Ocean Acidification Outpacing Organisms’ Ability to Adapt Catherine McLean | Forward Thinkers

German marine biologist Ulf Riebesell and his team are taking a deep dive into research on effects that could impact our food chain.


Ulf Riebesell

When German Chancellor Angela Merkel visited the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, in 2012, she was well-informed about the dangers posed by ocean acidification. What Merkel wanted to know was whether marine organisms could ever adapt to these changing conditions.

It’s an important question with potentially dramatic implications for our food chain—and one that German marine biologist Ulf Riebesell has spent the past seven years exploring through his research. What’s becoming increasingly clear is that while some organisms may be able to adapt to lower pH levels, many more will be challenged to do so when exposed to additional stressors such as global warming.

These developments could lead to a decline in biodiversity in the ocean, with some organisms losing the ability to compete and eventually disappearing from their ecosystem.

“Evolution will not be fast enough to replace those species that are lost,” says Riebesell, a professor of biological oceanography who joined GEOMAR in 2003. “With declining biodiversity, we expect ecosystems to be less tolerant to further change. Many of the ecosystem functions we rely on will change—many of them not for the better—and food production is likely to be one of them.”

Developing countries, where the population depends on fish and seafood for protein, would likely be hardest hit by any future declines in fish stocks, Riebesell warns. In contrast, he says, Europe, North America, and other wealthy countries should be able to find replacements such as aquaculture fish.

‘A Gigantic Carbon Sink’
Ulf Riebesell For the past eight years, Riebesell served as the architect and co-coordinator of BIOACID, a network of 20 German research institutes that has examined the biological impacts of ocean acidification. The project wrapped up in November 2017.

“Eight years ago, we had the basic understanding that acidification may not be good for organisms,” Riebesell says. “Today we know more of the details, and we understand better the responses to ocean acidification.”

Roughly one-third of manmade carbon dioxide emissions released into the atmosphere end up in the ocean, transforming the waters into what BIOACID calls “a gigantic carbon sink.” When dissolved in seawater, C02 forms carbonic acid, which causes the ocean to become more acidic as pH levels decline. These ocean acidity levels have likely risen close to 30% since 1850 in a process that is invisible to the human eye. 

But the changes are real and threaten sea life. Calcifying organisms such as mussels, for example, need carbonate to form their skeletons and shells, according to a BIOACID report. However, the absorbed C02 causes chemical reactions in the seawater, which leads to the loss of carbonate. And survival rates in the early stages of some fish species are negatively impacted by global warming and ocean acidification, according to BIOACID.

Herring Larvae Get a Boost
When it comes to ocean acidification, global warming and other local stressors such as pollution, there’s no one-size-fits-all reaction. Survival depends on many factors, such as generation times and how the food supply is affected, and it varies from species to species.

Riebesell’s research team, for example, has focused on evaluating data from experiments on how herring larvae fare when ocean acidification alters their food web. The initial results in a lab in Kristineberg, Sweden, in 2013 were surprising: The herring larvae actually did better under high C02 conditions. The team followed up with an experiment with the herring larvae in 2015 in the waters off Bergen, Norway, and came to a similar conclusion.

The herring larvae’s good fortune apparently came down to timing. Acidification boosted the growth of some phytoplankton, increasing biomass and the abundance of zooplankton at a critical time when the herring larvae were just beginning to feed.

Multiple Ocean Stressors Multiply Effects
The herring larvae benefited from changes in the food web during the experiments, but this may not always be the case in nature, Riebesell says. Survival was tougher for cod larvae, according to another BIOACID research project: When exposed to high levels of C02, survival rates were halved for cod from hatching to the development of their gills.

“That’s one of the results which got a lot of attention at the European Union,” Riebesell says. “If we want to maintain cod populations at sustainable levels, then these potential effects of acidification need to be included when determining fishing quotas.”

Acidification isn’t the only threat to the ocean today, with multiple other stressors including global warming, eutrophication, overfishing, and pollution to consider. When combined, these stressors can amplify each other and their effects on marine organisms, according to Riebesell.

To help marine life better cope with global stressors like ocean acidification and global warming, governments need to take steps to reduce local stressors like pollution, Riebesell says. It’s a message he is keen for Merkel and other leaders around the world to hear.


Catherine McLean is a freelance writer based in Switzerland. She writes about a wide range of topics for both Swiss and international publications.


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