Ocean acidification causing long-term effects on competition among species
As Sophie McCoy packed up samples stacked across a table in her old lab in the Zoology building at the University of Chicago, we joked that it was like moving out of your parents' house for good. McCoy finished her PhD in Ecology and Evolution at UChicago in 2014, and has since taken a new job as an assistant professor at Florida State University—but not before coming home to visit and pick up the rest of her stuff.
"The problem is, I can't fit it all in my car. There's too much," she said.
Those samples are fragile pieces of rock collected over the years from the coastline of Tatoosh Island, off the coast of Washington state. The rocks are crusted with various species of crustose coralline algae, the plants that became the focus of McCoy's work at UChicago.
They grow outer skeletons made of calcium carbonate, much like mussels and oysters, and have become a useful organism for studying the effects of climate change on marine life. As humans burn fossil fuels, they release extra carbon into the atmosphere. The ocean absorbs a big chunk of that extra carbon, changing the water's chemistry. It's causing pH levels to drop, making the water more acidic. Crustose coralline algae and shellfish have a hard time producing their shells in more acidic water, adding extra stress that's already changing the dynamics of competition among species in these waters.
In 2014, McCoy and her PhD advisor, Cathy Pfister, Professor in the Department of Ecology and Evolution, repeated experiments conducted in the 1980s by University of Washington biologist Robert Paine. They transplanted four species of algae to test sites to see how they grew and competed with each other, and saw that the dominant species in the 80s, Pseudolithophyllum muricatum, was evenly matched with others in modern experiments. What changed?
P. muricatum used to grow a skeleton that was twice as thick as the others, giving it a competitive advantage. But in current, more acidic waters, its skeleton was half as thick, or roughly equal to the other species.
In a new paper in the journal Proceedings of the Royal Society B based on her PhD work at UChicago, McCoy, Pfister, and Stefano Allesina, Professor in the Department of Ecology and Evolution, compared data from the 1980s to McCoy's recent field experiments, and created a model to forecast how these algae species would interact 20-30 years into the future. The model system predicted that a new dominant species, Lithothamnion phymatodeum, emerged, but not nearly as dominant as P. muricatum back in its glory days. In the past, invertebrate "grazers" that feed on the algae used to play a big role in shaping competition between species, but going forward, their role was also greatly diminished.
"The grazers helped level the playing field a little bit, or mix it up. They were introducing these 'rock, paper, scissors' dynamics where anybody could win at any given time," McCoy said.
What seems to be happening now is that ocean acidification is stressing the algae, not because more acidic water is literally dissolving their skeletons, but because they have to expend a lot more energy to produce them.
"That sort of knocked the feet out from under the competitive dominant, and let everyone else have a field day," McCoy said. "So now in the new system, since they're all still stressed by ocean acidification, that's preventing any one of them from becoming a true dominant."
The results also show that the ecosystem isn't stable: one previously dominant species has already been knocked off its perch (or rock), and projections show a new one emerging, though in slightly different ways. For McCoy, who first studied chemistry as an undergrad, this latest study shows how even the smallest changes can disrupt an ecosystem.
"It made me see the links between chemistry and the ocean, and how it could affect these living systems," she said. "Even these small changes can be really important. They can scale up in ways that you don't imagine, or have more long-term effects than you think."
As she sorted through the dozens of Ziploc bags and boxes containing algae samples from her work, trying to figure out how she could transport them to Tallahassee, she also talked about the value of being able to build upon decades of work of by other ecologists.
"A lot of that data is accessible, and there are a lot of samples like this in archives and museums that people just don't know what to do with," she said. "In the digital age, the fact that there are still these physical archives, especially as we consider what's going on in the world now and how it's changed over time, can still answer what I think are pretty modern questions."