crusts on seamounts, and also on polymetallic nodules on the abyssal plain (Ramirez-
Llodra et al., 2011). These forms of mining would involve removal of a large area of the
seafloor surface, and complete removal of any associated communities, along with the
generation of large sediment and tailing plumes that may impact filter feeding
communities at a distance from the mining activity (Ramirez-Llodra et al., 2011). On the
seamounts of the Kermadec Arc, some which have already been leased for mining, cold-
water coral communities consisted of scleractinian, schizopathid, stylasterid, primnoid,
and isidid corals primarily associated with inactive areas away from hydrothermal
venting (Boschen et al., 2015). Deep-sea corals are often found on the hard substrata in
inactive vent fields, and may be subject to significant impacts from their removal due to
their long life spans and low recruitment rates.
Global climate change is affecting every community type on Earth, and its effects are
already being felt in the deep sea. Ocean warming has been recorded in numerous
deep-water habitats, but is particularly significant in marginal seas, which are home to
many of the world’s cold-water coral reefs. In particular, there is evidence that the
Mediterranean has warmed by at least 0.1°C between 1950 and 2000 (Rixen et al.,
2005), and this change has been shown to impact the deep-sea communities there
(Danovaro et al., 2004). Cold-water corals are highly sensitive to warming waters
because of their upper thermal limits, and the temperature excursions around this
general upward trend are likely to be much higher.
Ocean acidification is another pervasive threat (see Chapter 5). Continued additions of
CO
2
into the atmosphere exacerbate the problem as the oceans absorb approximately
26 per cent of the CO
2
from the atmosphere (Le Quere et al., 2009). Because the
carbonate saturation state in seawater is temperature-dependent, it is much lower in
cold waters and therefore cold-water corals lie much closer to the saturation horizon
(the depth below which the saturation state is below 1 and carbonate minerals will
dissolve) than shallow-water corals. As ocean acidification proceeds, the saturation
horizon will become shallower, thus exposing more cold-water corals to undersaturated
conditions (Guinotte et al., 2006). Solitary corals of the South Pacific are already facing
saturation states below 1 (Thresher et al., 2011), and small reef frameworks constructed
by Solenosmilia variabilis grow in periodically undersaturated waters on Northeast
Atlantic (Henry and Roberts 2014; Henry et al., 2014) and New Zealand seamounts
(Bostock et al., 2015). The Lophelia reefs of the Gulf of Mexico lie very close to the
saturation horizon, at a minimum saturation state of approximately 1.2 (Lunden et al.,
2013). Since these recent studies represent the baseline for the deep-water carbonate
system, the extent to which anthropogenic CO
2
contributes to these low values remains
unclear.
Other possible effects of global climate change include deoxygenation and changes in
sea-surface productivity. Declines in oxygen availability are primarily linked to increasing
water temperature, but also to synergistic effects of pollution and agricultural runoff,
which are most significant in shallow water. However, because some cold-water corals
live at oxygen-minimum zone depths (Davies et al., 2010; Georgian et al., 2014), even
small changes in oxygen concentration could be significant. Because cold-water corals
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live below the photic zone and rely for their nutrition on primary productivity
transferred from the surface waters to depth, changes in surface productivity could
have significant negative impacts. In particular, the increased stratification of surface
waters above the thermocline will lead to decreased productivity in high latitude spring-
bloom and upwelling ecosystems (Falkowski et al., 1998). This includes the North
Atlantic, where the most extensive examples of the known cold-water coral reefs exist.
Through in situ habitat characterization as well as by experimental approaches, it has
become clear that acidification and the expansion of oxygen minimum zones, together
with rising temperatures, will affect the average metabolism and physiology of most
scleractinians (Gori et al., 2013; Lartaud et al., 2014; McCulloch et al., 2012; Naumann et
al., 2013). However, whether such changes will result in range shifts, massive extinctions
(as suggested by Tittensor et al., 2010), or if species possess the resources to cope with
variations through phenotypic plasticity or adaptive genetic changes, is still largely
unknown. The solitary coral Desmophyllum dianthus and colonial scleractinian
Dendrophyllia cornigera have shown resistance to high temperature in aquaria
(Naumann et al., 2013). The L. pertusa colonies from the North Atlantic and
Mediterranean have shown the ability to acclimatize to ocean acidification in long-term
experiments (Form and Riebesell, 2012; Maier et al., 2012). In other experiments,
certain genotypes of L. pertusa from the Gulf of Mexico were able to calcify at
saturation states as low as 1.0, suggesting a possible genetic basis to their sensitivity to
ocean acidification (Lunden et al., 2014). However, to date no long-term studies
combining acidification with temperature stress have been produced and long-term
effects on bare skeletal structure are unknown. In addition, some cold-water coral
species seem to be resilient to some of these processes, and may hold some of the
answers for coral survival in future global climate-change scenarios. Regardless, the
projected shoaling of the aragonite saturation horizon (Orr et al., 2005) threatens the
future integrity of deep-water scleractinian reef structures world-wide (Guinotte et al.,
2006).
The ability of these organisms to keep up with the pace of ocean change and disperse
into a new environment or to recolonize depleted areas depends on the capacity for
mid- or long-distance dispersal. This capacity has been demonstrated for L. pertusa by
isotope reconstruction and genetic analysis (Henry et al., 2014), supporting the
hypothesis of a post-glacial recolonization of the Atlantic by refugees in the
Mediterranean (De Mol et al., 2002; De Mol et al., 2005; Frank et al., 2009).
Overall, L. pertusa shows a pattern of relative homogeneity within regions (e.g. the
North Atlantic), and modest but significant differentiation among regions, both for the
Western Atlantic (e.g. Gulf of Mexico vs. Southeast United States vs. North Atlantic;
Morrison et al., 2011), as well as along Eastern Atlantic margins from the Bay of Biscay
to Iceland for both L. pertusa and M. oculata (Becheler, 2013). Previous studies on the
Eastern Atlantic margin had shown less extensive connectivity, possibly reflecting the
peculiar position of fjord populations in Sweden and Norway (Le Goff-Vitry et al., 2004).
Preliminary studies on D. dianthus suggest a lack of barrier to large-scale dispersal
(Addamo et al., 2012), although bathymetric barriers to gene flow are evident (Miller et
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