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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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
6.3.
Climate change
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).
(a)
(d)
(b)
(c)