Water quality mediates resilience on the Great Barrier Reef

Threats from climate change and other human pressures have led to widespread concern for the future of Australia’s Great Barrier Reef (GBR). Resilience of GBR reefs will be determined by their ability to resist disturbances and to recover from coral loss, generating intense interest in management actions that can moderate these processes. Here we quantify the effect of environmental and human drivers on the resilience of southern and central GBR reefs over the past two decades. Using a composite water quality index, we find that while reefs exposed to poor water quality are more resistant to coral bleaching, they recover from disturbance more slowly and are more susceptible to outbreaks of crown-of-thorns starfish and coral disease—with a net negative impact on recovery and long-term hard coral cover. Given these conditions, we find that 6–17% improvement in water quality will be necessary to bring recovery rates in line with projected increases in coral bleaching among contemporary inshore and mid-shelf reefs. However, such reductions are unlikely to buffer projected bleaching effects among outer-shelf GBR reefs dominated by fast-growing, thermally sensitive corals, demonstrating practical limits to local management of the GBR against the effects of global warming. Fitting a water quality index to survey-based estimates of coral resilience finds that reefs exposed to poor water quality are more resistant to bleaching but slower to recover from disturbance and more susceptible to disease outbreaks.