Some have called it “the other carbon dioxide problem.” As the world’s oceans absorb part of the anthropogenic increase in CO2, their chemistry is changing, becoming more acidic. Lower pH reduces the amount of calcium carbonate that marine invertebrates can use to build their shells. The impact on vulnerable species and food webs could be catastrophic—and could happen too quickly for these organisms to adapt. It’s not just a concern for tropical coral reefs and polar oceans: California is on the frontline.
Recent research indicates that the coastal upwelling that nourishes the productive ecosystem of the California Current is also bringing more acidic water from the depths to the surface. It enters estuaries like Tomales Bay and San Francisco Bay, which also receive low-pH water from runoff. According to UC Davis geologist Tessa Hill, this poses a double threat to shelled creatures — including commercially valuable oysters and mussels — as they transform from larvae to juveniles.
Hill is one of four scientists heading the Bodega Ocean Acidification Research (BOAR) program at the Bodega Marine Laboratory. They’re monitoring the chemistry of offshore, nearshore, and estuarine water, and rearing marine organisms under varying levels of acidity. Initial research has focused on native oysters (Ostrea lurida) and mussels (Mytilus californianus), which appear vulnerable. Previous studies elsewhere had documented reduced shell growth in commercially-farmed species.
Free-swimming organisms are not exempt. Fish exposed to acidic waters in early developmental stages grow more slowly and have lower reproductive rates. Acidification impairs blood oxygen transport and respiration in squid. The shells of pelagic snails, a major prey item for juvenile pink salmon in the North Pacific, can dissolve under acidic conditions.
“There will be winners and losers,” Hill notes. Crabs and sea urchins, whose shells are a mix of calcium carbonate and organic material, appear less susceptible than mollusks. Other reports suggest jellyfish, eelgrass, and cyanobacteria will thrive under more acidic conditions. Acidification may even change the acoustic properties of seawater, affecting whales.
“The science is very sound,” Hill says. There’s no debate as to whether ocean acidification is happening, only its relative importance among other stressors such as temperature changes, biodiversity loss, and invasive species. Analogies from deep time are ominous, though. A spike of acidification at the Paleocene-Eocene Thermal Maximum, 55 million years ago, is associated with a dieoff of marine microorganisms. Acidification was also implicated in an event 252 million years ago that claimed 96 percent of all marine species.