Catherine McLean Journalistin/Texterin/Übersetzerin

Ocean seafood failing the acid test

June 10, 2015

German marine biologist Ulf Riebesell says the unchecked pace of ocean acidification threatens to deplete future supplies of seafood and fish.

The increasing acidity of the world’s oceans is happening silently and invisibly for now. But the impact on our food chain—including declining numbers of certain edible species—will become more and more visible in coming decades, predicts German marine biologist Ulf Riebesell.

“The oceans aren’t pristine anywhere, anymore,” explains Riebesell, who is among a growing number of researchers carefully studying how ocean acidification is impacting marine ecosystems, benefiting some (the smallest plankton) and damaging others (mussels and calcifying microalgae).

“Ecosystems will change. We know there are going to be winners and losers…the next step from there is how will this affect human [food] consumption,” adds Riebesell, a professor of biological oceanography at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, and the architect and coordinator of BIOACID, a German national project on the Biological Impacts of Ocean Acidification. “By 2050, there will be major changes.”

Shell shocks

Along with global warming and lower oxygen levels in certain parts of the ocean, ocean acidification is a major human-influenced factor in the changing ocean ecosystem, says Riebesell. Roughly one-third of manmade carbon dioxide (CO₂) released into the atmosphere ends up in the ocean, he says. When dissolved in seawater, CO₂ forms carbonic acid, which in turn causes the pH levels in oceans to decline, making them more acidic.

Ocean acidification has occurred before in the Earth’s history, but the current process is happening 10 times as fast as during any other period over the past 55 million years. Ocean acidity levels have jumped 26 percent since 1850, although such changes can’t be seen with the naked eye because CO₂ doesn’t change the color of the seawater or the fish.

Riebesell admits that up to now, there are few clear examples where ocean acidification has had a direct impact on the human food chain. But he points to the oyster industry along the West Coast of the United States as one portent of the potentially devastating impacts of ocean acidification. During the past decade, falling pH levels have killed off billions of young oysters. Some hatcheries have been forced to relocate elsewhere. Others have introduced measures to limit the impact of ocean acidification, including seawater monitoring and controlling the acidity levels in oyster larvae tanks.

Indeed, mollusks such as oysters and mussels are expected to be among the biggest losers from ocean acidification, according to a recent symposium summary for policymakers that Riebesell helped write. Increased acid levels in seawater reduce the concentration of carbonate ions, dissolving the shells of mollusks, which are made up of calcite and aragonite. While fish in general appear to be less sensitive to ocean acidification than other organisms, the growth rates of their larvae and their food sources may be impacted, the summary suggests, which could affect the abundance of fish in the future.

The winners in ocean acidification, in contrast, are at the bottom of the marine food chain: pico phytoplanktons, which are smaller than 2 micrometers in diameter, according to Riebesell.

“They would increase their growth rate, they would increase biomass, and they would get a larger share of the available nutrients and take the nutrients away from others,” he says. “It’s kind of bad news that these tiny guys benefit the most.”

The dark side of CO2

Ocean acidification is still a relatively new area of research. The topic didn’t even factor into Riebesell’s post-doctoral research work at the Marine Science Institute at the University of California, Santa Barbara in 1992, following his completion of a doctorate at the University of Bremen, Germany, a year earlier. Instead, his focus was on CO₂ sequestration: the long-term storage of CO₂ in the deep ocean. A key question was whether extra CO₂ had a positive, fertilizing impact on phytoplankton. (Ocean pH levels then were considered irrelevant, and the term ocean acidification didn’t exist.)

“At that time, environmental impact was mostly thought to be pollution in coastal waters: human-produced substances introduced into near-shore waters that could do harm to organisms and communities,” Riebesell explains. “The open ocean was still considered mostly pristine at that time. That also reflected my research.”

He started to probe the negative side of extra CO₂—acidification—only when he returned to his native Germany as a scientist at the Alfred Wegener Institute in Bremerhaven in 1994. Working with coccolithophores—calcifying microalgae—Riebesell observed that low pH levels had a negative impact on them and that the calcification process became more difficult.

“That’s when I came to realize there’s more to it,” says Riebesell.

Open water effects

Riebesell believed he would have to change his research methods to effectively investigate the negative impacts of ocean acidification. Instead of studying single organisms in a laboratory and extrapolating the results, he needed to go out into the open waters and study the changes across entire marine ecosystems.

In 2003 he moved to GEOMAR, where he spearheaded work on a flexible and robust system that could be deployed anywhere in the world by research vessels. GEOMAR eventually built so-called mesocosms—giant floating structures that resemble test tubes. The mesocosms are open at the top and extend down 20 meters so that large volumes of water can be captured, enclosing many organisms and communities. Researchers add additional CO₂ to the mesocosms to test what will happen to these ecosystems in the future if CO₂ levels continue to rise. A full-diameter trap for sediment at the bottom of the mesocosms allows the researchers to collect samples.

The mesocosms that Riebesell uses are innovative in terms of their versatility: They can be used in various parts of the ocean and in different climates. They can also handle rougher conditions in the open water, with waves up to 2 meters in height, according to Riebesell. This “opens up the opportunity to study ecosystems beyond well-protected in-shore waters,” he says.

Recently Riebesell has teamed with other researchers to successfully deploy these mesocosms in various locations, including the waters off Norway and Spain’s Gran Canaria island. The study of CO₂ levels in open waters is already providing valuable new information about the impact of ocean acidification across entire ecosystems, according to Riebesell.

“Some organisms in the wild responded differently from what we had seen in the lab before,” he says. “If one [organism] has [even] small disadvantages, [another one] can take over, which we would never see in isolated lab experiments.”

He points to an experiment in Bergen, Norway in 2011, in which he discovered that ocean acidification had a “devastating” effect on a certain species of coccolithophores. “They could not fill their ecological niche anymore, and they completely got wiped out in the high CO₂ mesocosms,” Riebesell says. “It was the biggest eye-opener for me.”

Riebesell’s latest trip took him back to Norway in May 2015, where his research group is raising herring—a staple in Northern European cuisine—in ocean water with high levels of CO₂. This has been done in laboratories, but the fish there still had an adequate food supply. That may not be the case in the ocean itself, he says, as entire ecosystems adapt to declining pH levels. Riebesell believes the new research will help shed light on the impact of ocean acidification on the full marine food chain—and the repercussions for humans.

“We’re still missing the connection from changes at the base of the food web to really what matters to human society,” he says.