Historical comparison of competition among algae in waters around the Pacific Northwest provides more evidence for increased ocean acidification.
When we last spoke to Cathy Pfister and Tim Wootton about a year ago, they had just published an odd study they called their “No Hypotheses Are Supported” paper. Pfister and Wootton, biologists in the Department of Ecology and Evolution (who happen to be married to each other), have been studying the ecology of tiny Tatoosh Island off the coast of Washington state for decades, and have gathered a growing body of evidence that the ocean waters in this area are becoming increasingly acidic.
They’re convinced this is a result of climate change, as the ocean absorbs more and more fossil fuel carbon from the atmosphere. That “no hypothesis” paper addressed a number of alternate explanations for why this could be happening and knocked them down one by one. Now, a graduate student who has been working with them on Tatoosh has published another paper adding to the case for climate change, this time by studying one of the most basic organisms on the island: algae.
Sophie McCoy, a PhD candidate, examined competitive dynamics among crustose coralline algae, a group of species living in the waters around Tatoosh Island. They grow skeletons made of calcium carbonate, much like other shelled organisms such as mussels and oysters. As people burn fossil fuels and release carbon into the atmosphere, the ocean absorbs a big chunk of it, causing pH levels drop and make the water more acidic. Crustose coralline algae and shellfish have difficulty producing their skeletons and shells in such an environment, and can provide an early indicator of how increasing ocean acidification affects marine life.
“Coralline algae is one of the poster organisms for studying ocean acidification,” said McCoy. “On one hand, they can grow faster because of increased carbon dioxide in the water, but on the other hand, ocean acidification makes it harder for them to deposit the skeleton. It’s an important tradeoff.”
For this study, McCoy and Pfister repeated experiments conducted in the 1980s by University of Washington biologist Robert Paine. McCoy transplanted four species of crustose coralline algae to test sites to study how today’s ocean has changed how they compete with each other. The results, published this week in Ecology Letters, showed a reversal of competitive dominance among the different species, suggesting that increased ocean acidification is indeed altering biodiversity.In the previous experiments, one species, Pseudolithophyllum muricatum, was clearly dominant, “winning” almost 100 percent of the time over the other three species. In the current set of experiments, P. muricatum won less than 25 percent of the time, and no species proved dominant. McCoy called this new competitive environment “rock, paper, scissors dynamics,” in which no species has a clear advantage.
McCoy said that in the past, P. muricatum owed its dominance to being able to grow a much thicker skeleton than other species. Historical data show that in the 1980s it grew twice as thick as its competitors, but now P. muricatum no longer enjoys that advantage. Measurements from another recent study by McCoy in the Journal of Phycology show that it now grows half as thick on average, or roughly equal to the other species.
This decrease in thickness and loss of competitive advantage is most likely due to lower pH levels recorded over the last 12 years in the waters around Tatoosh.
“The total energy available to these organisms is the same, but now they have to use some of it dealing with this new stress,” she said. “Some species are more affected than others. So the ones that need to make more calcium carbonate tissue, like P. muricatum, are under more stress than the ones that don’t.”
McCoy said it’s crucial to continue studying the effects of ocean acidification in a natural context like Tatoosh Island instead of in the laboratory.
“This study shows different dynamics than what other people have found in lab studies,” she said. “Field sites like Tatoosh are unique because we have a lot of historical ecological data going back decades. I think it’s really important to use that in nature to understand what’s going on.”
McCoy S. & Pfister C. (2014). Historical comparisons reveal altered competitive interactions in a guild of crustose coralline algae, Ecology Letters, 14 JAN 2014 DOI: 10.1111/ele.12247