Environmental Protection Agency: Effects of Ocean and Coastal Acidification on Marine Life

By releasing carbon dioxide to the atmosphere, humans are rapidly altering the chemistry of the ocean and affecting marine life.

The acidity of the ocean has increased by about 25% since before the Industrial Revolution, greater than any other time within the last two million years. Given the speed at which humans are altering ocean chemistry, marine plants and animals may not have time to adapt or migrate as they did in the past to cope with changes to ocean chemistry over the history of the Earth.

As a consequence of acidification, marine life face a two-fold challenge: decreased carbonate availability and increased acidity. Laboratory studies suggest changing ocean chemistry will 1) harm life forms that rely on carbonate-based shells and skeletons, 2) harm organisms sensitive to acidity and 3) harm organisms higher up the food chain that feed on these sensitive organisms. However, we do not yet know exactly how ecosystems will be impacted.

Sea shell The minerals that animals build their shells out of are calcium carbonate compounds.

Building Shells and Skeletons: Calcifying Organisms

Many ocean plants and animals build shells and skeletons out of two chemicals that exist in seawater, calcium and carbonate. Organisms combine calcium and carbonate to form hard shells and skeletons out of the mineral calcium carbonate. Therefore, the plants and animals that use calcium carbonate for structure and protection are called calcifying organisms. Increased acidity slows the growth of calcium carbonate structures, and under severe conditions, can dissolve structures faster than they form.

The Struggle to Stay Healthy Under Increased Acidity

Just like humans, marine organisms require optimal conditions inside their bodies to stay healthy. If the acidity of seawater is beyond the optimum range for that organism, its body must use more energy to maintain healthy body fluid chemistry. Organisms can often compensate when faced with increased acidity, but this comes at the expense of using energy to grow critical body parts like muscle or shell. For example, scientists have found that mussels, sea urchins, and crabs start to dissolve their protective shells to counter elevated acidity in their body fluids. So even if an organism can adjust to survive increasing acidity its overall health can be impaired.

Under increasing acidity animals like this sea urchin must spend more energy to build and maintain shells, which could impair overall health.

Effects on Larvae

Many marine fish and invertebrates have complex life cycles. They spend their early lives as larvae while they develop and disperse to distant areas on ocean currents. Larvae are very small, which makes them especially vulnerable to increased acidity. For example, sea urchin and oyster larvae will not develop properly when acidity is increased. In another example, fish larvae lose their ability to smell and avoid predators. The vulnerability of larvae means that while organisms may be able to reproduce, their offspring may not reach adulthood.