Informing Management Decisions To Reduce GBR Coral Vulnerability Under Environmental Change

Dr. Ken Anthony, Dr. Nicholas Wolff, Prof. Peter Mumby, Dr. Eve McDonald-Madden and Dr. Michelle Devlin

The Great Barrier Reef faces a suite of environmental pressures, ranging from global climate change to local-scale impacts from declining water quality. The reported loss of coral cover on the GBR, however, has been attributed primarily to acute stress events: bleaching, cyclones and crown-of-thorns starfish. The impacts of chronic and cumulative pressures, which potentially increase coral sensitivity to stress and suppress coral recovery between disturbance events, have largely been discounted. Indeed, stress on the GBR is likely to accumulate, especially in light of rapid regional economic growth and current global warming predictions.

This collaborative and multidisciplinary project takes a forward look at GBR vulnerability under scenarios of both acute and chronic pressures. Specifically, using a model of coral growth, mortality and recovery, coral dynamics are predicted over 20-40 year horizons across 1,300 reefs, driven by global warming, cyclones, water quality and outbreaks of CoTS. Water quality is modelled based on river flows and nutrient loads propagated into the GBR lagoon using eReefs hydrodynamic models calibrated against observed chlorophyll dynamics. Further, the coral vulnerability model is integrated with structured decision analyses, providing a means to inform GBR spatial planning and decision-making (Figure 1).

Figure 1. Conceptual outline of the linked coral vulnerability model and decision-support framework (NERP TE Project 9.1). Arrows represent processes that are resolved quantitatively in the model. Ocean acidification (OA) is not formally included in the current model.

Under a business-as-usual scenario, climate change is predicted to become a strong driver of region-wide coral vulnerability as we approach year 2050, driven partly by systemic loss of coral resilience from ocean warming. Varying land-use management alternatives across catchments produce different spatial coral vulnerability patterns, opening new opportunities for targeted management. Our tentative initial conclusions suggest that the modelled scenario that has the strongest predicted impact on coral vulnerability is for agricultural development of the Normanby catchment (lower Cape York).  Here, high, simulated levels of nutrient export into Princess Charlotte Bay lead to high chlorophyll levels that initiate CoTS outbreaks propagating north into the Cape York region (Figure 2).

Figure 2. Modelled coral vulnerability (percent decline in coral cover) in the Cape York section of the GBR. Panel A shows predicted coral vulnerability over a 20-year horizon based on current water quality. Panel B shows coral vulnerability under a simulated 75% increase in dissolved inorganic nitrogen loading and 10% increase in suspended sediment exported by the Normanby River. Results are means of 100 simulations.

This project is the first to integrate drivers of global environmental change with chronic and acute pressures at the local scale within a linked predictive modelling and decision-support framework. The project team is now preparing to communicate the preliminary modelling results and the decision analyses to the GBR Marine Park Authority, Department of the Environment, Department of Environment and Heritage and other interested stakeholders.  With this consultation and engagement process we hope to achieve the following three objectives: (1) collaboratively explore the impacts of different local and regional development scenarios (e.g. changes in land-use practices), (2) examine options and scope for management interventions (e.g. spatial planning, direct CoTS control) and (3) determine their cost-effectiveness in reducing coral vulnerability and sustaining the GBR in the long term.

For more information, contact Dr. Ken Anthony (k.anthony@aims.gov.au) or Prof. Peter Mumby (p.j.mumby@uq.edu.au).