Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO2 levels to simulate doubling and three-to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO2 is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO2 induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO 2 scenario led to a 30% increase in productivity in Acropora, whereas high CO 2 lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO 2 leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.climate change ͉ global warming ͉ carbon dioxide ͉ Great Barrier Reef T he concentrations of atmospheric CO 2 predicted for this century present two major challenges for coral-reef building organisms (1). Firstly, rising sea surface temperatures associated with CO 2 increase will lead to an increased frequency and severity of coral bleaching events (large-scale disintegration of the critically important coral-dinoflagellate symbiosis) with negative consequences for coral survival, growth, and reproduction (2). Secondly, Ͼ30% of the CO 2 emitted to the atmosphere by human activities is taken up by the ocean (3, 4), lowering the pH of surface waters to levels that will potentially compromise or prevent calcium carbonate accretion by organisms including reef corals (1, 5), calcifying algae (6, 7) and a diverse range of other organisms (8). Ocean acidification research has focused mainly on the consequences of shifting ocean chemistry toward suboptimal saturation states of aragonite and calcite (9) and how this will affect the calcification processes of organisms in the pelagic (10) and benthic (11, 12) environments. Previous studies have shown dissolution of coral skeletons (13) and reduced rates of reef calcification (14) with increasing CO 2 concentrations. Ocea...

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Ocean warming and acidification from increasing levels of atmospheric CO2 represent major global threats to coral reefs, and are in many regions exacerbated by local-scale disturbances such as overfishing and nutrient enrichment. Our understanding of global threats and local-scale disturbances on reefs is growing, but their relative contribution to reef resilience and vulnerability in the future is unclear. Here, we analyse quantitatively how different combinations of CO2 and fishing pressure on herbivores will affect the ecological resilience of a simplified benthic reef community, as defined by its capacity to maintain and recover to coral-dominated states. We use a dynamic community model integrated with the growth and mortality responses for branching corals (Acropora) and fleshy macroalgae (Lobophora). We operationalize the resilience framework by parameterizing the response function for coral growth (calcification) by ocean acidification and warming, coral bleaching and mortality by warming, macroalgal mortality by herbivore grazing and macroalgal growth via nutrient loading. The model was run for changes in sea surface temperature and water chemistry predicted by the rise in atmospheric CO2 projected from the IPCC's fossil-fuel intensive A1FI scenario during this century. Results demonstrated that severe acidification and warming alone can lower reef resilience (via impairment of coral growth and increased coral mortality) even under high grazing intensity and low nutrients. Further, the threshold at which herbivore overfishing (reduced grazing) leads to a coral–algal phase shift was lowered by acidification and warming. These analyses support two important conclusions: Firstly, reefs already subjected to herbivore overfishing and nutrification are likely to be more vulnerable to increasing CO2. Secondly, under CO2 regimes above 450–500 ppm, management of local-scale disturbances will become critical to keeping reefs within an Acropora-rich domain.