Ocean Acidification on Marine Ecosystems

Ocean acidification results from the ocean absorbing carbon dioxide from the air--much of it emitted through human activities. Since around 1850, the ocean has reduced the amount of carbon dioxide humans have contributed to the Earth’s atmosphere by just over one-third, contributing greatly to reducing climate change. Also since that time, the amount of acid in the ocean has increased 26 percent, a rate 10 times any seen in the last 55 million years.

Ocean acidification affects the food web, ecosystems, and ecosystem services, all of which are closely intertwined. The marine ecosystem comprises crabs, sponges, oysters, sea urchins, and starfish, in addition to coral reefs and the numerous algae and plankton that are found there. Because marine life often gets its food source from other marine life, changes in the food web impact all sea life.

Whether close to shore or at the greatest depth of the ocean, coral reefs provide sustenance for the entire ocean ecosystem. Therefore, when ocean acidification impacts these reefs, the environment experiences a domino effect.

Coral reefs are calcium carbonate structures. Zooxanthellate corals and calcifying macroalgae are some of the major contributors of calcium carbonate to the reefs. Increased exposure to acidic seas can adversely impact reef growth by reducing calcification and reproduction rates; it can also contribute to complete loss of reef systems. This has further implications for human beings because reefs protect the shoreline from erosion during storm surges and cyclones.

One of the places where this is already happening is in the Great Barrier Reef, off the coast of Australia. While the precise role of ocean acidification has not been clearly defined, researchers found that there was a 14 percent decline in reef size between 1990 and 2005.

Ocean acidification also affects plankton systems, which serve as a food source. Beyond affecting phytoplankton calcification rates, it decreases the nutritional value of phytoplankton as a result of the increased rate of photosynthesis.

In terms of coastal systems, acidification directly impacts the growth and reproduction of organisms, as well as the development of various mollusks. In the case of crustaceans, acidification interferes with the density of their shells, making them less strong. Some experts liken the effects of acidification on their shells to a person developing osteoporosis. Sea urchins, in particular, become vulnerable to sea predators when their shells have grown to be deformed, thinner, and smaller as a result of exposure to high levels of acid in the ocean. Animals without protective shells also are vulnerable to ocean acidification, as it interferes with the development of eggs and larvae.

One of the major impacts resulting from acidification is a decrease in biodiversity. Most microalgae can survive acidification with only 5 percent loss in species diversity, but in tropical and subtropical zones, acidification reduces the thickness of coralline algae, which forms on the surface of rocks. This affects other marine life that settles and grows in the area. The acidification of seawater supporting macrofauna also contributes to a 30 percent decline in biodiversity when the pH levels drop from 8.1 to 7.8.

Ultimately, the waters that support marine life can become toxic as the result of absorbing too much carbon dioxide. This absorption has the potential not only to change marine life but also, in some cases, to decimate the existence of certain species.