Diversity Matters

The photographic time series from the coral reefs of Moorea provides some of the quantitative data MCR is collecting on how the reefs are changing through time. These pictures (both 0.5 x 0.5 m) from a site at 10 m depth on the north shore of the island show a representative area of reef in 2005 (left) and 2011 (right). Over this 6 year period, the reef changed dramatically after the coral was eaten by seastars (2007-2009) and broken by a cyclone (2010). Diversity matters -- nestled within the diverse assemblage on the left is a combination of winners and losers. The massive coral in the center of the left picture (a species of massive Porites) appears to be one of the winners -- it endures cyclones and is less affected by ocean acidification -- as is the yellow fire coral (Millepora spp.) in the lower center of the picture on the right. Understanding the mechanisms that allow these species to survive will be critical to understanding the kinds of reefs that will occur in the future.
Peter J. Edmunds

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.

Experiments by MCR scientists are elucidating the fundamental mechanisms determining which reef organisms are winners and which are losers under the current onslaught of environmental assaults. In an experiment conducted in 2010, the calcification of a species of massive Porites (a potential "winner") was compared to the calcification of Porites rus under a variety of conditions designed to simulate the conditions on coral reefs in the future. Critically, the species differ strikingly in the way that they respond to high CO2 concentrations (HCO2, = 780 µatm) versus low CO2 concentrations (HCO2 = 413 µatm) under combinations of high temperature (HT = 29.4°C) and low temperature (LT = 25.4°C) (left graph shows mean ± SE, n = 10). These results illustrate the complexity of responses to elevated CO2 concentrations in seawater that can be expected: the massive Porites is hardly affected, while the growth of P. rus is greatly reduced during high summer temperatures. The graph on the right shows the calcification responses of the crustose coralline alga Hydrolithon onkodes collected from different environments to ambient, elevated, and variable seawater CO2 treatments. Calcification is depressed in both the elevated and variable treatments, however algae collected from downstream environments where CO2 levels in seawater vary considerably each day (due to reef metabolism) show an intermediate response, suggesting that some acclimatization to higher CO2 levels might be possible.
Left: P.J. Edmunds in prep. Right: R.C. Carpenter in prep.
For further reading: 
Hofmann, G.E., J.P. Barry, P.J. Edmunds, R.D. Gates, D.A. Hutchins, T. Klinger, and M.A. Sewell. 2010. The effect of ocean acidification on calcifying organisms in marine ecosystems: an organism-to-ecosystem perspective. Annu. Rev. Evol. Syst. 41: 127-147.
For further information: 
Dr. Andrew J. Brooks
Contact email: 

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