Ocean acidification is a fundamental shift in ocean chemistry caused by the absorption of massive amounts of human-generated carbon dioxide (\(\text{CO}_2\)) from the atmosphere. Approximately one-quarter to one-third of the \(\text{CO}_2\) released by human activities is taken up by the world’s oceans. Once absorbed, the \(\text{CO}_2\) reacts with seawater to form carbonic acid, releasing hydrogen ions (\(\text{H}^+\)). This increase in hydrogen ions lowers the seawater’s pH. Since the start of the Industrial Revolution, the average pH of surface waters has dropped by about 0.1 units, representing a 30% increase in hydrogen ion concentration, setting the stage for widespread biological consequences.
Impact on Calcifying Organisms
The chemical changes associated with ocean acidification directly interfere with the ability of many marine organisms to construct and maintain their protective shells and skeletons. This process, known as biomineralization, relies on carbonate ions (\(\text{CO}_3^{2-}\)), which are the building blocks for calcium carbonate (\(\text{CaCO}_3\)) structures. As the ocean absorbs more \(\text{CO}_2\), the rising concentration of hydrogen ions binds with available carbonate ions, effectively reducing the concentration needed by calcifying species.
Scientists track this availability using the aragonite saturation state (\(\Omega_a\)), where aragonite is a highly soluble form of calcium carbonate used by many organisms. When the saturation state falls below 3, organisms experience significant calcification stress. If it drops below 1, conditions become corrosive, favoring the dissolution of existing shells and skeletons. Cold, high-latitude waters, such as those in the Arctic and parts of the Pacific, are particularly vulnerable to reaching these corrosive states faster than warmer, tropical seas.
One of the most profound impacts is seen in tropical reef-building corals, which secrete massive calcium carbonate skeletons that form the reef structure. Acidification specifically impairs the skeletal thickening process, which is necessary to reinforce the structure against wave action and bioerosion. This decline in skeletal density can be significant, with models projecting up to a 20% reduction in density for foundational coral species by the end of the century.
Mollusks and other invertebrates face similar structural challenges. Oyster larvae, for example, are highly sensitive during their earliest life stages, often struggling to form their initial shells, leading to mass mortality events in aquaculture. Similarly, the tiny planktonic snails known as pteropods, or “sea butterflies,” are extremely susceptible because their shells are made of aragonite.
Pteropods are found globally, but their thin shells are already showing signs of dissolution in areas where corrosive conditions are exacerbated by cold temperatures. Studies show that exposing pteropod shells to the low \(\text{pH}\) conditions predicted for the year 2100 causes them to dissolve within weeks. This structural failure impairs their defense, growth, and overall survival, representing a direct threat at the base of polar and subpolar food webs.
Disruption of Marine Physiology and Behavior
Beyond the structural damage to calcifying organisms, changes in ocean chemistry also affect the internal physiology and behavior of many marine animals, particularly fish. The increase in dissolved \(\text{CO}_2\) in the water leads to a corresponding increase of \(\text{CO}_2\) in a fish’s blood and tissues. While most fish can compensate for this internal change, regulating this acid-base balance requires energy, which is then diverted from other functions.
A notable consequence is the disruption of the neurological and sensory systems in various fish species. Olfaction, or the sense of smell, is the most consistently impaired sense under acidified conditions. This impairment affects fundamental survival behaviors, such as the ability of juvenile fish to detect and avoid the chemical cues released by predators.
This sensory confusion leads to maladaptive behavior, where fish exposed to future \(\text{CO}_2\) levels lose their natural fear response and are even attracted to predator scents. This is thought to be caused by the interference of elevated \(\text{CO}_2\) with neurotransmitter function in the brain, particularly the GABA-A receptor. Acidification has also been shown to affect the auditory system, with some fish species exhibiting reduced sensitivity to low-frequency sounds used for orientation and predator detection.
Impacts on growth and reproduction also manifest, though they vary widely among species and life stages. While adult fish often show resilience in basic metabolic functions, early life stages like larvae can suffer significant reductions in growth performance. This reduced growth is sometimes linked to decreased food intake or less efficient digestion.
Reproductive success can be indirectly affected, as organisms must allocate more energy toward maintaining internal chemical balance, leaving less energy for reproduction. Conversely, observations near natural \(\text{CO}_2\) vents show some fish species exhibiting increased reproductive output. This increase is possibly due to a shift in their energy budget, such as reducing energy spent on activities like foraging or aggression. These complex, species-specific responses highlight how behavioral changes can mediate the direct physiological stress of acidification.
Cascading Effects on Food Webs and Biodiversity
The individual effects of ocean acidification on calcifiers and fish ripple outward, leading to systemic changes that destabilize marine food webs and reduce overall biodiversity. The loss of foundational calcifying species directly translates into a loss of habitat and reduced energy available to higher trophic levels.
Coral reefs are a prime example of habitat loss, as their structural integrity declines due to reduced skeletal density. These reefs serve as shelter, spawning grounds, and nurseries for an estimated 25% of all marine life, including thousands of fish species. The physical degradation of a reef strips the ecosystem of its complexity, exposing dependent species to greater predation risk and reducing the habitat’s capacity to support diverse life.
At the base of the food chain, the decline of pteropods has far-reaching consequences, particularly in polar and subpolar waters. These tiny zooplankton are a primary food source for many commercially important fish, including salmon, cod, and herring, as well as larger marine mammals. A shortage of pteropods due to shell dissolution creates a critical bottleneck that affects energy transfer up the food chain, threatening the stability of fisheries.
Ultimately, ocean acidification drives a global reshuffling of ecological communities, leading to a loss of functional diversity. The stress of lower \(\text{pH}\) favors species that are either non-calcifying or naturally more resilient to the chemical changes, such as certain types of algae, seagrasses, and jellyfish. As sensitive species, like corals and shelled mollusks, decline, they are often replaced by these resilient organisms, resulting in a simplified ecosystem structure.
This shift from complex, diverse habitats to simpler ones reduces the resilience of the marine environment to other stressors, such as rising ocean temperatures. The long-term trajectory is toward a less diverse ocean, where fewer species perform necessary ecological functions, impacting everything from nutrient cycling to global food security.