Ocean acidification reduces the seawater availability of carbonate ions needed for shell formation and alters growth patterns in shellfish, with consequences for ecology, fisheries, and coastal communities. Evidence compiled by Richard A. Feely at NOAA Pacific Marine Environmental Laboratory shows long-term decreases in surface ocean pH and carbonate saturation states associated with rising atmospheric carbon dioxide. Experimental and field research led by Kristy Kroeker at University of California, Santa Cruz demonstrates that these chemical shifts translate into biological impacts across many calcifying species.
Mechanisms: chemistry and calcification
When atmospheric carbon dioxide dissolves into seawater it forms carbonic acid, lowering pH and shifting the balance of dissolved inorganic carbon. The key effect for shell-building organisms is a decline in carbonate ion concentration and the related decrease in aragonite and calcite saturation states, which are the chemical conditions that favor shell and skeleton formation. Reduced saturation makes it physiologically more difficult and energetically costly for organisms such as oysters, mussels, clams, and some plankton to precipitate calcium carbonate. Early life stages including larvae are especially sensitive because they must build an initial shell while allocating limited energy to growth and development.
Ecological, economic, and cultural consequences
Reduced calcification and slower growth can produce thinner, more fragile shells that increase predation risk and lower survival. Changes in growth rates alter population dynamics and can cascade through food webs, affecting species that depend on shellfish as prey or that rely on reef structures for habitat. Coastal fisheries and aquaculture industries face productivity losses when juvenile mortality rises or harvest sizes decline. Indigenous and local communities that depend on shellfish for food, cultural practices, and livelihoods may experience disproportionate impacts where resources are central to identity and economy. Local factors such as upwelling, freshwater runoff, and temperature interact with global acidification, producing strong regional variability in outcomes.
Management responses draw on monitoring and experimental work from NOAA and academic groups and include selective breeding for resilient stocks, hatchery water treatment, and reductions in other local stressors like pollution and overharvesting. Because the root driver is atmospheric carbon dioxide, mitigating global emissions remains essential to restore favorable carbonate chemistry and protect the long-term integrity of shellfish populations.