Ocean acidification is a global environmental concern, characterized by the ongoing decrease in the pH of the Earth’s oceans. This change is driven by the absorption of excess atmospheric carbon dioxide. The impacts are far-reaching, especially for marine organisms that form shells, such as shellfish.
Understanding Ocean Acidification
The ocean absorbs atmospheric carbon dioxide (CO2). When CO2 dissolves into seawater, it forms carbonic acid, which then releases hydrogen ions into the water. This increase in hydrogen ions causes the ocean’s pH to drop, making it more acidic.
While the ocean remains alkaline, it is becoming less so, which means it is moving towards being more acidic. This process also reduces the availability of carbonate ions, a crucial component for many marine organisms. Since the industrial era, the ocean’s average surface pH has decreased.
Impacts on Shell Formation
Ocean acidification primarily hinders shell formation, a process known as calcification. Shellfish, such as oysters, clams, and mussels, build their shells from calcium carbonate, which requires available carbonate ions. As ocean acidity increases, hydrogen ions bind with carbonate ions, making them less available for shell-building organisms.
This reduced availability of carbonate ions forces shellfish to expend more energy to build and maintain their shells, diverting resources from other essential biological functions like growth and reproduction. In severe cases, the increased acidity can even cause existing shells to dissolve. For instance, the shells of pteropods, tiny sea snails, have been observed to dissolve in more acidic waters.
Juvenile shellfish and larvae are particularly vulnerable because they are rapidly forming their initial shells. Oyster larvae, for example, build about 90% of their body weight as shell within 48 hours, a process that becomes energetically costly under acidified conditions. Some mussels have been observed to produce thinner shells or shells with altered mineral composition, making them more susceptible to damage and predators.
Broader Biological Effects on Shellfish
Beyond shell formation, ocean acidification impacts various other biological processes in shellfish. Growth rates are affected, leading to slower development and smaller adult sizes. Clam and scallop larvae, for example, may exhibit reduced growth and take longer to develop into juvenile stages.
Reproduction is also compromised, with acidification leading to impaired larval development, reduced fertility, and lower survival rates of offspring. For instance, mass mortality events in oyster hatcheries have been linked to ocean acidification. Increased energy expenditure to cope with changing conditions can also negatively impact metabolism, reducing overall health and market quality.
Behavioral changes, such as altered swimming patterns, feeding behaviors, and predator avoidance, have been observed in some shellfish species. These changes can make them more vulnerable to predation and less efficient at finding food. The immune response of shellfish can also be weakened, making them more susceptible to diseases and parasites, as their bodies struggle to maintain internal acid-base balance.
Ecosystem and Human Implications
The decline of shellfish populations due to ocean acidification creates ripple effects throughout marine ecosystems. Shellfish often serve as a vital food source for other marine animals, and their diminished numbers can disrupt marine food webs. Many shellfish species also act as filter feeders, playing a critical role in maintaining water quality, so their decline can negatively impact overall ecosystem health.
The consequences extend to human activities and economies, particularly commercial fisheries and aquaculture industries. Coastal communities that rely on these resources face significant challenges, including potential economic losses. For example, the oyster industry in the Pacific Northwest has experienced substantial losses due to ocean acidification affecting larval survival. Shellfish harvests in the U.S. could decrease, impacting livelihoods and seafood supplies.