One of the most critical effects of increasing ocean acidity relates to the production of shells, skeletons, and plates from calcium carbonate (CaCO3), known as calcification. Acidification shifts the equilibrium of carbonate chemistry in seawater, reducing pH and the abundance of carbonate ions available for corals and other marine calcifiers to use to build their skeletons. This decreases the rate and amount of calcification among many marine organisms that build external skeletons and shells, ranging from plankton to shellfish to reef-building corals.
The reduction of dissolved carbonate ions in seawater has many implications for coral reef ecosystems. Since reef-building corals need carbonate to build their skeletons, decreasing carbonate ion concentrations will likely lead to weaker, more brittle coral skeletons and slower coral growth rates. In the future this may cause coral reefs to erode faster than they can calcify, thus decreasing the ability of coral species to compete for space1. At current rates of CO2 emissions, coral reef production will decrease by 30% within the next 40 years. For corals and other calcifiers such as sea urchins and shellfish, reductions in calcification may:
- diminish coral resilience from bleaching and disease
- decrease the ability of organisms to fend off predators
- inhibit their potential to compete for food and habitat
- alter behavior patterns
Laboratory studies have examined the effects on many types of corals and coralline algae, revealing a range of responses from a 3% to 60% decline in calcification rate for a doubling of atmospheric CO22. A recent study of brain corals in Bermuda found that calcification rates have declined by 25% over the past 50 years and ocean acidification is a likely contributing factor3.
Other potential effects include an increased susceptibility to coral bleaching, a reduced capacity to tolerate ultraviolet radiation, and increased bioerosion rates.
To date, most of the elevated CO2 response studies on marine biota, whether for calcification, photosynthesis, or some other physiological measure, have been short-term experiments that range from hours to weeks. Chronic exposure to increased pCO2 may have complex effects on the growth and reproductive success of calcareous plankton and could induce possible adaptations that are not observed in short-term experiments. While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that many calcifying species will be adversely affected. There are longer-term studies currently underway to better determine the likely impacts. The implications of these studies will be included in this Toolkit as they become available.
VIDEO: Ocean Acidification: How are ocean animals affected? Dr. Richard Feely, Oregon Sea Grant