Ocean acidification is the ongoing decrease in the pH of the Earth’s oceans, caused by the absorption of excess atmospheric carbon dioxide. This change in chemistry is altering the marine environment at a rate faster than seen for millions of years. Biodiversity, the variety of life in the ocean, faces substantial threats from this chemical shift, which affects the structure and function of entire marine ecosystems.
The Chemical Process of Ocean Acidification
The ocean absorbs a significant amount of the carbon dioxide (CO₂) released into the atmosphere by human activities, affecting seawater chemistry. When CO₂ dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃), setting off a chain of chemical changes.
The carbonic acid dissociates, releasing hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). This increase in hydrogen ions lowers the ocean’s pH, making the water more acidic. Ocean surface waters are already about 30% more acidic than they were at the beginning of the industrial era.
This process also impacts carbonate ions (CO₃²⁻). The excess hydrogen ions bond with available carbonate ions to form more bicarbonate, reducing the concentration of free carbonate ions. These ions are the building blocks marine organisms use to construct their shells and skeletons.
Direct Impacts on Marine Organisms
The reduction of available carbonate ions has a direct impact on calcifying organisms. Corals, clams, and oysters rely on calcium carbonate minerals to build their protective shells and skeletons. These organisms must expend more energy to build their structures, often resulting in thinner, weaker shells that are more vulnerable to damage.
The effects of ocean acidification extend beyond shell-building species to include fish and other non-calcifying animals. Changes in seawater pH can disrupt the internal acid-base balance of marine animals, affecting physiological processes. Altered pH levels can have neurological consequences for fish, impairing their sense of smell, hearing, and vision, which hinders their ability to detect predators and find food.
The response of marine plants to increased carbon dioxide is varied. Some species, such as seagrasses and certain types of algae, may experience enhanced photosynthesis and growth due to higher CO₂ concentrations. This can allow them to grow more rapidly and outcompete other species for space and resources.
Ecosystem-Wide Consequences
The health of individual organisms is tied to the stability of the broader marine ecosystem. For example, pteropods—tiny sea snails—are a food source for fish like salmon and herring, as well as for whales. The struggle of these snails to build their shells in more acidic waters threatens their populations, which in turn impacts the larger animals that depend on them for food.
Coral reefs are vulnerable to ocean acidification. Corals are ecosystem engineers that build complex structures providing shelter, food, and nursery grounds for thousands of other species. As acidification hinders coral growth and structural integrity, these habitats degrade, leading to a direct loss of the biodiversity they support.
The changing ocean chemistry leads to significant shifts in species composition. While calcifying organisms struggle, some forms of macroalgae may flourish in high-CO₂ environments. This can lead to algae overgrowing and replacing coral communities, resulting in a less diverse ecosystem dominated by a few tolerant species.
Compounding Environmental Stressors
Ocean acidification does not occur in a vacuum; it interacts with other environmental changes, like rising ocean temperatures and decreasing oxygen levels. These stressors often act together, creating combined effects that are more severe than any single factor acting alone.
The relationship between ocean warming and acidification is damaging for coral reefs. Warmer waters can cause coral bleaching, where corals expel the symbiotic algae living in their tissues. While corals can recover from bleaching events, ocean acidification makes it harder for them to rebuild their skeletons, hindering their recovery.
The combined pressures of acidification, warming, and deoxygenation challenge marine biodiversity. These multiple stressors can push organisms beyond their physiological limits, leading to population declines and altering the composition of marine communities.