Ocean acidification results from the absorption of vast amounts of atmospheric carbon dioxide (\(\text{CO}_2\)) into seawater. Since the start of the Industrial Revolution, the ocean has absorbed approximately one-quarter of human \(\text{CO}_2\) emissions. This chemical shift poses a risk to invertebrates, particularly corals, which are foundational species for entire reef ecosystems. The stability of these organisms is tied directly to marine biodiversity and global economic activities like fisheries and coastal protection.
The Chemistry of Ocean Acidification
When carbon dioxide dissolves into seawater, it initiates chemical reactions that alter the water’s balance. The dissolved \(\text{CO}_2\) reacts with water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). This acid quickly dissociates, releasing hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^-\)) into the water.
The resulting increase in free hydrogen ions defines the process of acidification, causing the ocean’s pH level to drop. These excess hydrogen ions readily bond with carbonate ions (\(\text{CO}_3^{2-}\)), a natural component of seawater. This process “ties up” the carbonate ions, making them less available for organisms that need them to build skeletons and shells.
Impact on Calcification and Skeletal Structure
The altered chemistry primarily impacts calcifying invertebrates, which build hard structures from calcium carbonate (\(\text{CaCO}_3\)). Reef-building corals construct their massive skeletons using aragonite, a form of calcium carbonate. The availability of carbonate ions determines the aragonite saturation state (\(\Omega_{aragonite}\)), which measures how easily corals can build and maintain their structures.
As acidification reduces carbonate ion concentration, the aragonite saturation state declines, making calcification energetically more costly. Corals must expend more energy to build new skeletal material, resulting in reduced growth rates and weaker skeletons. Studies of massive Porites corals show a reduction in skeletal density since the 1950s, increasing their susceptibility to physical damage and bioerosion.
Other invertebrates are similarly affected, including mollusks like oysters, clams, and pteropods, or “sea butterflies.” These organisms rely on carbonate ions to form protective shells, which can become thinner, more porous, and prone to breakage. In severe cases, seawater can dissolve existing calcium carbonate structures faster than the organism can rebuild them. Larval stages are particularly susceptible to these structural defects, preventing proper development.
Physiological and Behavioral Disruptions
Ocean acidification disrupts the physiological balance and behavior of many invertebrates, beyond physical damage to shells. Organisms like crustaceans, cephalopods, and worms must regulate the pH of their internal body fluids. Facing a lower external pH, these animals expend significant energy on acid-base regulation, often by accumulating bicarbonate ions internally.
This increased energy expenditure means less energy is available for other biological functions, leading to metabolic stress and reduced growth or reproduction. For instance, some species exhibit reduced sperm motility and decreased fertilization success at lower pH levels. Larval development in creatures like sea urchins and oysters is also often impaired under acidified conditions.
Acidification can interfere with neurological function and sensory systems, causing observable behavioral changes. Some invertebrates show an impaired ability to detect or avoid predators due to changes in chemical sensing mechanisms. Behaviors like habitat selection and anti-predator responses can be negatively impacted, as seen in organisms like the California sea hare.
Broader Ecological Implications
The localized impacts of acidification on individual invertebrates affect entire marine ecosystems. Corals are “ecosystem engineers” because their skeletons create the physical structure of reefs, providing shelter and feeding grounds for a quarter of all marine species. If coral calcification rates decline and skeletons weaken, reef degradation leads to a loss of habitat for fish and crustaceans.
The decline of small, shelled invertebrates like pteropods triggers cascading consequences through the marine food web. Pteropods are a primary food source for commercially important fish species, seabirds, and whales. A reduction in these shelled plankton affects predator-prey dynamics and diminishes the productivity of fisheries. The stress experienced by invertebrates translates into a loss of species richness and a disruption of marine environments.