Operators at a shellfish hatchery on the Oregon coast discovered years ago that their oyster larvae would not survive if they just pumped in incoming Pacific water. Before the water came into contact with the tanks, it had to be analyzed, its chemistry measured, and its acidity evaluated. The larvae fail to build shells within hours of the water shift on days when upwelled deep water—which is colder, more acidic, and deficient in carbonate ions—reaches the surface and is drawn into the intake. Early adopters of this strategy modified their operations and managed to survive. Before anyone could pinpoint the cause, those who were unaware of what was going on lost entire batches of seeds.
Ocean acidification is the process causing the issue. Approximately 25% of the CO2 that people release into the atmosphere each year is absorbed by the ocean. Hydrogen ions are released into the water when the CO2 dissolves in seawater and reacts to generate carbonic acid. Lower pH is caused by more hydrogen ions. Lower pH results in less carbonate ions, which are the chemical building blocks used by marine organisms that make shells to create and preserve their calcium carbonate structures. The average pH of the ocean has decreased by roughly 0.1 units during the Industrial Revolution. That figure may seem little until you realize that pH is logarithmic, which means the shift corresponds to a roughly 30% increase in acidity. The ocean is changing more quickly than at any other time in the last 800,000 years of ice core records, and it is far more corrosive than it was two centuries ago.
This poses a direct structural risk to oysters, clams, mussels, sea urchins, and the billions of tiny pteropods that float across open water. In order to create calcium carbonate, carbonate ions must be extracted from the surrounding water and combined with calcium. The process becomes more difficult and costly in terms of energy when the concentration of carbonate ions decreases. Animals still achieve it, but at a cost—maintaining their shells takes up energy that could be used for immune system function, development, or reproduction. Furthermore, existing shells don’t simply fail to form correctly in the most acidic conditions. They start to disintegrate.
A larger-scale variation of the same issue affects coral reefs. Calcium carbonate is used by stony corals to construct their skeletons, and reef structures develop over centuries of gradual calcification. Calcification rates decrease as acidification lowers the amount of carbonate in the water. Structural density begins to decline in reefs that took hundreds of years to form. They start to break more easily. Headlines typically highlight bleaching events, which are caused by warming water, but acidification is also occurring in the same water, making it more difficult for the reef to regenerate in between bleaching episodes. As a result, the reef system is more difficult to harm and takes longer to heal.
Although less obvious, the behavioral consequences on fish are becoming more well-documented. Fish travel, communicate, identify predators, and locate food by using chemical signals in the water. These signals become distorted as the chemistry of the water changes. Fish bred in more acidic water make different decisions, according to studies; they approach predators instead of avoiding them, react inappropriately to danger warnings, and lose the ability to distinguish between known and unfamiliar scents. These impacts are significant. Impaired chemical sensitivity is a major disadvantage in an environment where survival depends on a split-second response to a predator.
Throughout the system, the effects of the food web compound. Salmon, herring, and a variety of other commercially significant fish rely on pteropods, which are little shelled zooplankton frequently referred to as sea butterflies. Their shells are especially susceptible to acidification, and populations in regions where acidification is caused by upwelling have been reported to exhibit shell breakdown. When there are fewer pteropods, the species that rely on them have less food, which spreads up the food chain to the fish that support marine ecosystems and the fishing industries that provide food and jobs for millions of people.
