Tools to support resilience-based management in the face of climate change

The goal of this project (MTSRF Project 2.5i.4) was to develop quantitative modeling tools that help to explain the risks posed to the linked GBR social-ecological system due to the effects of climate change.

The simulation model, BleachWQ(1.0), developed as one of these tools, investigated the contributing role of reef water quality in co-determining the bleaching tolerance of symbiotic reef corals to increasing ocean temperatures (Fig. 1). The model allows various scenarios to be run which test how different percentages of nutrient reductions might help to raise coral bleaching thresholds (Fig. 2); thereby reducing the likely incidence and severity of future bleaching events due to rising water temperatures as a result of climate change. The simulated improvement in the future survival prospects of viable hard-coral-dominated reefs, due to this enhanced ‘resistance’ to bleaching impacts, is taken as a measure of coral reef resilience.

The crucial factors in the model are: water quality, as measured by chlorophyll α (Chl α) to indicate levels of dissolved inorganic nitrogen (DIN); the movement of water, as indicated by flood plumes; and temperature, as measured by sea surface temperatures (SSTs).

GBR Resilience Model

Quantitative linkage between upper thermal bleaching limits (°C) and the degree of exposure to nutrient enriched terrestrial waters (after Wooldridge, 2009a). The bleaching limits are based on a 5-day exposure period. Coastal reef waters with high DIN-enriching impact are characterised by a higher annual exceedence probability (AEP) of [Chl α 0.9 µg.L-1. The quantitative linkage implies that local management efforts to lower end-of-river DIN loads could benefit reefs by acting to raise their thermal bleaching limits.

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Within the GBR, DIN loading is typically highest at coastal locations exposed to terrestrial runoff, lowest at mid-shelf locations, and moderate at offshore locations (where upwelling of deep nutrient-rich water occurs sporadically). Therefore it is predicted that the mid-shelf reefs of the GBR will be affected the least by warming ocean temperatures. Because local land management strategies have the greatest capacity to reduce DIN loading at coastal reefs locations, scenarios have focused on these areas.

Within the coastal zone, it is important to understand that the initial impact of high nutrient runoff is experienced as a short-term (days to weeks) pulse of water, a flood event (as opposed to a steady river flow). The effect of the pulse lasts much longer (weeks to months) as the nutrients then cycle through the pelagic food webs. The mixing behaviour for dissolved nutrients means that their range of influence may extend hundreds of kilometres from river mouths, hence the importance of flood plume distributions. The enriching impact of summer runoff events typically coincides with the period of annual maximum SSTs.

GBR Resilience Model

Fig. 2: Simulated increased in the upper thermal bleaching limits (°C) of inshore corals due to specified (uniform %) reductions in end-of-river DIN loading for the numerous basins that drain the GBR catchment (after Wooldridge, 2009a)

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Results

The predictive capacity of BleachWQ(1.0) enables the future survival ‘envelope’ for coral reefs to be mapped as a function of local land management imperative and global warming scenarios. For example, for inshore reef areas on the central GBR, the model indicates that an 80% reduction in DIN may permit the maintenance of a viable (hard-coral-dominated) reef state for an additional ~50 years beyond current mortality projections for the No mitigation (‘business-as-usual’) warming scenario. Moreover, when coupled with the CO₂ Mitigation (‘Stabilisation at 450 ppm’) warming scenario, the model indicates that enforcement of ~50% reduction in DIN is sufficient to ensure the long-term persistence of the impacted reefs.

GBR Resilience Model

Fig. 3: Projected annual exceedence probability (AEP) for 50% coral mortality events (LD50) given a 20%, 40%, 60% and 80% reduction in end-of-river DIN load. The projections are run for the No mitigation (‘business-as-usual’) and CO₂ Mitigation (‘Stabilisation at 450 ppm’) global warming scenarios. The 10yr ARI (grey line) is indicative of the maximum bleaching disturbance frequency for the maintenance of a viable hard-coral-dominated reefscape. The results highlight that, as an absolute minimum, local reductions in DIN load of ~50% AND global CO₂ stabilisation at ~ 450 ppm are needed to ensure the long-term persistence (viability) of the inshore reefs of the central GBR.

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Implications

BleachWQ(1.0) indicates that regional-scale reductions in ambient DIN loads, which can be managed at the local (catchment) level, represent a crucial strategy for ameliorating climate change effects on coral reefs. An additional simulation module developed as part of Project 2.5i.4 aimed to quantify whether the economic cost associated with achieving such necessary water quality targets could be offset by the economic benefit of safeguarding future reef tourism (Thomas et al., 2009).

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Further Reading

Thomas CR, Gordon IJ, Wooldridge S, van Grieken M, and Marshall P. (2010) The development of an integrated systems model for balancing coral reef health, land management and tourism risks on the Great Barrier Reef.

Wooldridge SA and Done TJ. (2009) Improved water quality can ameliorate effects of climate change on corals. Ecological Applications 19(6): 1492-1499 [doi:10.1890/08-0963.1]

Wooldridge SA. (2009) Water quality and coral bleaching thresholds: Formalising the linkage for the inshore reefs of the Great Barrier Reef, Australia. Marine Pollution Bulletin 58(5): 745-751 [doi:10.1016/j.marpolbul.2008.12.013]

Wooldridge SA. (2009) Managing local water quality to help combat climate change impacts on the Great Barrier Reef, Australia. Report to the Marine and Tropical Sciences Research Facility. Reef and Rainforest Research Centre Limited, Cairns (23pp.).

Wooldridge S. (2009) Modelling the improved resilience of inshore coral reefs to climate change due to terrestrial water quality improvements: A case study from the Burdekin River catchment. Report to the Marine and Tropical Sciences Research Facility, 30 January 2009. Reef and Rainforest Research Centre Limited, Cairns.

MTSRF Project 2.5i.1 webpage