Ocean acidification is reshaping marine food webs

The depths of the ocean are slowing climate change, but at a cost. Seawater acidity is increasing as the oceans absorb carbon dioxide from fossil fuels.

Average surface ocean pH is now 30% more acidic than in pre-industrial times. In a business-as-usual scenario, by 2100, ocean water could be over one-and-a-half times more acidic.

The effects of ocean acidification can be grouped into “known knowns, known unknowns, and unknown unknowns,” says Wiley Evans, a chemical oceanographer leading ocean acidification research for the Hakai Institute. Evans is riffing on an old Donald Rumsfeld quote because, although the underlying chemistry is straightforward, the full consequences of ocean acidification are uncharted. “A lot of what is unknown is at the species level,” he says. Only a handful of organisms have been thoroughly tested for their response to rising acidity. “And then there’s the ecosystem response – how impacts on one organism is going to affect the whole food web, and the ecosystems that those organisms reside in. So pretty quickly it snowballs into a lot of unknowns.”

Biologists, chemical and physical oceanographers, geneticists, and other experts are now working together to investigate these unknowns. Research into ocean carbonate chemistry has been done since the 1970s, but only in 2003 did the term ocean acidification come into use. It’s now an “exploding field,” says Evans. “It really is interdisciplinary, it relies on the physics and the biology, and the ecosystem level [research], and so ecologists and modellers and everyone is really working together to try and understand the breadth of the problem.”

The dynamic ocean environment makes the research complex. “The pH of the ocean changes constantly,” says Mark Spalding, president of the Ocean Foundation, an environmental organization based in Washington, DC. “It fluctuates daily, it fluctuates seasonally, it fluctuates with El Niño events, it fluctuates when you have an upwelling from the deep ocean.” Ocean acidity also differs by location. Natural factors dominate the exchange of atmospheric carbon dioxide into the ocean, but just as with atmospheric carbon, additional emissions have tipped the scales.

Ocean acidification also has to be disentangled from other co-occurring climate change stressors such as warming water and deoxygenation. Each effect is worrying on its own; combined, these multiple stressors are greater than the sum of their parts.

Despite this complexity, ocean acidification is caused by simple chemical reactions.

“It’s pretty much just like high school level chemistry,” says Evans. “As you increase carbon dioxide in the atmosphere, it’s not all going to stay there, it’s going to want to move into an area of lower concentration, and CO2 gas will dissolve into seawater.”

Gavin MacRae, Sentinel, 12 April 2019. Article.

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