A More Acidic Ocean, a Different Oyster

Ocean chemistry is changing, and the change has been measured. The ocean absorbs about 30% of the carbon dioxide released into the atmosphere. Dissolved CO₂ forms carbonic acid, which releases hydrogen ions and lowers the water's pH. Since the start of the Industrial Revolution, surface ocean pH has fallen by about 0.1 units. Because the pH scale is logarithmic, that is roughly a 30% increase in acidity.¹ Climate models project a further fall of 0.06 to 0.32 units by 2100, depending on how much more CO₂ is emitted.²

For oysters, acidification has already done damage, and at scale, on the Pacific Northwest coast of the United States. In July 2007, larval deaths at Whiskey Creek Shellfish Hatchery in Netarts Bay, Oregon, reached levels unprecedented in its thirty-year history, and they continued until October. Managers first blamed a bacterial pathogen, but the die-offs went on after it was eliminated. Researchers working with the hatchery then showed that larval survival tracked the aragonite saturation state of the incoming seawater, a measure of the carbonate chemistry of the CO₂-rich deep water that upwelling brought to the coast.³⁴ The industry adapted by monitoring water chemistry in real time, timing intake around upwelling and buffering tank water with sodium carbonate, and those systems have since supplied billions of extra larvae. The chemistry that caused the die-offs is still there.

Why Larvae Are Vulnerable

Acidification hits oysters first and hardest as larvae, because of how they build their first shell. Within a day or two of fertilization, a larva must build that shell, the prodissoconch, running only on the energy reserves of the egg. It builds mostly from aragonite, a form of calcium carbonate that dissolves more easily than the calcite of adult shell. Experiments on Pacific oyster larvae show that the aragonite saturation state of the seawater, rather than pH or CO₂ alone, decides whether that first shell forms.⁵

Research on the biochemical consequences has found effects beyond simple dissolution. A metabolomic and transcriptomic study of oyster larvae found that experimental acidification significantly down-regulated genes involved in protein metabolism, glycolysis, lipid breakdown, calcium transport and shell-matrix formation during the formation of the first shell. The authors concluded that acidification disrupts the larvae's energy budget, and that insufficient energy supply then suppresses the mobilization of calcium, delaying or preventing shell formation.⁶

What Acidification Does to Flavor Compounds

The consequences for adult oyster flavor are more subtle than larval mortality, and only partly measured. Two routes matter.

First, acidification and warming together reduce the protein, fat and carbohydrate in adult oysters. A twelve-week experiment exposed Pacific and European flat oysters to six scenarios built around 3 °C of warming and 350–600 ppm more atmospheric CO₂, in line with mid- to end-of-century projections. Both species became less nutritious, with lower protein, fat and carbohydrate and fewer calories. The effect was strongest in the Pacific oyster, which also took up more copper under the future conditions.⁷ Less carbohydrate in an adult oyster means less glycogen, and less glycogen means less body in the flesh.

Second, the phytoplankton community on which oysters feed is itself sensitive to changing ocean chemistry and temperature. Because an oyster's aroma is built largely from the oxidation of fatty acids it acquires from its diet, any lasting shift in which phytoplankton dominate a growing area would change the raw material of flavor. How coastal phytoplankton communities will reorganize is still an open research question, and the consequences for oyster aroma have not been measured directly, so this second route is the more speculative of the two.

Where It Is Happening Now

The Pacific Northwest is an acidification hotspot because seasonal upwelling brings deep, CO₂-rich water to the surface. It is also where most of the West Coast's oyster seed comes from. Three major hatcheries (Whiskey Creek in Oregon, and Taylor Shellfish and Coast Seafoods in Washington), together with smaller ones, produce 40–60 billion eyed larvae a year for an industry worth about $270 million annually.⁴ The hatcheries that adapted after 2007 installed continuous water-chemistry monitoring, spawned around upwelling events, moved production earlier in the year and buffered incoming water with sodium carbonate.⁴ Those are engineering workarounds for a chemical problem that keeps getting worse.

Upwelling is not the only cause. Where coastal water receives nutrient pollution, algal blooms sink and decompose, using up oxygen and releasing CO₂ into the bottom water. Research in the northern Gulf of Mexico and the East China Sea showed that this makes subsurface coastal water markedly more acidic than atmospheric CO₂ alone would.⁸ So the oyster regions at risk include the most productive estuaries, often the ones taking in the most nutrients from the land around them, as well as remote upwelling coasts.

At NOAA Fisheries' Milford Laboratory in Connecticut, eastern oysters kept for ten weeks at low pH had significantly lighter shells than oysters kept at the normal pH of Long Island Sound. The team also tracked feeding and respiration.⁹ Work on Sydney rock oysters found that a selectively bred line coped better with acidified water than wild stock,¹⁰ so breeding for resilience is a realistic, if slow, way to adapt.

The Ten-Year Horizon

For buyers who think about where their oysters will come from in ten years, acidification is a sourcing question now. The Pacific Northwest will stay under pressure, and so will nutrient-loaded estuaries elsewhere. Ask growers whether they monitor carbonate chemistry at their site: the ones who know their pH swings are the ones who can plan around them.

For adult oysters in most regions, the flavor effects at today's pH are still small. The studies point one way, though. More acidic water means less aragonite, a higher energy cost for building shell and less energy left for glycogen, and perhaps a changing food supply too. The oyster you eat in 2035 may come from the same farm as today's and have a little less body than today's.