Geoscience Reference
In-Depth Information
actively chew, rasp, break, or penetrate carbonate
substrates by releasing CO
2
or other acids (Fig. 7.1;
e.g. Alexandersson 1975 ; Tribollet 2008 ). This proc-
ess is referred to as bioerosion. The absolute extent
and rate of CaCO
3
dissolution from any given shal-
low environment is poorly quantii ed owing to the
fact that most studies are only able to characterize
the net effect of calcii cation minus dissolution
( Langdon
et al
. 2010). The relative magnitude and
importance of metabolic dissolution and bioero-
sion are not well known. Nonetheless, assuming
that the biological processes generating bioerosion
and metabolic dissolution remain unaffected by
ocean acidii cation and climate change, carbonate
dissolution as a result of these processes is likely to
increase because the initial seawater carbonate
saturation state will become progressively lower
as a result of ocean acidii cation. Recent experi-
mental results indicate that bioerosion may indeed
become increasingly efi cient in breaking down
carbonate material under higher CO
2
conditions
( Tribollet
et al
. 2009 ).
It is highly likely that carbonate dissolution will
increase due to ocean acidii cation, which is impor-
tant because this consumes CO
2
and acts as a sink
of anthropogenic CO
2
, albeit a small one on the
decadal to centennial timescale. It also increases
total alkalinity, which increases the ability of sea-
water to absorb CO
2
. Hypothetically, if dissolution
of shallow-water carbonate minerals could keep up
with the oceanic uptake of anthropogenic CO
2
, this
process could act as a buffer and prevent major
changes in surface-seawater pH and carbonate sat-
uration state resulting from this process. However,
it has been demonstrated that the rate of dissolu-
tion is too slow relative to the rate of uptake of
anthropogenic CO
2
and the time seawater resides
in shallow regions in contact with carbonate miner-
als to produce a signii cant buffer effect on times-
cales of decades to centuries (Andersson
et al
. 2003 ,
2005 ; Morse
et al
. 2006). In addition, the size of the
reactive coastal ocean carbonate reservoir is too
small to enhance the buffer capacity substantially
( Morse
et al
. 2006 ). On longer timescales of several
thousands of years, dissolution of carbonate sedi-
ments, particularly in the deep sea, will be the ulti-
mate sink of anthropogenic CO
2
( Archer
et al
. 1998 ;
see Chapter 2 ).
Effect on major benthic organisms
Until the late 1990s, only a few studies had been con-
ducted to investigate the response of benthic organ-
isms such as corals and algae to seawater CO
2
conditions anticipated as a result of anthropogenic
ocean acidii cation (e.g. Smith and Roth 1979;
Agegian 1985 ; Gao
et al
. 1993). For the past decade,
and concurrent with the rising awareness and con-
cern about the problem of ocean acidii cation,
numerous studies and experiments have been con-
ducted with a range of different taxa of benthic
organisms (see Chapter 1). Nonetheless, although
some observed trends appear relatively consistent
for some organisms, such as the dependence of coral
calcii cation rates on seawater Ω (Fig. 7.2), there are
still inconsistencies and substantial variations
between results, and there are many important
groups of organisms for which we have a poor
understanding or a lack of data on how they might
respond to rising CO
2
. Table 7.1 summarizes some of
the major results reported to date for a range of
marine benthic organisms including algae, bivalves,
corals, crustaceans, echinoderms, foraminifera, and
seagrasses exposed to elevated CO
2
conditions
under different experimental settings and durations.
The focus is mainly on results from studies con-
ducted under
p
CO
2
and pH conditions anticipated
as a result of present and future anthropogenic ocean
acidii cation, although some of these studies have
extended their observations and treatments well
beyond these conditions. Nonetheless, Table 7.1 is
not an exhaustive list of studies and contains a sub-
set of the published results available. For additional
discussion on the effects of ocean acidii cation on
benthic organisms and physiological effects in gen-
eral see also Chapters 9 and 8, respectively.
7.3.2
Effect on shallow benthic communities
A limited number of ocean acidii cation studies
have been conducted at the community scale. The
majority of these investigations have been con-
ducted on subtropical or tropical calcifying commu-
nities in the natural environment (e.g. Yates and
