Geoscience Reference
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Halley 2006 ; Silverman
et al
. 2007 ; Bates
et al
. 2010 )
or in experimental mesocosms (e.g. Leclercq
et al
.
2000 , 2002 ; Langdon and Atkinson 2005 ; Jokiel
et al
.
2008 ; Andersson
et al
. 2009) as well as in the
Biosphere 2 facility (Langdon
et al
. 2000 , 2003 ). In
general, studies on benthic communities have
shown that there is a strong positive coupling
between seawater carbonate saturation state and
the net community calcii cation rate (
G
). That is,
under elevated CO
2
and lower pH conditions, the
daily net community calcii cation rate and deposi-
tion of CaCO
3
were signii cantly lower relative to
the rate and deposition at ambient conditions in
these experiments (Fig. 7.3). Based on observed
relationships between net community calcii cation
and seawater aragonite saturation state, a reduction
in average Ω
a
from 3.5 to 2.5 could result in a decline
in net community calcii cation of about 15 to 130%
(-51% to -73%, Langdon
et al
. 2000 ; -18%, Leclercq
et al
. 2000 ; -15%, Leclercq
et al
. 2002 ; -97 to -102%,
Yates and Halley 2006; -133%, Silverman
et al
. 2007 ;
-50 to -64%, Andersson
et al
. 2009 ).
In some experiments, negative net community
calcii cation has been observed (i.e. carbonate dis-
solution exceeded gross calcii cation) at times of
very low carbonate saturation state occurring pre-
dominantly during the night in the absence of pho-
tosynthesis (Leclercq
et al
. 2000 ; Langdon
et al
. 2003 ;
Yates and Halley 2006 ; Andersson
et al
. 2009 ).
However, the chemical threshold in terms of the
seawater
p
CO
2
and carbonate saturation state at
which different systems become subject to net dis-
solution varies signii cantly and is a function of a
range of properties such as community composi-
tion, amount of reactive organic material, and
sediment mineral composition. Nevertheless, as
the surface-seawater carbonate saturation state con-
tinues to decrease as a result of anthropogenic
400
300
200
100
0
1
2
3
Aragonite saturation state
4
5
6
Coral community - light (Leclercq
et al
. 2002)
Coral community - dark (Leclercq
et al
. 2002)
Sand community - light (Leclercq
et al
. 2002)
Soft bottom community - light (Boucher
et al
. 1998)
Biosphere 2 - long term (Langdon
et al
. 2000)
Biosphere 2 - short term (Langdon
et al
. 2000)
Coral community - light (Leclercq
et al
. 2000)
Coral community - dark (Leclercq
et al
. 2000)
Coral community (Ohde and Woesik 1999)
Figure 7.3
Net community calcii cation as a function of seawater aragonite saturation state in a range of different experimental settings and
environments. Positive rates imply net calcii cation and net accretion of CaCO
3
whereas negative rates imply net dissolution and net loss of CaCO
3
(from
Andersson et al. 2005, reprinted by permission of the American Journal of Science ) .
