Global Warming Effects Around the World

Virgin Islands (U.S.)

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Oceans (Ocean chemistry)

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Ecosystems (Salt water)

Temperature (Ocean)

Increasing acidification of the Caribbean Sea makes it difficult for Elkhorn coral to reproduce

Increased ocean acidification poses risks to the very initial stages of life of the already threatened elkhorn coral (Acropora palmata), a key reef builder in the Caribbean Sea that provides critical habitat for other reef organisms. The earth's oceans are now absorbing excess CO2 from the atmosphere, tipping the chemical balance of sea water toward sour (acidic). 1

Key Facts

Once the dominant reef builder of the Caribbean Sea, elkhorn coral (Acropora palmata) can be found as far north as Florida and as far south as Venezuela.2 It typically lives on the side of the reef facing the open ocean, thereby taking the brunt of waves kicked up during storms and protecting communities on the nearby shoreline.

  • Hurricane damage, overfishing, pollution, and higher ocean temperatures that cause coral bleaching have resulted in elkhorn coral receiving "threatened" status under the Endangered Species Act.2
  • Elkhorn coral now faces a looming threat from increasing ocean acidification: new research shows that exposing these corals to higher levels of acidity reduces successful fertilization and settlement of larvae on reefs (where they can produce new corals)—around 52 percent in a moderately acidic scenario and 73 percent in a more highly acidic scenario.3,4
  • These conditions are projected to be reached by the middle and end of the century respectively if we continue on a path of high carbon dioxide emissions.5,6


As their name suggests, elkhorn coral (Acropora palmata) grow in a branching pattern similar to the horns of an elk. The branches of elkhorn and related staghorn corals (Acropora cervicornis) provide critical habitat for other reef organisms in the Caribbean Sea.7

In 2005, during the hottest average decade on record,8 low-wind conditions known as "the doldrums" combined with very high ocean temperatures to cause massive coral bleaching in the Virgin Islands.9 This was followed by a particularly severe outbreak of at least five coral diseases in the Virgin Islands, resulting in a decline in coral cover of about 60 percent.9 There is some indication that higher ocean temperatures—between 86 and 95 degrees Fahrenheit (30 to 35 degrees Celsius)—promote optimal growth of several coral pathogens.9 Other research showed that elkhorn coral post-bleaching had larger disease lesions than unbleached specimens, suggesting that bleaching may increase the corals' susceptibility to disease.9,10

What the Future Holds

As if coral bleaching and a host of new diseases were not bad enough, a souring ocean creates new threats. Many studies have demonstrated the risks that ocean acidification pose to marine organisms, such as coral dissolving in more acidic water.6 However, new findings suggest that the August and September time period could be particularly challenging for the earliest life stage of elkhorn coral—an important reef-forming coral of the Caribbean—if we continue on a path of high carbon dioxide emissions.5 Ordinarily each August or September elkhorn corals flood the water with eggs and sperm (gametes) for sexual reproduction.2

Specimens of elkhorn coral living in water with excess carbon dioxide have been studied for fertilization rates, ability of larvae to settle on reef substrate (where they produce new corals), and subsequent growth and survival.3 Three levels of carbon dioxide were tested, corresponding to concentrations today, at mid-century, and at the end of the century on a high-emissions path.3,5 At the mid-century concentration, the ability of fertilization to occur and for larvae to settle successfully on the reef was significantly reduced: around 52 percent, and the decline intensified to about 73 percent at the late-century concentration.3 The corals' ability to survive over the long run declined as well, by an average of 39 percent and 50 percent respectively.3,4

This already threatened species might have a chance to survive if existing protections are enforced in the Virgin Islands National Park and other places. Meanwhile, experiments aimed at helping corals adapt to changing conditions are under way, such as one that proposes shading to help some coral species recover after bleaching or disease.11 Perhaps the biggest help of all would be lowering atmospheric carbon dioxide below 2010 levels.



  1. Photograph: Caroline Rogers, National Park Service Virgin Islands. Online at viis/ forkids/ images/ ElkhornCoralCRogersOST.jpg, accessed November 14, 2010.
  2. NOAA Fisheries Office of Protected Resources. Elkhorn Coral (Acropora palmata). Online at pr/ species/ invertebrates/ elkhorncoral.htm, accessed November 15, 2010.
  3. Albright R., B. Mason, M. Miller, and C. Langdon. 2010. Ocean acidification compromises recruitment success of the threatened Caribbean coral Acropora palmata. Proceedings of the National Academy of Sciences. November 8. Online at cgi/ doi/ 10.1073/ pnas.1007273107.
  4. Specifically the study tested conditions under current acidity (~400 µatm pCO2), middle (~560 µatm pCO2), and high (~800 µatm pCO2) acidity conditions in the water.
  5. The emissions scenarios referred to here are the Intergovernmental Panel on Climate Change's high-emissions paths known as A1FI and A2.
  6. Schubert, R., H.-J. Schellnhuber, N. Buchmann, A. Epiney, R. Grießhammer, M. Kulessa, D. Messner, S. Rahmstorf, and J. Schmid. 2006. The future oceans: Warming up, rising high, turning sour. Special report of the Wissenschaftlicher Beirat der Bundesregierung Globale Umweltveränderungen (German Advisory Council on Global Change). Online at
  7. Federal Register. 2005. Endangered and Threatened Species; Proposed Threatened Status for Elkhorn Coral and Staghorn Coral, Department Of Commerce, National Oceanic and Atmospheric Administration, 50 CFR Part 223, Docket No. 050304058-5113-02; I.D. 060204C, RIN 0648-XB29, Volume 70 Number 88 Monday May 9, 2005, Proposed Rules. Available at pr/ pdfs/ fr/ fr70-24359.pdf on November 15, 2010.
  8. Arndt, D.S., M.O. Baringer, and M.R. Johnson (eds.). 2010. State of the climate in 2009. Bulletin of the American Meteorological Society 91(6):S1-S224. Online at bams-state-of-the-climate, accessed November 15, 2010.
  9. Miller J., E. Muller, C. Rogers, R. Waara, A. Atkinson, K.R.T. Whelan, M. Patterson, and B. Witcher. 2009. Coral disease following massive bleaching in 2005 causes 60% decline in coral cover on reefs in the US Virgin Islands. Coral Reefs 28:925-937.
  10. Muller, E.M., C.S. Rogers, A.S. Spitzack, and R. van Woesik. 2008. Bleaching increase likelihood of disease on Acropora palmata (Lamarck) in Hawksnest Bay, St. John, US Virgin Islands. Coral Reefs 27:191-195.
  11. Muller, E.M., and R. van Woesik. 2009. Shading reduces coral-disease progression. Coral Reefs. 28:757-760.
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