How rising ocean acidity could send us into a downward spiral

Our oceans are getting more acidic, and it’s having big effects on some very small animals—with worrying implications.

Ocean acidification, a result of excess carbon dioxide in our atmosphere, can disrupt plankton blooms, according to new research published in Nature Geosciences. It’s a troubling finding, scientists say, because those blooms are helping mitigate some of the effects of carbon dioxide pollution.

The plankton in question are coccolithophores, single-celled organisms smaller than the pixels on your monitor that make their energy with photosynthesis. Coccolithophores are known for their scaly armor of round calcium carbonate plates, the stuff of sea shells. They produce about half of all the calcium carbonate in the oceans.

Calcium carbonate, as the name suggests, is part carbon. Animals like coccolithophores get that carbon by ingesting carbon dioxide, which dissolves from the atmosphere into seawater. That’s where ocean acidification enters the story.

The oceans absorb about a quarter of the carbon dioxide we dump into the atmosphere by way of smokestacks and car exhausts. Carbon dioxide makes water more acidic (that’s why sparkling water has a little bite to it) and since the industrial revolution we’ve decreased the oceans’ pH from about 8.2 to 8.1. That might not sound like much, but if your blood pH fell by one-tenth of a point you’d start getting sick. Before humans’ influence, ocean pH had been stable for thousands of years.

Higher acidity makes it harder for coccolithophores and other organisms to build their shells because it limits the supply of carbonate. Research has shown that our more acidic oceans are corroding the shells of some animals, including oysters, and impacting their populations. The effect on coccolithophores may have much wider reaching consequences.

Like many plankton, coccolithophores have an annual bloom, a period of massive reproduction and growth when conditions are right. Some coccolithophore blooms can be seen from space. When it happens, the plankton suck up a tremendous amount of the dissolved carbon dioxide near the sea surface.

When they die shortly thereafter, they take that carbon with them to the ocean floor, locking it away so it can’t return to the atmosphere and contribute to climate change. It’s one of the only natural processes we know of that can sequester carbon for a long time.

Except it doesn’t happen in more acidic seawater. When study author Ulf Riebesell and his colleagues increased carbon dioxide levels to what we might see near the middle or end of this century, the coccolithophores just didn’t bloom.

Riebesell says his group believes the problem begins before the bloom starts. “You can imagine being a phytoplankton cell you have to survive the whole year round, and then you wait for this very short period when you explode,” he says. They found that this pre-bloom population had decreased by a factor of about five under more acidic conditions.

“When the time was right for bloom the seed population was way too low to even get going,” says Riebesell, an oceanographer at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany.

The lack of a bloom goes beyond coccolithophores. In normal conditions they tend to clump together with other organic matter—dead plankton, poop, that kind of thing—and drag it all down with them, taking even more carbon out of the system.

Without that, a feedback loop can develop that amplifies the effects of carbon dioxide, explains Christopher Gobler, an oceanographer at Stony Brook University.

“As the ocean acidifies, there’s going to be fewer coccolithophores, less carbon sinking in the ocean and therefore more going into the atmosphere,” says Gobler. “It’s really one of the true fears that scientists have about climate change.”

Without a coccolithophore bloom to remove carbon from near the ocean surface, that water is already saturated with dissolved carbon dioxide, meaning it has less capacity to absorb CO2 from the atmosphere. More carbon dioxide in the atmosphere will make our world heat up faster and make the oceans more acidic, perpetuating the cycle.

And Riebesell thinks we should be thinking beyond this one species of plankton. “For me the coccolithophores are kind of the canary in the coal mine,” he says. The changes his group saw in coccolithophores may happen to other animals as well, with just as broad effects.

Like many scientists these days, Gobler is looking beyond the research.

“It would appear that on the policy front this country is at a pivot point with regards to how we deal with carbon emissions. There are important decisions to be made and very real consequences. I think everybody needs to be aware of the consequences so these decisions are made in the light of reality,” Gobler says.

Sam Lemonick, Forbes, 13 December 2016. Article.

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