Mapping Variations in Thermal Stress in Order to Help Manage Coral Reefs

As ocean temperatures continue to increase in the next few decades, resultant mass coral bleaching will significantly threaten reefs worldwide. Differences in thermal stress among reefs may play an important role in determining the design of marine reserves. By locating reserves in areas less prone to thermal stress, the corals inside reserves may benefit from reduced physical and biological stress, thus maximizing their overall resilience. In their study, Mumby et al.(2011) used maps of variations in thermal stress to develop hypothesis about the future response of corals to each stress scenario. Additionally, they incorporated spatially realistic predictions of larval connectivity among reefs and applied reserve design algorithms in order to create potential reserve networks for a warming environment. The results showed that reef larval dispersal is sufficient to connect reefs from desirable thermal stress conditions into a reserve network. Although such a network is viable, the reserve design is limited in its ability to account for phenotypic and genetic adaptations in corals. Mumby et al. seek to demonstrate how the design of marine reserve networks is influenced by the corals’ ability to adapt to climate change. They provide two hypotheses: that adequate larval dispersal allows marine reserve design to be stratified, and that uncertainty about coral adaptation to rising sea temperatures is a crucial component in designing marine reserve networks.
Mumby, P. J., Elliott, I. A., Eakin, C. M., Skirving, W., Paris, C. B., Edwards, H. J., Enríquez, S., Iglesias-Prieto, R., Cherubin, L. M. and Stevens, J. R. 2011, Reserve design for uncertain responses of coral reefs to climate change. Ecology Letters, 14: 132–140. doi: 10.1111/j.1461-0248.2010.01562.x

            Peter Mumby collaborated with marine ecologists from various institutions to provide an adaptive approach to reef management. All field data were collected at a depth of 710 m from 58 Bahamian sites. Several different strategies were used to test aspects of their hypotheses. A 20-year climatology of satellite-derived sea surface temperature (SST) was used to examine spatial patterns of thermal stress across reef ecosystems in the Bahamas. The information collected was then developed into four contrasting thermal stress regimes. Principles derived from previous laboratory studies were used to hypothesize the response of corals to climate change in each thermal regime. Thus, the authors predict that corals in regime A, experiencing high chronic and low acute stress, will have the most resistance to bleaching. In order to account for uncertainty over coral adaptation and acclimation, Mumby et al. considered three scenarios for testing their hypotheses. They assumed that the response of corals to thermal stress present today will continue into the future, that corals will exhibit limited local acclimation inadequate to adapt to climate change, and a “bet-hedging strategy” where larval connectivity between reefs is identified. To test the feasibility of the proposed reserve networks, the research team employed a 3D individual-based model of larval dispersal adapted for the Caribbean ecosystem. Reserve-selection algorithms were then used to design a conservation network that minimizes the cost of the reserve system and meets the conservation objectives.
            The results from this study demonstrate that a small proportion of the 58 sites, about 15 percent, are a good selection for inclusion into a reserve network. However, it is difficult to develop an optimal strategy that accounts for all the conditions present in the three scenarios. Data show that acute stress is highest in the central Bahamas, while chronic stress is highest to the west near the Gulf Stream. Generally, larval dispersal showed a west-east pattern across the Caribbean. By including thermal stratification and larval connectivity into reserve design, the authors determined that overall network performance greatly improved by as much as sixfold.

            The first hypothesis posited by Mumby et al. proved to hold true; the spatial distribution of thermal stress and larval connectivity are similar enough that networks may be stratified according to the response of corals to bleaching. Furthermore, satellite measurements of SST reveal that there is enough larval supply to generate a reserve network. Results from the second hypothesis indicate that the key difference in response scenarios is not corals ability to adapt to global warming, but the differences between the alternate scenarios. Although the authors provided potential selection sites for reef reserves based on coral adaptation to stress, they maintain that further research is needed in this area. The framework Mumby et al. develop may be adapted to future improvements in research regarding larval connection and coral response to climate change.

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