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synergistically with another major stressor (coral
reefs).
the biological pump from the surface is mineralized,
releasing CO 2 into waters close to the seabed. These
deep CO 2 -rich waters have low pH and a sufi ciently
low carbonate ion concentration that they are under-
saturated with respect to CaCO 3 (see Chapter 3 ),
making it difi cult for calcifying organisms to live
there. With additional atmospheric CO 2 entering the
ocean surface, the horizon that separates saturated
from undersaturated waters is shoaling. The global-
mean depth of the aragonite saturation horizon is
projected to shoal from its pre-industrial level of 1090
m to 280 m in 2100 under the IS92a scenario (see
Chapter 3 ).
These changes in saturation may affect aragonitic
cold-water corals. Nearly all of these corals now live
in deep waters where Ω a > 1, but it is projected that by
2100 under the IS92a scenario 70% of them will be
bathed in waters where Ω a < 1 (Guinotte et al. 2006 ).
Shell- or skeleton-forming animals that live in deep
water but above this horizon can currently calcify but
may be amongst the earliest to be affected by ocean
acidii cation (Turley et al. 2007). It is projected that as
the saturation horizon moves past them they may no
longer be able to calcify. Despite the sensitivity of
deep-sea environments to ocean acidii cation, very
few data are available on the biological responses. A
recent short-term experiment has shown that the
important cold-water coral Lophelia pertusa seems to
be able to calcify in slightly undersaturated water but
its rate of calcii cation decreases by 50% when kept in
high-CO 2 seawater (Maier et al. 2009 ). While tropical
coral reefs are built by a large number of species, cold-
water coral communities are constructed by one or
two species but provide shelter for many others. It is
therefore likely that the combined effect of lower cal-
cii cation and increased dissolution of pre-existing
skeletons will have a negative impact on the biodiver-
sity of cold-water coral communities. Deep-sea organ-
isms other than corals may also be affected by ocean
acidii cation as well as by the future reduction in the
oxygen concentration (Brewer and Peltzer 2009).
15.3.1 Polar seas
High-latitude oceans are cold water bodies which
have naturally low carbonate concentrations and are
therefore most sensitive to ocean acidii cation (see
Chapter 3). During the 21st century, their surface
waters will become chemically corrosive to arago-
nite, i rst in the Arctic Ocean and a few years later
the Southern Ocean. These severe conditions could
prevail over much of the surface Arctic Ocean by the
end of the century. Furthermore, the aragonite satu-
ration horizon is shoaling (moving upwards) at a
rate of 4 m yr -1 in the Iceland Sea, exposing each year
800 km 2 of seal oor to waters undersaturated with
respect to aragonite. Despite the high vulnerability
of polar areas to ocean acidii cation, the biological,
ecological, and biogeochemical consequences are
not well documented and very few perturbation
experiments have been conducted. The i rst results
were obtained on the pteropod (pelagic marine
snail) Limacina helicina which has an important role
in the food chain and the functioning of Arctic and
sub-Arctic marine ecosystems. Its aragonitic shell
serves as a ballast, enabling large vertical migra-
tions, and as a protection against predators. The
gross CaCO 3 precipitation of L. helicina decreases
logarithmically as a function of decreasing aragonite
saturation state, but still occurs, at a low rate, in
undersaturated waters (Comeau et al. 2010 ).
However, dissolution of CaCO 3 was not measured
in this experiment and the saturation level up to
which a positive balance between gross CaCO 3 pre-
cipitation and dissolution can be achieved is
unknown. The recruitment of the benthic life stages
of the spider crab Hyas araneus was shown to be
affected by ocean warming and acidii cation
(Walther et al. 2010). Knowledge about the response
of polar organisms and ecosystems to ocean acidii -
cation is still in its infancy and it is critical to gather
data on these particularly threatened ecosystems.
15.3.3 Coral reefs
Coral reefs are CaCO 3 structures located at or near
sea level constructed by scleractinian corals and cor-
alline algae. The skeletons of both types of reef
builders are particularly soluble because they are
15.3.2 Deep-sea environments
The depths of the ocean are generally rich in CO 2
because much of the organic material exported by
 
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