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suggesting that laboratory selection experiments
can be used to predict the response of natural popu-
lations. Second, the evolutionary response differed
in both direction and magnitude from the short-
term (physiological) response to elevated CO
2
as it
involved an increase in both photosynthesis and
growth. The fact that short-term responses may not
scale up predictably to longer timescales is worri-
some, as current projections of the consequences of
ocean acidii cation are almost exclusively based
on short-term experiments. It highlights the need to
perform experimental tests of evolutionary res-
ponses to elevated CO
2
in marine phytoplankton
(Bell and Collins, 2008). Genomics, transcriptomics,
proteomics, and assessment of the expression of
specii c marker genes for crucial functions are
promising methods that are or soon will be availa-
ble to tackle these issues.
i ed in the presence of competitive or trophic
interactions. Hence, results obtained in isolation
from other relevant inl uences need to be verii ed
under more realistic conditions with multiple inter-
acting variables. This calls for an extension of com-
munity-level investigations (see also Section 15.4.6).
For an integrated assessment of the effects of ocean
acidii cation on marine life it will therefore be cru-
cial to cover the entire range of possible interacting
inl uences and to employ a wide spectrum of
approaches from subcellular to ecosystem-level
experimentation and modelling.
15.4.5 Lack of i eld evidence other than
around CO
2
vents
The abrupt changes and short duration of perturba-
tion experiments are key limitations (Section 15.4.3)
and it is critical that the predictions and projections
made using such experiments can be corroborated
using i eld data. Two approaches, observations
across natural CO
2
gradients and retrospective stud-
ies, have been used with mixed success; just a few
examples are provided in this section. The north-
south shift from 'overcalcii ed' to weakly 'calcii ed'
cells of the coccolithophore
E. huxleyi
in the Southern
Ocean rel ects a shift in dominance from one eco-
type to another, rather than the environmental effect
of decreased carbonate ion concentrations and
calcite saturation state on a single 'apparently
cosmopolitan' population (Cubillos
et al.
2007 ).
Observations at CO
2
vents (e.g. Hall-Spencer
et al.
2008) have demonstrated that ocean acidii cation
does have signii cant effects on benthic primary
producers, calcii ers, and biodiversity. Retrospective
studies have provided outcomes that are much less
clear, because attribution to a single environmental
factor has proven difi cult. Grelaud
et al.
( 2009 )
found an increasing carbonate mass in coccolitho-
phore shells from 1917 to 2004 concomitant with ris-
ing
p
CO
2
and sea-surface temperature in the region
of the Santa Barbara Basin, California. Although
perturbation experiments suggest that coral calcii -
cation may have decreased by about 10% since the
beginning of the Industrial Revolution, the evidence
for this has not yet been found in i eld samples.
Some (e.g. De'ath e
t al.
2009), but not all, retrospec-
tive studies show decreasing trends in calcii cation
15.4.4
Interactions with other stressors
Ocean acidii cation occurs simultaneously with
changes in other environmental variables associ-
ated with anthropogenic climate change, including
ocean warming, expanding hypoxia, changes in
salinity, physical disturbance, mixing, and stratii -
cation. There are many examples in the geological
record of pronounced extinction events when sev-
eral environmental stressors were imposed at once
(see Chapter 4). Future concurrent changes in some
environmental variables may have synergistic
effects with ocean acidii cation, amplifying or
dampening the sensitivities of organisms to reduced
pH and increased
p
CO
2
. Several of these variables
act together in constraining the window for per-
formance of organisms (Pörtner 2010). For instance,
as discussed by Pörtner and Farrell (2008), ocean
acidii cation can narrow the thermal window of
some animals, thereby further reducing their geo-
graphical distribution in a warming ocean. So far
ocean acidii cation research has given little atten-
tion to multiple interacting environmental stressors.
Future studies should therefore expand to test for
synergetic effects of environmental stressors with
ocean acidii cation.
In addition to interference from environmental
stressors, the response of an organism to a perturba-
tion such as ocean acidii cation may also be modi-
