| 英文摘要: | The deep sea hosts some of the world's largest, oldest, and most sensitive ecosystems. Climate change and ocean acidification are likely to have severe implications for many deep-sea ecosystems and communities, but what, if anything, can be done to mitigate these threats is poorly understood. To begin to bridge this gap, we convened a stakeholder workshop to assess and prioritize options for conserving legislatively protected deep-sea coral reefs off southeast Australia that, without management intervention, are likely to be severely degraded within decades as a result of climate change. Seventeen possible options were explored that span biological, engineering and regulatory domains and that differed widely in their perceived costs, benefits, time to implementation, and risks. In the short term, the highest priority identified is the need to urgently locate and protect sites globally that are, or will become, refugia areas for the coral and its associated community as climate change progresses.
Recent instrumental data are consistent with global models that predict that anthropogenic climate change will have long-term effects on the physics and chemistry of the deep ocean. These effects include warming of Antarctic Bottom Water1, cooling and freshening at intermediate depths at high latitudes3 and possible warming and freshening at lower latitudes4, declining inputs of surface-derived particulates5 and declining carbonate ion concentrations6. The consequences of these changes for deep-sea ecosystems could be substantial, particularly in the light of the relative constancy of the physical/chemical environments these systems have historically experienced and the typically long life spans and generation times of the biota7, 8. The latter potentially constrains the capacity of deep-sea organisms to evolve adaptations in the face of relatively rapid environmental changes. The scope for mitigating the impacts of climate change on these organisms and ecosystems is highly uncertain, in part due to the multifaceted nature of the threat, the sparse data on the physiology, ecology, and responses of deep-sea organisms, and uncertainties associated with forecasting deep-oceanic environments under climate change scenarios. Even if these issues could be overcome, however, the overarching concern is likely to be the severe logistical constraints of dealing with organisms and systems found kilometres below the sea surface. Are these constraints so difficult that mitigation efforts are effectively futile? This question was explicitly addressed at a workshop of deep-sea ecologists, oceanographers and marine reserve managers that was convened in Hobart, Tasmania. The workshop focused on the potential fate and management options for deep-sea coral reef communities in the Huon Commonwealth Marine Reserve (HCMR), off southeast Australia. Among deep-sea communities, cold-water coral reefs have been highlighted as particularly vulnerable to climate change due to projected shoaling of water undersaturated with respect to aragonite, the isomorph of calcium carbonate that constitutes the coral skeleton and consequent reef matrix. Although many deep-sea taxa physiologically compensate for and calcify in undersaturated conditions9, 10, the colonial scleractinians that build the matrix seem to be particularly sensitive to low carbonate ion concentrations and rarely occur far below the modern aragonite saturation horizon11, 12. This observation has led to predictions that the world's deep-sea coral reefs are at high risk of extinction in the near future as a result of ocean acidification11, 13.
Analyses of the HCMR reefs are consistent with this prediction. Between ≈1,000 and 1,300 m depth, the seamounts in the modern HCMR are essentially completely covered by the reef-forming stony coral Solenosmilia variabilis (Supplementary Fig. 1), which in turn supports the reserve's highest biodiversity14. The reef matrix is at least metres thick, accumulates at a rate of about 0.3 mm yr−1, and has been present since before the Last Glacial Maximum15. Directly measuring the impacts of climate change on this reef requires long-term monitoring (Supplementary Fig. 2). In the interim, we assessed the magnitude of the threat by determining the environmental tolerance ranges of S. variabilis, using a suite of direct and indirect methods, and then mapping these tolerances ranges with respect to future ocean conditions as predicted by a state-of-the-art regional geophysical model (Supplementary Information). Multiple lines of evidence suggest minimum values for reef development for temperature and carbonate saturation state (Ωarag) of ≈2.5 °C and 0.9 (that is, water 10% undersaturated with respect to aragonite), respectively. Our analyses also suggest that the coral rarely occurs in the South Pacific at water temperatures higher than 7° C. Mapping these constraints on to modern oceanographic conditions captures the known distribution of live S. variabilis around southern Australia (Fig. 1a); all samples were collected at sites with a habitat suitability predicted to be >80%, qualitatively validating the approach. The distribution of areas suitable for reef development in 2099 is shown in Fig. 1b for the range of possible carbon emission scenarios (Representative Concentration Pathways; RCPs) adopted by the IPCC. The scenarios range from very optimistic (RCP2.6; emissions peak in 2020 and then decline) to moderately optimistic (RCP4.5; emissions peak in 2040 and then decline) to less optimistic (RCP8.5; emissions continue to increase throughout the twenty-first century). Even under the moderately optimistic scenario, by the end of the century low carbonate saturation levels alone severely reduce areas with a habitat suitability >80%; at RCP8.5, they are eliminated completely, other than a small stretch of soft-sediment shelf edge adjacent to Western Australia. Imposing an upper temperature tolerance of 7 °C. essentially eliminates all habitat with a suitability >40% by 2099 (Fig. 1). Seamount peaks, which might logically be considered refugia from shoaling undersaturated seawater, become too warm for the coral to survive (Supplementary Fig. 8). In all three scenarios, aragonite saturation levels fall below that required for reef development (Ωarag < 0.9) between 2060 and 2080; by 2099, at the depths of the modern reef, they fall below the level apparently required for the coral to even survive (Ωarag < 0.84) (Supplementary Fig. 8). Although these predictive models inevitably suffer incomplete process understanding of global climate change and its impacts of deep-water circulation, chemistry and physics16 and the mechanisms that might underpin or mitigate a taxon's response to those changes17, the predicted fate of the HCMR reef appears to be robust and dire.
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