Caldeira Lab

Ocean Acidification and Warming Will Lower Coral Reef Resilience

Kenneth R. N. Anthony, Jeffrey A. Maynard, Guillermo Diaz-Pulido, Peter J. Mumby, Paul A. Marshall, Long Cao & Ove Hoegh-Guldberg

Coral reefs are affected by numerous processes and stresses, including global climate change and local conditions. Using model-simulated ocean warming and acidification as well as a dynamic community model, this study quantitatively shows that in a world with warmer and acidified oceans, coral reefs will be more vulnerable to local-scale stresses.

Anthony, K. R. N., J. A. Maynard, G. Diaz-Pulido, P. J. Mumby, P. A. Marshall, L. Cao, and O. Hoegh-Guldberg, 2011: Ocean acidification and warming will lower coral reef resilience. Global Change Biology, 17, 1798–1808, doi:10.1111/j.1365-2486.2010.02364.x.


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.



Figure: Projected frequency distribution of relative abundances of corals (Acropora, panels a–c) and fleshy macroalgae (Lobophora, panels d–f) as a function of CO2 forcing and herbivore grazing rate (columns, as annual mortality rate of macroalgae). Arrows indicate direction of attraction within a sequence of three regimes: coral dominance, alternate stable states (shaded areas) and algal dominance. Simulations were run using ocean acidification and grazing as press-type disturbances whereas warming and resulting coral bleaching and mortality were modelled as acute (stochastic) events with varying frequency and intensity. Dashed vertical lines indicate combinations of coral abundance projections, CO2 regime and grazing rates subjected to sensitivity analyses.