The ocean is becoming increasingly acidic as climate change accelerates and scientists are ramping up investigations into the impact on marine life and ecosystems. In just a few years, the young field of ocean acidification research has expanded rapidly – progressing from short-term experiments on single species to complex, long-term studies that encompass interactions across interdependent species.
“Like any discipline, it takes it time to mature, and now we’re seeing that maturing process,” said Shallin Busch, who studies ocean acidification at the National Oceanic and Atmospheric Administration’s (NOAA) Northwest Fisheries Science Center in Seattle.
As the ocean absorbs carbon dioxide from the burning of fossil fuels, the pH of seawater falls. The resulting increase in acidity hinders the ability of coral, crabs, oysters, clams and other marine animals to form shells and skeletons made of calcium carbonate. While the greenhouse gas effect from pumping carbon dioxide into the atmosphere has been known for decades, it wasn’t until the mid-2000s that the impacts of ocean acidification became widely recognized. In fact, there is no mention of acidification in the first three reports from the United Nations Intergovernmental Panel on Climate Change, issued in 1990, 1995 and 2001. Ocean acidification did receive a brief mention in the 2007 report summarizing the then-current state of climate science, and finally was discussed at length in the latest edition released in 2014.
But about halfway through that brief dozen years of acidification research, a shift started taking place.
“The early studies were just a first step and often quite simple,” said Busch of ocean acidification research. “But you can’t jump into the deep end before you learn how to swim.”
That started to change about five or six years ago, according to Philip Munday, who researches acidification effects on coral reefs at Australia’s James Cook University. “The first studies were often single species tested against ocean acidification conditions, often quite extreme conditions over short periods of time,” he said. “Now people are working on co-occurring stresses in longer-term experiments.”
That includes studying how acidification could change how organisms across a community or ecosystem interact – in other words, how the impacts on one species affect those it eats, competes with or that eat it. It also means looking at how impacts could change over time, due to species migrating or adapting, either in the short term or across a number of generations and how such effects may vary within the same species or even with the same population.
Nine examples of this new generation of acidification research are included in the latest issue of the journal Biology Letters. One study, for example, found that the ability to adapt to pH changes differed in members of the same species of sea urchins based on location. Another discovered that a predatory cone snail was more active in waters with elevated carbon dioxide levels but was less successful at capturing prey, reducing predation on a conch species. Another highlights that an individual organism’s sex can affect its response to acidification.
Munday, who edited the series of papers, said one of the major takeaways is that researchers are increasingly studying the potential for species to adapt to ocean acidification and finding those adaptations can be quite complex.
He pointed to a study on oysters. Previous work had shown that oysters whose parents were exposed to acidification conditions do better in those conditions than those whose parents weren’t. But in a new study, researchers found that when they exposed the offspring to additional stressors – such as hotter water temperatures and higher salinity – those adaptive advantages decreased.
All the studies call for including often-overlooked factors such as sex, location or changes in predation rate in future studies. Otherwise, researchers warn, impacts will be increasingly difficult to predict as the ocean continues to acidify.
“It’s far too early to make any sort of generalities,” Munday said.
The latest paper from NOAA’s Busch also cautions against generalities. By building a database of species in Puget Sound and their sensitivity to changes in dissolved calcium carbonate, she found that summarizing species’ sensitivity by class or order rather than the specific family can result in overestimating their sensitivity.
She compared it to similarities between people in the same immediate family versus people who are distant cousins. “There would be a lot more variation among those people because they’re not super closely related,” she said. “But when people started summarizing data really early in the field, there wasn’t much data to pull from. So it was done at a class level.
“Now that we have many more studies and information to pull from, how we draw summaries of species response should be nuanced,” she added.
Acidification research is likely to get only more nuanced in the years ahead. From the broad initial projections of average, ocean-wide surface acidity, for instance, researchers have started to pinpoint local pH projections, local impacts and local adaptations.
“We know the ocean is changing in a number of ways,” said Busch. “So just studying one of those factors without looking at the other changes in what’s going on in the ocean is not going to yield useful results.”