As carbon dioxide changes the sea, shellfish biologists work to adapt

To anyone who has spent a languid summer afternoon tumbling in the waves on South Beach or watched the earth’s closest star dip into the horizon at Menemsha, the ocean can seem eternal and unchanging. But scientists are increasingly discovering that human activity is transforming what was once thought to be an invulnerable resource. The ocean is getting warmer, more acidic, louder and filled with the detritus of civilization. What effect these changes will have on the ocean’s inhabitants in the decades to come is unclear.

But while global warming gets all the headlines, there is a much less talked-about problem with carbon dioxide that makes many shellfish growers nervous: ocean acidification.

Acidifying Oceans

“Human emissions are roughly 10 billion metric tons of carbon a year and the ocean takes up roughly a quarter of that,” said Woods Hole Oceanographic Institution senior scientist Scott Doney, who was President Obama’s pick for chief scientist of the National Oceanic Atmospheric Administration before his nomination was blocked for over a year in Congress.

Put simply, atmospheric carbon dioxide reacts with seawater to create carbonic acid. As a result, after soaking up centuries’ worth of carbon dioxide, the ocean is now 30 per cent more acidic than it was in pre-industrial times, and it is headed toward levels not seen in tens of millions of years. For calcifying animals like shellfish, whose juveniles simply dissolve outside of a very narrow pH range, the results could be devastating. According to one study carried out by researchers at Stony Brook University, the impacts are already here. That study looked at quahaugs reared in different tanks of seawater with varying levels of dissolved CO2, representing the pre-industrial ocean, the modern ocean and the future ocean. The study found that not only were the shells reared in water with acidity levels projected for the end of the century smaller, more defective and thinner, but that modern-day quahaug shells paled compared to those raised in pre-industrial seawater, which were far hardier and healthier. It could mean that modern oysters are much more vulnerable to disease than in decades past.

“Acidification is really a chronic illness that builds up over a long period of time and is background stress on animals,” said Mr. Doney. “We’re expecting to see very rapid change and very large change in the next several decades. One of the things we don’t know yet is if there is a tipping point.”

In the Pacific, oyster growers have gotten a sneak preview of that tipping point in recent years, as upwelling events that have brought acidic water to the ocean surface have wiped out whole oyster farms and businesses. Mr. Karney does not expect such catastrophic acidification to happen for decades, but he said development around the Island’s estuaries can simulate those acidic environments. When runoff from fertilizer or discharge from septic tanks hits Island ponds, it spurs algae growth which dies and is consumed by bacteria. The bacteria can rob the ponds of oxygen while they metabolize, but they also emit CO2, making the waters more acidic.

But the Vineyard is not without a defense.

Last summer Vineyard Vision fellow Jessie Kanozak began a mission to return Island oyster shells to their homes, driving from restaurant to restaurant and collecting hundreds of buckets of used oyster shells as part of the Martha’s Vineyard Shellfish Recovery Partnership, which she founded. This summer she has upgraded her program with a pick-up truck and hopes to install a mechanical lift. While Vineyarders have been taking millions of wild oysters out of the saltwater ponds in recent years, which is good news for consumers, very few shells were going back in the water, which is bad news for juvenile oysters. Most spent oyster shells from Island restaurants had wound up in the trash. Now more than ever, the calcium carbonate shells are vital to the life of the ponds. Not only do they provide a hard surface for juvenile oysters to set (known as culch), but they can also buffer against increasing acidity.

Or as Mr. Karney puts it:

“Shell is essentially the Rolaids of the water.”

In a forum earlier this year, Ms. Kanozak explained her young program.

“The community really likes the idea of harvest-to-table and back,” she said. “This program is simple and it closes the loop.”

Curiously, not all shell-bearing animals react the same to more acidic water, as former Woods Hole researcher and North Carolina professor Justin Reis discovered. Mr. Reis subjected 18 different animals to high CO2 water, from urchins to corals. While oysters, clams and bay scallops struggled, crustaceans such as lobsters and crabs, which are able to adjust their internal pH, were able to use the dissolved carbon to make more shell and grew faster.

“I’ve actually been contacted by some aquaculture people who have begun to bubble CO2 into their systems to enhance their growth rates of blue crabs, which was kind of an ironic impact of the study,” he said. “But this study did hit home that the bivalve shellfish industry will be hit the most hard.”

Peter Brannen, The Vineyard Gazette, 17 August 2012. Full article.

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