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made of aragonite (corals) or high-magnesian calcite
(coralline algae). Coral reefs are distributed in waters
with a relatively high aragonite saturation state (Ω
a
=
3.3 on average; Kleypas
et al.
1999 ). The tropics and
subtropics will see the biggest absolute changes in
Ω
a
with a projected drop from 4.2 in the year 1820 to
2.3 in 2100 under the A2 scenario (see Chapter 3).
Steinacher
et al.
(2009) found that under the A2 sce-
nario, waters with Ω
a
> 3 will disappear by 2070.
Coral reefs in the eastern tropical Pacii c are good
indicators of what the future of coral reefs could be.
Manzello
et al.
(2008) observed that cements in
intraskeletal pores were almost absent. Consequently,
these reefs, which experience naturally high CO
2
and
low Ω
a
as a result of upwelling, are poorly developed,
and are subject to high rates of bioerosion. The abun-
dance of cement appears to be correlated to the seawa-
ter aragonite saturation state and inversely related to
measured rates of bioerosion.
Corals and coralline algae are probably the organ-
isms that have been investigated the most with
respect to ocean acidii cation (see Chapter 7 ).
Although the dependence of calcii cation rates on
the carbonate chemistry is widespread in coral reef
organisms, a few seem to be resistant to ocean acidi-
i cation. Increased dissolution and lower calcii ca-
tion will lead, at some point in time, to a transition
from net calcii cation and CaCO
3
accretion to net
dissolution and net loss of CaCO
3
. Additionally,
zooxanthellate corals lose their endosymbiotic algae
at elevated temperature, leading to coral bleaching
and high rates of mortality (e.g. Hoegh-Guldberg
1999). The combination of ocean acidii cation and
warming undoubtedly makes coral reefs the eco-
system most threatened by global environmental
change (Hoegh-Guldberg
et al.
2007 ). Several criti-
cal issues require better investigation: the
mechanism(s) that enable some corals to resist
ocean acidii cation, the interaction between coral
bleaching and ocean acidii cation, and the response
of natural communities through long-term pertur-
bation experiments in the i eld.
demonstrates that many uncertainties remain. The
present section briel y reviews the main reasons for
this relatively poor knowledge and provides sug-
gestions on what can be done to make faster
progress in the near future.
15.4.1
Limited workforce and funding
In the 1990s, each year, between 7 and 42 individual
authors published on the issue of changes to the car-
bonate system in seawater and their impacts on
marine organisms and ecosystems (Fig. 15.2). A sig-
nii cant number of these papers were looking at the
general effects of pH on physiological processes,
sometimes at very high levels of acidity; fewer than
25 scientists were involved in investigating the effects
of the changes in the carbonate system generated by
the uptake of anthropogenic CO
2
. Ocean acidii ca-
tion was not a topic of great interest to scientists,
indeed the term had not even been coined. The
number of individual authors increased considerably
starting in 2005 and reached over 550 individual
authors in 2010. This huge increase in the workforce
600
500
400
300
200
100
0
1920
1940
1960
1980
2000
Year
Figure 15.2
Number of individual authors having published at least one
paper addressing ocean acidii cation per year. The bibliographic database
compiled by Gattuso and Hansson (see Chapter 1) was used. The complete
list of references is available as online Appendix 1.1 at
http://ukcatalogue.
oup.com/product/9780199591091.do.
Only scientii c papers were
considered and duplicate authors were removed.
15.4 Past limitations and future
prospects
Synthesis of our present knowledge about ocean
acidii cation and its consequences (Section 15.2),
