The rise and fall of some species of algae competing in the coastal waters around the Pacific Northwest suggests ocean acidification is already impacting marine life.

The impact is an “early warning sign,” according to lead study author Sophie McCoy, a doctoral candidate in ecology and evolution at the University of Chicago.

Why algae? It is an “indicator species” sensitive to increasingly acidic water as the ocean absorbs more carbon dioxide from the atmosphere, McCoy said.

It is the first step in looking more broadly at how global climate change is altering biodiversity, according to the new University of Chicago study published Wednesday in Ecology Letters.

“In general, coralline algae are at the base of the food network so they can affect the ecosystem in ways that others couldn’t,” McCoy said.

Crustose coralline algae are commonly found in all of the world’s oceans and recognized by its typically pink color. Like oysters and other shelled organisms, the species grows skeletons made of calcium carbonate, a main ingredient in chalk.

“Calcified algae are the ones likely to suffer most quickly,” said Megan Dethier, a research professor of zoology at the University of Washington. “It begins to answer the question, ‘What is ocean acidification going to do to us?’”

McCoy and Cathy Pfister, professor of ecology and evolution at the University of Chicago, began the study in 2010 with a series of boat trips to Tatoosh Island, Wash. They roamed the rocky beach just before dawn to transplant algae to test sites.

The island’s extensive ecological history provided a baseline to study ocean changes. McCoy repeated biologist Robert Paine’s experiments from the 1980s as a “before” snapshot to compare to the modern data.

“Sophie’s work is terrific in documenting in dynamics of coralline algae changing through time,” said Paine, professor emeritus at the University of Washington.

For two years McCoy examined how quarter-sized pieces of four different species of crustose coralline algae grew on top of one another to compete for dominance.

“They have a fantastic system for looking at long-term change without a time machine,” Dethier said. “Many of us wish we had time machines for looking back at what it was like 20, 50, years ago.”

Yet unlike past results, there are no clear winners and losers. Instead of a hierarchy determined by the survival of the fittest, currently no species has an advantage. The competition more closely resembles “rock, paper, scissors dynamics,” McCoy said.

Historically, the reigning champion, Pseudolithophyllum muricatum, dominated almost 100 percent of the time. But McCoy reveals the species’ success rate dropped to less than 25 percent due to ocean acidification that impacts its skeletal development.

No longer the alpha algae, the species lost its edge because of thinning skeletons, measured by McCoy in another recent study.

She attributes the changes to lower pH levels recorded during the last 12 years in the water around Tatoosh Island, which is a result of ocean acidification stressing the species. Carbon dioxide causes faster growth, but makes it more difficult to deposit skeletons.

After comparing the present to the past, McCoy is looking to the future. She plans to create computer models with the experimental data to interpret what it could mean for local biodiversity.

“Things are clearly changing out there,” Paine said. “Every marine biologist in the world should be concerned with ocean acidification.”

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