Evaluating the Effects of Climate Change and Chemical, Physical and Biological Stressors on Nearshore Coral Reefs: A Case Study in the Great Barrier Reef, Australia.

An understanding of the combined effects of climate change and other anthropogenic stressors, such as chemical exposures, is essential for improving ecological risk assessments of vulnerable ecosystems. In the Great Barrier Reef, coral reefs are under increasingly severe duress from increasing ocean temperatures, acidification and cyclone intensities associated with climate change. In addition to these stressors, inshore reef systems, such as the Mackay Whitsunday coastal zone are being impacted by other anthropogenic stressors, including chemical, nutrient and sediment exposures related to more intense rainfall events that increase catchment runoff of contaminated waters. To illustrate an approach for incorporating climate change into ecological risk assessment frameworks, we developed an adverse outcome pathway network to conceptually delineate effects of climate variables and PSII herbicide (diuron) exposures on scleractinian corals. This informed the development of a Bayesian network to quantitatively compare the effects of historical (1975-2005) and future projected climate on inshore hard coral bleaching, mortality, and cover. This Bayesian network demonstrated how risk may be predicted for multiple physical and biological stressors including temperature, ocean acidification, cyclones, sediments, macroalgae competition, and crown of thorns starfish predation, as well as chemical stressors such as nitrogen and herbicides. Climate scenarios included an ensemble of 16 downscaled models encompassing current and future conditions based on multiple emission scenarios for two thirty-year periods. It was found that both climate-related and catchment-related stressors pose a risk to these inshore reef systems, with projected increases in coral bleaching and coral mortality under all future climate scenarios. This modelling exercise can support the identification of risk drivers for the prioritisation of management interventions to build future resilient reefs.