Coral reefs are under pressure from a suite of global and local-scale disturbances. Ocean warming and acidification are principal global threats to reefs, and in some regions local-scale disturbances including overfishing and eutrophication continue to be significant stressors to coral reef ecosystems. Our understanding of global and local impacts on reefs is growing, but their relative contribution to reef resilience and vulnerability in the future is unclear. Here, we analyse how the resilience of the benthic reef community, as defined by its capacity to maintain and recover to coral-dominated states, will be affected by different combinations of ocean acidification and warming scenarios and pressures from herbivore overfishing and nutrients. We use a dynamic community model integrated with the growth and mortality responses for branching corals (Acropora) and macroalgae (Lobophora). The response function for coral growth (calcification) was parameterised by ocean acidification, coral bleaching and mortality by warming, macroalgal mortality by herbivore grazing rate and macroalgal growth rate by nutrient loading. The model was run for combinations of two contrasting climate change scenarios by the IPCC, the energy conscientious B1 and the fossil-fuel aggressive A1FI, and low and high fishing pressures (on herbivores) and nutrient loading. Model outputs were probability distributions based on Monte Carlo analyses of relative coral and macroalgal abundances as a function of time and CO2 concentration.
Results/Conclusions
Results demonstrated that impairment of coral growth by ocean acidification and increased coral mortality under warming dramatically lowers reef resilience. Acidification and warming alone can drive reefs to macroalgal dominance even under high grazing intensity and low nutrients. However, under low CO2, thresholds at which coral reef resilience is lost are mostly determined by grazing intensity and indirectly by fishing pressure. Sensitivity analyses indicated that system shifts from coral to macroalgal dominance are equally comparably sensitive to bleaching, acidification and nutrient loading, but that the locations of such sensitivity peaks vary strongly with climate-change scenario and grazing intensity (fishing pressure). These findings demonstrate two important points: Firstly, reefs already subjected to overfishing and nutrification are more sensitive to climate change; and secondly, in the longer-term, management of local disturbances alone cannot significantly offset the climate change threat. To sustain coral dominance on reefs this century will requires that atmospheric CO2 concentrations as well as local or regional disturbances are minimised.
