Ocean acidification is already driving changes in Northwestern marine ecology

 

For a time, Pseudolithophyllum muricatum was king of the kelp forest understory around Tatoosh Island, a rocky blip of land off the northwestern tip of the Olympic Peninsula. In experimental "bouts" staged there by famed ecologist Bob Paine that pitted the crusty, milky red algae against other species of coralline algae it lived amongst, P. muricatum "won" almost 100 percent of the time, growing more abundantly than any of its competitors. Its edge was its especially thick crust, which allowed it to slip over the lip of its more thinly crusted neighbors and overtake them.

In 2012, Sophie McCoy, a doctoral candidate at the University of Chicago, collected samples of P. muricatum from Tatoosh Island and compared them with samples Paine had collected in the 1980s. Her samples were only half as thick as his. Now, by repeating Paine's experimental plant battles, McCoy has shown that P. muricatum has indeed lost its competitive edge. "It's now winning only about a quarter of the time," she says. "It loses to basically everybody some of the time. That's a huge change."

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The experimental bouts between different species of coralline algae on Tatoosh Island are performed by transplanting specimens onto marine cement in kelp beds and watching their growth over time. Courtesy Sophie McCoy.
The cause of this paradigm shift? Most likely, says McCoy, it's the downward creep of the ocean's pH, caused in large part by the vast amounts of carbon dioxide the ocean has absorbed since humans began burning fossil fuels. This phenomenon, known as ocean acidification, was once described by former National Oceanic and Atmospheric Administration chief Jane Lubchenco as the "equally evil twin" of climate change.

The waters that lap Tatoosh Island are already experiencing pH creep at a rate 10 times faster than models predicted. Combine that fact with the decades of detailed ecological data collected there by Paine and his disciples, and you've got an ideal place to study the ecological consequences of changing ocean chemistry.

Coralline algae are particularly good organisms to probe to begin to understand these consequences. They sit at the bottom of the food chain, meaning changes in their competitive relationships -- which control the abundance and composition of different species -- are likely to have ripple effects. Coralline algae are also expected to be particularly sensitive to changes in water chemistry. That's because they're highly dependent on bicarbonate for photosynthesis and carbonate to build their hard skeletons. As the water's pH changes, bicarbonate levels go up and carbonate levels go down. So far, McCoy says, whether coralline algae ultimately benefit from increased bicarbonate, or suffer from decreased carbonate "seems to be really species dependent." For instance, a decrease in carbonate means the algae have to expend more energy to maintain their skeletons, which are at greater risk of dissolving in the acidifying water. This explains why P. muricatum has begun to thin: It's become too taxing to build and maintain such a thick skeleton. Thinner algae, on the other hand, need less energy to maintain themselves, and therefore might take less of a hit.

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P. muricatum overgrowing L. phymatodum, another species of coralline algae that McCoy studied. Courtesy Sophie McCoy.

But for this recent study, McCoy was less interested in how individual species were reacting than in how those reactions altered the competitive dynamics of entire communities, and whether those changes might impact the overall diversity of plants and animals in the long run. How the loads of carbon we're pumping into the atmosphere will alter global and local biodiversity is a big question, and this study provides an early look at how marine life in the Northwest is already changing.

That said, the ultimate implications for biodiversity are still uncertain. Though acidification is negatively affecting P. muricatum, that's not necessarily a bad thing for overall diversity. Though in the past Tatoosh Island's coralline algae had a simple, ladder-like hierarchical structure, with P. muricatum on the top rung, it didn't blanket the kelp forest's understory in a monoculture like cheatgrass in the Great Basin. Rather, its abundance was kept somewhat in check by snails and urchins that grazed it and, in turn, made room for other plants. Now, these predators don't seem to have much of an effect on the composition of the plant community. "What I think is happening is that those changes in seawater chemistry are more stressful than the (snails and urchins)," says McCoy. In other words, one source of stress has overwhelmed the other, and proven a more formidable foe for P. muricatum. That's not necessarily a bad thing for overall diversity, assuming other species of algae are less impacted. If ocean acidification eventually causes the populations of P. muricatum to decrease, which seems likely, it could open up more room for other species of algae to colonize than the snails and urchins made available.

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Courtney Stepien, left, and Sophie McCoy, right, in the field on Tatoosh Island. Courtesy Cathy Pfister.
The most important insight to be gleaned from McCoy's study, however, is perhaps more simple: The natural world is changing rapidly, often out of view, in ways we are only beginning to understand. Change tends to happen very slowly in natural systems, and starts with processes that are invisible to the naked eye. It is especially hard to pick up on changes in communities of long-lived, slow-growing species like coralline algae through observation alone -- by simply surveying how many of each algae species there are around Tatoosh Island, for instance. For now, P. muricatum, much of it established years, decades, or even centuries ago, is still the most common coralline algae in the area. But the new experimental bouts showed that it's no longer the strongest. As McCoy writes in her paper: "The direct measurements of altered competitive dominance we observed experimentally may be the first indications of pervasive ecological change in this system."

Cally Carswell is the assistant editor at High Country News. She tweets @callycarswell.

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