Mesophotic Coral Ecosystems

6.3. Climate change

Increasingly stressful conditions from natural and anthropogenic factors are producing significant population declines and community shifts in shallow coral reef communities. Increasing ocean temperatures as a consequence of increasing atmospheric carbon dioxide levels (IPCC 2014) have led to mass coral bleaching, an increase in the number and prevalence of diseases (produced by pathogens), and ocean waters becoming more acidic, which is predicted to severely affect growth and survivorship of calcifying marine organisms by mid-century (Hoegh-Guldberg et al. 2007, Hoegh-Guldberg and Bruno 2010, Pandolfi et al. 2011, van Hooidonk et al. 2013, Maynard et al. 2015). Bleaching and disease can cause substantial mortality in corals and can result in shifts in the composition and structure of coral communities over relatively short timescales (Miller et al. 2009, Weil et al. 2009a, b). Ocean acidification, a decrease in the pH level in seawater caused by increasing carbon dioxide levels, will reduce calcification rates in reef-building organisms, accelerate reef erosion problems and directly impact the

growth rates of coral and calcifying algae (Feely et al. 2004). Additionally, as ocean temperatures rise, it is expected that the intensity of tropical storms will increase (IPCC 2013), leading to increases in both direct (e.g. damaging storm waves) and indirect (e.g. sedimentation) impacts on MCEs. There is little understanding of the extent of the impact of increasing ocean temperatures and carbon dioxide levels on MCEs due to the difficulties of sampling at depth and a lack of long-term monitoring records for most MCEs. Like shallow reefs, MCEs are likely to show considerable spatial variability in susceptibility to climate change, depending on factors such as local-scale environmental conditions, historical exposure to extreme conditions, and differing susceptibility to stressors among different reef species. Locations where long-term data do exist, including Palau and the U.S. Virgin Islands, provide valuable insights into environmental processes affectingMCEs that can help to guide monitoring, research and management programs. For example, from long-term datasets we have now

(a)

(d)

(b)

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Figure 6.4. The loss of colour from these coral colonies is indicative of coral bleaching. (a) Leptoseris sp. in Palau at 90 m (photo Patrick L. Colin). (b) Goniopora sp. in Eilat, Israel at 60 m (photo Gal Eyal). (c) and (d) Agaricia sp. in the U.S. Virgin Islands at 60 m and 40 m, respectively (photos Tyler Smith).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 70