A strong correlation is known to exist between coral bleaching and anthropogenic climate change. Expected increases in sea surface temperatures (SST) are predicted to drastically affect the biodiversity, ecology, and tourism of coral reef ecosystems. The coastal reefs in the Great Barrier Reef (GBR) are particularly susceptible to the threat from increased ocean warming and acidification; estimates suggest that coastal reefs in the central GBR area may lose a large portion of coral structures by as early as 2030. Additionally, studies suggest that poor water quality may exacerbate effects of heat stress by lowering the thermal bleaching threshold of certain coral species. For this reason, the study by Wooldridge et al.(2011) seeks to identify appropriate management interventions that would influence thermal tolerance, improving coral reef survival rates from thermal bleaching. Using dissolved inorganic nitrogen (DIN) levels as a measure of water quality, the authors study whether elevated DIN concentrations exacerbate the damage to reef structure caused by thermal bleaching. Furthermore, they develop a modelling framework that maps coral bleaching risks when considering two factors: local land management and global warming; this modelling framework is used to support coral reef management that aims to increase climate change resilience of reefs through localized water quality management.
Wooldridge, S.A., Done, T.J., Thomas, C.R., Gordon, I.I., Marshall, P.A., Jones, R.N. August 2011. Safeguarding coastal coral communities on the central Great Barrier Reef (Australia) against climate change: realizable local and global actions. Climatic Change, doi: 10.1007/s10584-011-0229-z.
Scott Wooldridge collaborated with various scientists to identify appropriate coral reef management options for the central Great Barrier Reef (GBR) in Australia. Wooldridge et al. employed two different software packages in order to model changes to reef ecosystems. “ChloroSim,” a decision support tool, was used to model the beneficial effects of reductions in dissolved inorganic nitrogen (DIN) concentrations. ChloroSim functions by providing a relationship between DIN concentrations, flood intensity, and bloom of phytoplankton biomass for different regions. Additionally, the software “ReefClim” was used to map future sea surface temperature (SST) warming patterns on the GBR. Using eight different climate models, “ReefClim” provides regional-scale SST estimates for two alternative global CO2 emission scenarios: “no mitigation” and CO2 mitigation leading to atmospheric CO2 concentrations of 450 ppm. The SST projections were then combined with coral mortality thresholds under different DIN to simulate future coral mortality rates up to 2100.
The results provided by this study suggest that reducing end-of-river DIN levels can considerably increase the future survival rates of locally-impacted reefs on the GBR. The simulations indicate that the potential improvement in bleaching threshold for the reef sites with the worst water quality typically require large reductions in DIN. For example, an 80% reduction in DIN results in the maintenance of the coral-dominated reef state for 50 years beyond the current coral mortality projections under the “no mitigation” warming scenario. In addition, under a CO2 mitigation warming scenario, a 50% reduction in DIN ensures the long-term survival of the impacted reefs. Thus, a combination of local and global management can help guard the long-term viability of hard coral communities within the central GBR. Regionally, the study found that the projected SST warming in the central-southern GBR is proportionately higher than the rest of the GBR.At the local level, the authors propose best methods to achieving reduced DIN concentrations; they maintain that local agricultural policy should focus on no over-application of fertilizers, reduced tillage, split fertilizer application, and removing production from the least productive soil types; doing so would eliminate over 80% of agricultural DIN exports, such as fertilizer loss. Wooldridge et al. emphasize the need for a global climate policy agreement between the largest CO2-emitting countries that will ensure atmospheric CO2 levels stabilize below 450 ppm.
A key finding of this study points to lowered DIN levels in reef waters as crucial for enhancing reef survivability during thermal stress, and aiding its recovery afterstress. Given that DIN loading is typically lowest at mid-shelf locations, it is predicted that the mid-shelf reefs of the central GBR should display the highest resistance to heat stress; therefore, this area has important implications for the designation of a future network of marine protected area (MPA). Additionally, the results indicate that the current bleach frequency in many coastal reef communities on the central GBR inhibit the reef-building capacity of these ecosystems. This further emphasizes the significant role of good water quality in enhancing reef-building capacity between distrubance events.