Virgin Islands (U.S.)
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
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.
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.