Scientists, shellfish growers wary of rise in ocean acidification

NARRAGANSETT — As more carbon dioxide is pumped into the earth’s atmosphere, approximately 25 percent of it is being absorbed by the ocean, where it forms carbonic acid and changes the pH balance of seawater. Ocean acidification is a less-studied effect of the complex process of climate change, but one that many researchers believe is important to understand.

Scientists are trying to determine how a more acidic ocean will affect the organisms that live there. Some of them, oysters, clams and sea scallops in particular, are commercially important to Rhode Island, and a lower pH makes it harder for the mollusks to obtain the calcium carbonate they need to build their shells.

“The impact that has gotten the most attention is the effect on organisms that use calcium to build skeletal structures,” said Jason Grear, a research ecologist at the Environmental Protection Agency Laboratory in Narragansett. “That would be things like shellfish and corals. But there are also plankton that use calcium in building their cellular structures, and there are even marine snails that swim in the water column. All of those are organisms that use calcium minerals to build their structures are therefore really sensitive to changes in pH, because that change in pH changes the availability of those calcium bio minerals.”

Robert Rheault, executive director of the East Coast Shellfish Growers Association, which represents growers from Maine to Virginia, is waiting for more scientific evidence to show how an increasingly acidic ocean will affect oysters and clams.

“Right now, the science on the impacts is weak,” he said. “The only thing that we know for sure is that the larvae, in that first 48-hour period before they start feeding, are tremendously susceptible to dissolution. Their energy budget goes negative because they haven’t started to feed yet, and if they haven’t got enough energy in that egg and they’re starting to dissolve, then it takes extra energy to lay down shell, and they sometimes don’t make it.”

Another threat to shellfish larvae lurks in muddy, coastal ocean bottoms. Nutrients that accumulate in the mud consume oxygen and create more acidic conditions, and Rheault said quahog and soft-shell clam larvae were particularly vulnerable.

“An algae bloom or seaweed rotting, that rotting produces CO2 and the muds, the black, anoxic mayonnaise that stinks like sulfur dioxide, it’s so acidic that the larvae, they swim for a few weeks in the water column, they look to settle down, and if they come upon muds like that, they’ve got a choice,” he said. “They either sit on the surface and get eaten, or they dig in and dissolve. It’s not a great choice for this dust speck-sized baby clam.”

Shellfish growers are able to alter the water chemistry, but wild populations are more vulnerable.

“Just meter in a little bit of carbonate, and we can restore the balance and fix the pH and they can form their shells fine,” Rheault said. “It really doesn’t matter where the CO2 is coming from. If you’re a larva swimming around, if it’s too acidic, too hard to form a shell, you’re screwed.”

The impact of acidification is intensified in coastal environments, where runoff from land adds more nutrients to the water. These in turn can produce a low oxygen condition called hypoxia, which suffocates organisms that are unable to swim away.

“If you think about an organism breathing, you can think of an ecosystem the same way,” Grear explained. “When we breathe, we’re consuming oxygen, just like those organisms on the bottom of the bay that are causing hypoxia. They’re breathing in oxygen, but they’re exhaling carbon dioxide. That’s a different process that is contributing carbon dioxide to subsurface waters in the coastal environment, and that can potentially exacerbate the ocean-driven part of acidification.”

Plankton, microscopic plants and animals at the base of the food chain, also affect pH levels. Just like plants that grow on land, phytoplankton, which are also plants, absorb carbon dioxide during the day, raising pH levels, and then respire at night, giving off carbon dioxide and lowering pH levels once again.

Susanne Menden-Deuer, a biological oceanographer at the University of Rhode Island’s Graduate School of Oceanography, studies plankton, and how they react to changing pH levels.

“We were interested in how ocean acidification affects whole communities,” she said. “Prior research had looked at individual species. Some of them did better when pH was lower and some of them did worse, so there was no clear picture of how the community would respond. We have done one study in Narragansett Bay on the whole community, showing that it is not a clear-cut ‘things will be better or worse’ in terms of plankton production with ocean acidification. There is a lot more research needed.”

The Northeast Regional Association of Coastal and Ocean Observing Systems monitors pH levels in the Northeast and coordinates the Northeast Coastal Acidification Network. Members of the New Hampshire-based network include industry representatives, scientists, and federal and state government representatives.

“We have different monitoring,” said the association coordinator, Cassie Stymiest. “In the Gulf of Maine, we actually help fund the only long-term buoy off the Isles of Shoals in New Hampshire and Maine that is documenting CO2 change.”

Stymiest agreed that more science is needed to understand and mitigate the effects of acidification.

“We’re doing a lot of measurements in Narragansett Bay,” she said. “We have a monthly survey that we do that includes measurements of acidification and nutrients. In addition to that, there’s a buoy network in Narragansett Bay that is established by a consortium of different institutions, and a lot of that is coordinated by URI. They’ve begun to put pH sensors on their buoys at different times of the year. We’re not advocates. We’re just pushing to prioritize the questions that need to be answered and to do the research and make the recommendations of the work that needs to be done.”

Rheault said sea level rise presented the greatest and most immediate threat to the shellfish industry.

“Sea level rise is going to hit us faster and harder than we thought, and talk about a real change to my industry, that’s going to be real. Because all the sewage treatment plants are within 50 feet of seawater, right in that sweet spot of the next Hurricane Sandy.”

Grear said, “In the open ocean, I do think the evidence is clear, and there’s reason to be concerned. It gets more complicated in the coastal environment, because we haven’t been working on this problem as long in the coastal environment. We don’t understand it as well, and we don’t know when we see these biologically driven variations in pH, how new are they for these organisms? How new is that dynamic for these organisms? Is that part of the normal variation that you would expect to occur in the coastal environment, or is it somehow the variation and excursions somehow getting more and more extreme? And is it interacting with ocean acidification in a way that is something we should be concerned about?”

Menden-Deuer said studies of the entire ocean ecosystem were needed.

“I speak as a person, not as a scientist,” she said. “I think we have a dire need to understand our oceans and coastal ecosystems better. Ocean acidification is a very important part of that, but the oceans are facing so many challenges, increases in temperature, increased utilization of coastal areas, pollution — I personally think that we need to study the system as a whole and its response to a multitude of stressors. I wouldn’t put a hierarchy on what is most important. They’re all fundamentally important for our well-being.”

Cynthia Drummond, The Westerly Sun, 2 December 2015. Article.


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