Water Planet Chemistry: Ocean Acidification Is More Pervasive Than Previously Thought
New research by an international team of oceanographers has found that ocean acidification has significantly compromised 40% of the global surface ocean, and 60% of the subsurface ocean to a depth of 656 feet (200 meters).
The degree of acidification indicates there has been considerable declines in suitable habitats for important marine species that build hard shells or skeletons from dissolved calcium and carbonate ions, including economically important species of crabs, oysters, mussels and other bivalves, corals and small sea snails known as pteropods.
The finding by an international team that included scientists from Plymouth Marine Laboratory in Great Britain, NOAA’s Pacific Marine Environmental Laboratory, the Cooperative Institute for Marine Ecosystem Research at Oregon State University, and the Earth System Science Interdisciplinary Center at the University of Maryland, was based on a detailed analysis of ocean carbon system observations, models and biological assessments. The research was published in the journal Global Change Biology.
The new analysis provides a global assessment of the saturation state of a form of the mineral aragonite. Aragonite is a form of calcium carbonate that marine organisms like corals and shellfish use to build their shells and skeletons. NOAA uses aragonite saturation state as a measure of how easily aragonite will dissolve in seawater, in order to track ocean acidification.
As pH levels drop, causing seawater to become more acidic, calcifying species such as corals, oysters, mussels, and tiny sea snails called pteropods struggle to maintain their protective structures, leading to weaker shells, and slower growth. In 2016, NOAA research linked human-caused emissions of carbon dioxide (CO2) to dissolving sea snail shells off U.S. West Coast, reduced reproduction and decreased survival rates.
“Our research has found that since the pre-industrial era, the aragonite saturation state has declined by a fifth in 40% of surface waters and in 60% of subsurface waters to a depth of 200 meters, which means that we have gotten close to or crossed the boundary of “safe living space” of good habitats for some calcifying species in many regions of the ocean,” says Richard Feely of NOAA’s Pacific Marine Environmental Laboratory.
The largest change in surface waters has been in the polar regions, while the largest change in deeper waters has been in the sub-polar regions and along the west coast of North America from Canada to Mexico.
The researchers estimate that some tropical and subtropical coral reefs have lost 43% of their suitable habitats, sea butterflies (pteropods, a key food web species) in polar regions have lost up to 61% of their habitat, and coastal shellfish species have lost 13% of their global coastline habitats in which they can sustain their essential biological processes.
Additional findings of a NOAA study published in February 2025 in the Canadian Journal of Fisheries and Aquatic Sciences show correlations between changes in ocean pH in the Bering Sea and recent declines of Bristol Bay red king crab. The study, led by Mike Litzow at NOAA’s Alaska Fisheries Science Center Kodiak Lab, looked at whether increased acidity, sea ice, or warming could explain the decline in southeast Bering Sea red king crab. Results showed that ocean acidification may be a driver. Increasing acidity explained about 21% of the population decline over the 1980 to 2023 period, and roughly 45% of the decline since 2000, the study found. Litzow and his co-authors note that the study shows a correlation, and that it’s very difficult to apply lab-based results on wild populations because ecosystems are complex.
The Bering Sea red king crab fishery experienced a two-year closure from 2022 to 2024 due to low stock abundance, and reopened in October 2023 with a significantly reduced quota.
In 2020, a NOAA study, led by Nina Bednaršek of Oregon State University, documented for the first time that ocean acidification along the U.S. Pacific Northwest coast is impacting the shells and sensory organs of some young Dungeness crab, another prized crustacean that supports the most valuable fishery on the West Coast.
In the late 2000s, changing ocean chemistry rocked shellfish hatcheries in the Pacific Northwest, driving an estimated $110 million loss for the industry until hatchery managers began to buffer the water used in their tanks. to adjust the pH.
Ocean acidification 101
The ocean acts like a carbon sponge, absorbing about 25% of the carbon dioxide (CO2) that is released in the atmosphere by human activity or natural systems. As levels of atmospheric CO2 increase, the amount of carbon dioxide absorbed by the ocean also increases. When CO2 is absorbed in seawater, it reacts to form carbonic acid, which then breaks down into bicarbonate and hydrogen ions. The increased concentration of hydrogen ions lowers the pH of the ocean and reduces the availability of carbonate ions.
Carbonate ions are an important building block of skeletal structures for “calcifying” organisms such as oysters, clams, sea urchins, shallow water corals, deep sea corals, and certain kinds of plankton. Decreases in carbonate ions can make building and maintaining shells and other calcium carbonate structures more difficult. Some marine life like these calcifiers spend more energy building and maintaining their shells and skeletons – energy that could be used for feeding, avoiding predators and reproducing.
Ocean acidification may contribute to recruitment failure of Bering Sea red king crab, Canadian Journal of Fisheries and Aquatic Sciences (open access)
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