As the effects of human society on the global environment intensify, notably to elevate atmospheric concentrations of carbon dioxide, coral reefs and reef-forming corals find themselves the colloquial “canary in the coal mine” in providing early warning of the dire effects of these changes. Nearly 30 years ago, scientists raised the alarm over the effects of unseasonably warm seawater in causing corals to bleach (lose their symbiotic algae) and die, and now the insidious effects of increasing carbon dioxide in acidifying seawater through dissolution are being detected. The gravity of this problem becomes apparent with the realization that coral reefs are built on the efforts of a diversity of calcifying organisms — ranging from corals, to algae, clams, snails, crustaceans, and sponges — whose efforts create a vast calcified matrix, which is the defining feature of these important ecosystems. Early research has suggested that these critical systems could be functionally extinct within 100 years due to their inability to calcify in more acidic seawater.
The Moorea Coral Reef LTER plays a major role in the global effort to evaluate the effects of climate change on coral reef ecosystems because it provides an unrivalled record of how reefs are changing over time, and facilitates the experiments and observations necessary to understand causes of these changes. The MCR has documented dramatic changes in the reefs, largely a result of brief disturbance events. Critically, MCR scientists are finding that not all coral species are responding in the same way to these events (both in terms of which species die and which recover), and there are clear signs of ecological “winners” and “losers”; corals in the genus Porites – particularly those with massive growth forms – appear to be remarkably resistant to environmental assaults. Furthermore, the location in the reefscape where calcifying organisms grow may affect how they respond to environmental challenges.
Observations of winners and losers among the components of the reefs of Moorea have motivated a program to explore mechanisms of resistance to environmental stress, and our initial efforts are focusing on the effects of ocean acidification. MCR scientists have fabricated in Moorea a state-of-the-art system for growing corals, algae and other calcified organisms under different concentrations of carbon dioxide, thereby creating one of the most sophisticated systems of its kind in any tropical location. MCR scintists already have found that corals and calcified algae differ in striking ways in their response to high levels of carbon dioxide. Massive species of Porites are, indeed, clear winners compared to their close relative, Porites rus , and thus massive forms of Porites are more likely to dominate on the reefs of the future. The response of an important crustose coralline alga (Hydrolithon onkodes) to elevated and variable levels of carbon dioxide in seawater depends on where they were growing on the reef. The kinds of studies MCR is conducting demonstrate that responses to changing environments by reef organisms will be diverse, and show which resistant species will build the reefs of tomorrow.