeochemical oceanographer at Oregon State University. "But this is the first time this particular seed-stock problem has been tied to CO2."

Photo courtesy of James F Clay

For the Whiskey Creek Shellfish Hatchery, a small commercial operation on the shores of Netarts Bay, Ore., the problem began in 2007, when large numbers of larval oysters began dying. Other growers suffered similar crop failures. Taylor Shellfish Farms lost 60 percent of the oyster larvae at its hatchery on Dabob Bay in Quilcene, Wash., in 2008.

The industry scrambled to figure out what was going on, collaborating with scientists who investigated and abandoned one potential culprit -- the bacterium Vibrio tubiashii -- before zeroing in on the chemistry of seawater flowing into hatchery tanks.

It's more acidic than it used to be, a consequence of the increasing amounts of carbon dioxide oceans have absorbed from the atmosphere as humans' output of the greenhouse gas has risen.

Acidic water makes shell-growing difficult

Along the Pacific Northwest coast, that ongoing process is intensified by a natural ocean circulation pattern, the seasonal upwelling that pulls water from the deep ocean onto the continental shelf each spring. According to a 2008 study by researchers at the National Oceanic and Atmospheric Administration, the water stirred up by that natural upwelling has reached levels of acidity that scientists once believed wouldn't occur until 2050.

But that fast-forward is also a look backward. The deep ocean water stirred by the annual upwelling is the surface water of 50 years ago, increasingly richer in CO2 as the proportion increased in the atmosphere.

Now, as that old water resurfaces, it is creating new problems for shellfish growers.

With Hales' help, Whiskey Creek installed instruments to monitor the chemical profile of ocean water flowing through the lines that feed tanks of oyster larvae.

The new study, published in the journal Limnology and Oceanography, examines the data collected by those instruments and records of oyster larvae growth in 2009.

The scientists were able to compare the fate of oysters spawned during periods when the coastal upwelling was ongoing with oysters spawned when the upwelling had temporarily ebbed.

They found that more acidic water made it harder for the young oysters to develop shells, slowing their growth or killing them during key stages in the middle of their life cycle.

Concerns about a biological 'break-even'

The effect was apparent days after water was piped into hatchery tanks.

Researchers believe the water may be interfering with an explosive period of shell growth that occurs after a fertilized oyster egg hatches. In just two days, the shell-less hatchling transforms into a larva with a shell that accounts for 70 percent of its body weight.

Hales and his colleagues think that ocean acidification may be interfering with that initial shell formation. "It's like poor childhood nutrition in pre-K leading to low SAT scores later," he said. "If you're energetically strapped in the important early developmental stages, you pay for it later."

If CO2 emissions continue growing at the current rate, Hales says, the Whiskey Creek hatchery -- and others like it -- could reach a point where the oysters in its tanks at the end of a growing cycle weigh less than the mass of larvae it started with.

What Hales calls a biological "break-even" point could arrive in 20 to 30 years he says. But oyster growers may reach an economic break-even point even earlier.

"That's a break-even point for organisms, but as far as the hatchery is concerned, they can't make money at that rate," Hales said.

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