September 19, 2012
Oceans of Change
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Researchers wonder whether critical sea species will be able to adapt to global ocean acidification from carbon emissions in our lifetime
The tiny invertebrate Thecosomata has several names. Sometimes marine researchers call them sea butterflies for the delicate, wing-like lobes they use to propel themselves through the water. Other scientists know them as pteropods or “flapping snails.”
Dr. Gretchen Hofmann, an eco-physiologist at UC Santa Barbara, is concerned that the tiny sea butterfly of the Southern Ocean near Antarctica has become an unlikely harbinger of change.
“They’re these not terribly charismatic little gastropods,” Hofmann explains, “about the size of a peppercorn. They make this beautiful shell, and they occur in clouds in the water, and they’re an incredibly important part of the food chain because fish eat them. And then other things eat the fish.”
But now the sea butterfly is a canary in a coalmine. As more and more anthropogenic carbon dioxide (CO2 produced by human activity) cycles through the Earth’s atmosphere and into its oceans, the fundamental chemistry of the oceans is changing. The world’s oceans are becoming at least 30 percent more acidic than they were a century-and-a-half ago. Ocean acidification is a problem that has scientists all over the world worried, because it could mean the end of the line for sea creatures like mollusks, urchins, corals, and sea butterflies.
The Southern Ocean is already a tough place to make a living as a marine animal: because of the sub-zero water temperatures, it’s also the world’s most acidic ocean. “It’s more acidic down there because cold water absorbs more carbon dioxide gas. The water is just right above the freezing point of seawater, at minus 1.86°C [28.7°F],” which, Hofmann points out, is “really cold!”
The pH of the world’s oceans is a product of atmospheric carbon dioxide being absorbed into the water. In the water, dissolved carbon dioxide molecules join water’s hydrogen and oxygen atoms, and is strung into a longer chemical called carbonic acid (H2CO3).
This means that under normal circumstances, a lot of the free-flowing carbon atoms in the water are being recruited to make carbonic acid. “It’s just chemistry,” Hofmann explains.
“I can change the pH of a glass of water just breathing into it. CO2 from my breath dissolves right into the water. Ocean acidification is like that, but on a larger scale.”
Nearly half of the atmosphere’s carbon is dissolved into the world’s oceans. This carbon moves in a cycle from carbon dioxide to carbonic acid to carbonate, the mineral used by mollusks, corals, and other sea creatures to build shells and skeletons. The carbonic acid not used as carbonate sinks and eventually settles on the ocean floor, where it remains sequestered for decades.
Studies have shown that this cycle has changed since the Industrial Revolution. As humans create more carbon dioxide, the oceans absorb much of the excess carbon. A third to half of the atmosphere’s excess carbon dioxide ends up in the ocean – sometimes referred to as the oceanic sink. These surplus carbon atoms become surplus carbonic acid.
“Even though these organisms are adapted to this environment, will we start hitting the guardrails of their tolerance? How acidic is too acidic?”GRETCHEN HOFMANN
And because more CO2 dissolves into the Southern Ocean’s freezing, already-acidic water, the process of ocean acidification is expected to accelerate around Antarctica. “More gas will dissolve into that cold water,” Hofmann says, “and over time the rate of decline of pH, the reduction of the carbonate ions, is going to happen in Antarctica first, and possibly faster and sooner in time than anywhere else.”
This is bad news for the pteropod, and other creatures like it that use the ocean’s carbon to build their shells. “The saturation of calcium carbonate in the water in Antarctica is very low already,” she continues, “so making a shell there, if you’re a pteropod or a scallop or a mussel, is challenging. Their shells are incredibly brittle and thin.” In fact, Hofmann accidentally crushed the first Antarctic urchin she picked up.
Pteropods are the bottom rung of a delicately constructed food chain in Antarctica, one that goes all the way up to penguins, elephant seals, albatrosses, and whales. Hofmann explains, “pteropods are an important pressure point for the whole ecosystem in terms of food.”
By 2050, the world might see what happens when an ocean becomes too acidic for species like the sea butterfly to survive. The big question is, for Hofmann, “Even though these organisms are adapted to this environment, will we start hitting the guardrails of their tolerance? How acidic is too acidic?”
Urchins and Upwelling
Hofmann and her team’s interest in the effects of changing ocean pH began quite literally in their own backyard with the purple urchins at Campus Point, a stone’s throw from their lab on UCSB’s campus.
The California purple urchin is important for two reasons. The first is because urchins are among the only sea creatures adapted to regular changes in pH. The acidity of their Pacific Coastal waters, from the Santa Barbara Channel north nearly to Alaska, fluxes on a cyclical basis.
“It’s a well known and well studied oceanographic process that there’s a global circulation of the ocean,” says Hofmann. At certain times of the year, and in certain areas of the world’s oceans, winds blowing across the surface of the water cause deeper water to be pulled up toward the surface. This process is called “upwelling,” and all of the creatures up and down the western Pacific coast are used to this periodic change in their water. They’ve even adapted to cope with the changing chemical composition caused by the movement of the water.
“That deep water holds a lot of CO2,” Hofmann continues. “Oceanographers can tell you that the CO2 in the deep ocean is on average about 50 years old. As that CO2–rich water comes up from deep in a process called ‘shoaling’, it drifts up onto the coast.”
At any given time, the winds and the upwelling can cause the pH of the Santa Barbara Channel to fluctuate. “So we can’t just say that the average pH in the Channel is 8.1,” says Hofmann. “A lower pH might only last for three, maybe five days, but then it goes away. And then it comes back again when the winds come. It’s episodic.”
Marine creatures, such as the purple urchin, are physiologically adapted to this shoaling of acidic water. They’ve learned how to wait out the pulses, and gather the carbon they need to make their skeletons and shells after the shoaling passes. The trouble is that the urchins, oysters, and other marine life along the Pacific coast are used to pulses of water containing levels of CO2 from a half-century ago. Researchers began to wonder how urchins will cope with water that contains twenty-first century levels of CO2.
This concern made Hofmann and her team wonder: could they create a facsimile of the Pacific Ocean 50 years in the future in their lab? And once they did that, how would they know if the variations in acidity and temperature were irrevocably harming the urchins?
This is the second important physiological point of interest about purple urchins: They have the distinction of being one of the only creatures to have their embryonic development recorded by microscope, and they were the first chordate whose entire genome was sequenced. Knowing the entire purple urchin’s sequenced DNA code means scientists could theorize whether changes in its environment, for example, will be immediately visible in its genes.
Completed in 2003, the U.S. Human Genome Project developed new techniques to test which genes were responsible for which characteristics, from eye color to cancer predisposition. “As they developed these techniques,” says Hofmann, “animal ecologists and biologists like me were excited to co-opt that technology for our research. So we asked: can we pick up signatures of physiological response to stress from the environment by using genomics?”
The answer was: Yes.
Simulating Future Oceans
Hofmann’s team has recreated a variety of future oceans in their lab. Dr. Pauline Yu, the lab’s senior post-doc and an expert on larvae, studies the physiological responses of larval urchins to a variety of scenarios based on projections of anthropogenic warming and ocean acidification. Yu and the Hofmann team “ask” the urchin larvae about stress responses by extracting their RNA and checking for a physiological response to stress.
They discovered that laboratory conditions of increased dissolved CO2 inhibit the ability of the urchin larvae to complete certain molecular processes necessary for their development. At both increased CO2 levels and higher temperatures, many larvae died before they’d even had a chance to develop.
The lab’s interest in urchins isn’t just ecological: the urchin is hugely important to local fisheries. Uni, or urchin gonad, is considered a sushi delicacy and its market price reflects that. In recent years, the California urchin catch—which averages 10 million pounds per year—has been valued at over $15 million wholesale.
Urchin fishermen all along the Pacific coast began to notice that their carefully managed populations of urchins were either shrinking in size or disappearing completely. Several local urchin fishermen asked Hofmann if something about the ocean was changing. So they began working with local fishermen to answer their questions about changes to the Pacific purple urchins’ environment.
Meanwhile, Hofmann’s colleague at Scripps Institute of Oceanography at UC San Diego, Andrew Dickson, was also interested in the ocean change going on right in his backyard. He and other oceanographers wondered what was going to happen when anthropogenic CO2 dissolving into the ocean met other stressors, such as fertilizer run off, in the “urban ocean” between San Diego and Santa Barbara. They used pH sensors called SeaFETs — an ion sensitive field effect transistor — to monitor the pH of ocean water around the world, and offered to share the SeaFETs’ data with researchers who were trying to answer similar questions.
Both Hofmann and Dickson wanted to learn more about what was going on chemically to cause these changes, and whether the effects were localized or diffused across a broader region. Hofmann presented a proposal for a multi-campus research initiative to the UC Office of the President, and it was funded. She brought in colleagues from the Bodega Marine Laboratory at UC Davis who were investigating how ocean acidification was affecting northern California oyster and mussel hatcheries.
“We immediately understood that studying ocean acidification would be a multi-disciplinary effort over a longer period of time,” Hofmann says of the consortium’s origins. “We needed to train students and post-docs in this new, synergistic field where we integrate a knowledge of oceanography—and the SeaFET sensor technology—with its significance in biology.”
The consortium has also set the stage for other collaborations including OMEGAS [Ocean Margin Ecosystems Group for Acidification Studies] and C-CAN [California Current Acidification Network]. OMEGAS is funded by the National Science Foundation (NSF), and includes researchers from the original three UCs, as well as UC Santa Cruz, Oregon State University, Stanford, the University of Hawai’i, and the Monterey Bay Aquarium Research Institute. C-CAN is a collaborative project between researchers like Hofmann and Dickson and people with an economic stake in ocean acidification, such as owners of oyster hatcheries and urchin divers. C-CAN’s goal is to identify the causes of shellfish depletion along the Pacific coast and explore solutions that can help sustain populations.
Soon, groups like the National Forest Service and National Oceanic and Atmospheric Administration became involved. With funding from the National Parks Service, UCSB graduate student Lydia Kapsenberg installed SeaFETs in the Channel on two islands in the Channel Islands National Park. Other sensors in the area are run by Dr. Carol Blanchette, a research biologist at the Marine Science Institute at UCSB, and Libe Washburn, a professor of geography who studies coastal circulation and air-sea interaction.
“Any single group attempting to understand the genetics, physiology, and oceanography at once would be spread too thin, explains Hofmann. This is a collaboration that not only extends across many labs, but many universities, several oceans, and, as it turns out, many continents.
The Hofmann Research Team (L to R): Oliva Turnross, Tyler Evans, Jacqueline Padilla-Gamino, Morgan Kelly, Emily Rivest, Professor Gretchen Hofmann, Lydia Kapsenberg, Pauline Yu, and Paul Matson. Not pictured: Geoff Dilly.
The Ocean’s Lungs
Antarctica and the Pacific Coast are not the only two outposts of investigation for Hofmann and her team. On Moorea, an island in French Polynesia nine miles from Tahiti, several of Hofmann’s graduate students are studying the effects of acidification on corals at an LTER — Long Term Ecological Research laboratory.
The National Science Foundation funds 26 LTERs all over the world. Over the last two decades, the intention of the LTERs has evolved from studying ecology to studying environmental change. The laboratories range from the Arctic and Antarctic, to stations in the deserts, grasslands, mountains, and oceans — including the Santa Barbara Channel — of North America, all the way out to Moorea, the only LTER dedicated to studying the ecological processes of a coral reef.
In Moorea, graduate student Emily Rivest examines how the dual stressors of high temperature and acidification affect coral larvae. Much like they have done in their UCSB lab, Rivest and her fellow researchers built a system of seawater filled buckets, and calibrated each to a particular range of stressors — high temperature, low pH, or some combination thereof. As with the urchins, the team checks the coral larvae’s physiological response to these changes.
Corals are very narrowly adapted to their environment. While urchins have adapted to the pulses of acidic water washing up from the deeper Pacific, coral larvae will only flourish in water that fits into a narrow band of pH and temperature. Thanks to the SeaFET sensors, researchers now know that the pH of coral reefs is diurnal.
This means that the pH of the water in a coral reef is regulated by the respiration of the Symbiodinium — the algae that are symbionts in the cells of the invertebrate coral — on a daily basis. Coral reefs are therefore among the only places in the world where the pH of the water is not due to outside forces, such as upwelling on the Pacific coast or the extreme temperatures of the Southern Ocean. Hofmann points out, “Out there on the reef, it’s biology driving the exchange of carbon in the water.”
This “diurnal signature of respiration” in coral reefs is significant not only because it creates its own pH levels, but because coral reefs are, in a certain sense, the lungs of the ocean. The massive exchange of CO2 between reefs and the atmosphere is hugely important for the health of the atmosphere, as well as the oceans.
Hofmann reminds us, “You could also argue that coral reefs are economically important, too, because a lot of nations are dependant upon the health of coral reefs for their economy.” Besides being home to nearly a quarter of the entire world’s marine species, tropical coral reefs are major tourist attractions.
Globally, travel and recreation associated with tropical coral reefs is valued at over 100 billion dollars. As with urchin harvesting and oyster farming, Hofmann feels that the degradation of tropical reefs by ocean acidification could mean economic disaster for people who depend on the biodiversity there not just for food, but for tourism, too.
“The Maldives and little island nations are in big, big trouble,” said Hofmann. The people of these nations rely on the ocean for their economy.” Despite their small carbon footprint, people in small island nations like French Polynesia, New Guinea, or Indonesia could have their whole lives upended by the effects of ocean acidification.
The Bottom Line
Ocean acidification is not just a pollution problem: it’s an economic and food security problem. This rings true for Hofmann, her research lab, members of the consortium — and people who run fisheries, urchin harvesters, or tour operators.
“Global-change biologists talk about four outcomes when environments start to change,” Hofmann explains. “One option is that the organism can migrate — change its range, go somewhere else where conditions are most hospitable to them. They can also acclimate or climatize, use their plasticity and their flexibility to stay where they live and keep functioning. Or, they can truly adapt, come up with a new genetic solution to the problem. And the fourth option is extinction. Populations that are less able to adapt or climatize are very vulnerable to extinction.”
The same set of choices might be what’s in store for human beings.
Economically, says Hofmann, when changes due to acidification start to happen “the intensity of the impact will be local, really focal and intense.” Bleached or algae-covered corals will translate to less revenue for people who rely on tourism or fishing, and more acidic waters will stunt or exterminate the larvae of pteropods, urchins, and corals.
“If ocean acidification is allowed to take hold, people who depend on the ocean for their livelihood might have to give up and move away. Further down the line, people who love seafood might have to adapt to a world with fewer options and less food security. It’s not an exaggeration to say,” she warns, “that there could be some things that aren’t around in a hundred years that used to be around. If things get bad enough, important seafood will be really challenged.”
The question of future food security is just one way Hofmann and her team make the threat of anthropogenic ocean acidification real for the public. In Hofmann’s experience, “I haven’t heard a lot of skepticism about it. When I talk to people, it clicks really quickly. People love the oceans, and ocean acidification is basic chemistry.”
“That’s how I explain ocean acidification to the average person. And it resonates.”