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Corals and Other Marine Calcifiers
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Key Points:
- Marine calcifiers, such as corals and marine snails, play important roles in nearly all oceanic ecosystems.
- Ocean acidification makes it more difficult for marine calcifiers to build shells and skeletons and will likely threaten ecosystem dynamics in areas where marine calcifiers play dominant roles in the food web.
- The effects of elevated greenhouse gas concentrations could cause substantial coral reef loss on tropical and sub-tropical reefs by 2050.
Increasing atmospheric concentrations of greenhouse gases are changing the physical and chemical properties of the oceans in ways that impact the health of marine calcifiers—species that makes shells and plates from calcium minerals (IPCC, 2007). Two ongoing changes are particularly consequential: Increasing sea surface temperatures, due to radiative forcing, and decreasing carbonate saturation, due to ocean acidification. This page describes the impacts of elevated greenhouse gas concentrations on corals and other marine calcifiers.
Corals
Corals are anthozoans, the largest class of organisms within the phylum Cnidaria. They most often exist as colonial organisms composed of thousands of individuals, called polyps. All species of coral secrete calcium carbonate (CaCO3), and the majority of coral species form reef structures over time. Coral reefs harbor more than 25 percent of all known fish and provide our oceans with the highest biodiversity of any marine ecosystem (IPCC, 2007).
Surface warming and acidification of the oceans adversely affect the health of coral reefs. Surface warming increases the likelihood of coral bleaching (stress-induced expulsion of unicellular algae resulting in the loss of coral color) and, if conditions are warm enough for long enough, can cause reef mortality (IPCC, 2007). Ocean acidification lowers the saturation states of aragonite and other carbonate minerals, making these materials less available for construction of the calcified structures reefs require to survive (Raven et al, 2005).
According to the IPCC (2007):
- Warm-water coral cover has been reduced by 30 percent or higher since the beginning of the 1980s. Many studies link coral bleaching to warmer sea surface temperature.
- Annual or bi-annual exceedance of water temperature thresholds for bleaching is projected at the majority of reefs worldwide by 2030 to 2050.
- Corals could become rare on tropical and sub-tropical reefs by 2050 due to the combined effects of increasing dissolved carbon dioxide (CO2) and increasing frequency of bleaching events.
- Since the impacts of increased CO2 are greater at higher latitudes, cold-water corals are likely to show large reductions in geographical range this century.
Other Marine Calcifiers
Corals are perhaps the best known marine calcifier, but many other marine organisms also rely on calcification, including crustaceans (e.g. shrimp), echinoderms (e.g. starfish), large calcareous algae, and some phytoplankton (Raven et al, 2005). Marine calcifiers play important roles in the food chains of nearly all oceanic ecosystems, help regulate ocean chemistry, and are an important source of biodiversity and productivity. For example, marine snails, called pteropods, are an important food source for salmon, mackerel, herring, and cod. According to the IPCC (2007), climate change and ocean acidification will impair a wide range of calcifiers that use aragonite (a form of calcium carbonate) to make their shells or skeletons, with impacts likely to be particularly acute in the Southern Ocean (IPCC, 2007).
According to a report on ocean acidification by the Royal Society (Raven et al, 2005):
- Polar and sub-polar surface waters and the Southern Ocean are projected to be aragonite under-saturated by 2100, and Arctic waters will be similarly threatened.
- Ocean acidification will likely threaten ecosystem dynamics in the areas where marine calcifiers play dominant roles in the food web and in carbon cycling.
- Lower pH in the oceans may inhibit the ability of echinoderms, such as sea urchins and mollusks, to form skeletal material.
- Ocean acidification could have large consequences for benthic (bottom-dwelling) organisms, including in particular benthic calcifiers, by changing the pH levels sediment communities are exposed to and by causing changes in the biological pump.
- Some unicellular species that form the base of marine food chains could experience reduced growth and fitness as a result of decreased pH levels.
- The overall reaction of marine biological carbon cycling and ecosystems to a warm and high-CO2 world is not yet well understood, but there is a risk that a decrease in calcifier productivity could lead to cascading effects throughout the food chain.
Marine calcifiers (including corals) are not impacted by the effects of elevated greenhouse gas concentrations in isolation. Other factors, such as over-fishing and pollution, affect calcifiers in multiple ways that are both difficult to distinguish from climate change and likely to reduce resiliency to it.
Figure 1: Existing coral locations and estimated aragonite saturation states of the surface ocean for the years 1765, 1995, 2040, and 2100 for a business-as-usual (i.e. no mitigation beyond current measures) CO2 emissions scenario. Many calcifiers use aragonite to make shells and skeletons and are likely to be affected by sub-optimal saturation levels.
Source: Kleypas et al, 2006.
References
- IPCC, 2007: Climate Change 2007: Impacts, Adaptation, and Vulnerability
. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E.Hanson, Eds., Cambridge University Press, Cambridge, UK, 976pp. - Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, and L.L. Robbins, 2006. Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research , report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 pp.
- Raven, J., K. Caldeira, H. Elderfield, O. Hoegh-Guldberg; P. Liss, U. Riebesell, J. Shepherd, C. Turley, and A. Watson. 2005. Ocean Acidification due to Increasing Atmospheric Carbon Dioxide. The Royal Society, London, UK.