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in abundance or the loss of benthic calcii ers at
lower pH. Similarly, changes in coral reef communi-
ties, in both cold and warm waters, appear likely as
community calcii cation rates will decrease with
decreasing saturation state of CaCO 3 . In the case of
tropical coral reefs, changes in the balance between
calcii cation and erosion will ultimately result in the
degradation of the reef structure and hence spatial
heterogeneity, a factor likely to amplify shifts in
community composition away from calcii ers
towards algal-dominated ecosystems.
Potential shifts in community structure for pelagic
open-ocean systems are more elusive. For example,
while marine photosynthesis appears to benei t from
increased levels of CO 2 , the extent of this CO 2 fertili-
zation effect depends on the physiological character-
istics of individual phytoplankton groups. Groups
with a comparable inefi cient carbon acquisition
pathway, such as certain coccolithophores, are
favoured. Climate change is projected to enhance
stratii cation and to reduce mixed layer depth, which
in turn reduces the injection of macronutrients to the
euphotic zone, thus creating conditions more favour-
able for nanophytoplankton, also including cocco-
lithophores. On the other hand, several studies have
reported reduced calcii cation in response to decreas-
ing calcite saturation state for the coccolithophore
Emiliania huxleyi , which ranks among the main calci-
i ers in the pelagic realm. From the preceding, it can
be concluded that future physico-chemical condi-
tions might favour nanophytoplankton at the
expense of diatoms. The case of E. huxleyi exemplii es
the interplay between climate change and ocean
acidii cation in shaping future pelagic communities.
Its outcome is highly uncertain.
The level of coni dence that ocean acidii cation
will change the composition of communities is high,
mostly due to the robust evidence available for ben-
thic communities. There is a need to collect better
information on non-calcii ers in the palaeorecord
and to determine the magnitude of the changes in
present key ecosystems.
ocean acidii cation on organisms at the base than at
the top of the food web. The relatively low sensitiv-
ity of nektonic, active organisms to ocean acidii ca-
tion is related to their high capacity for acid-base
regulation (see Chapter 8). Elevated CO 2 in body
l uids could nevertheless reduce their i tness, for
example by depressing foraging, growth, and repro-
duction. For example, ocean acidii cation was
shown to alter the behaviour of larvae of two coral
reef i sh and dramatically decrease their survival
during recruitment to adult populations, potentially
decreasing the sustainability of i sh populations
( Munday et al. 2010 ).
Possible changes in the lower trophic levels
(Section 15.2.2.6), either as shifts between dominant
phytoplankton groups or altered food quality (e.g.
altered elemental composition of phytoplankton),
are likely to spread across the food chain and affect
higher organisms that feed on them. For example,
pteropods can represent as much as 93% of the total
zooplankton biomass in high-latitude regions (Hunt
et al. 2008) and are a key food resource for many
predators such as herring, salmon, whales, or sea-
birds. The pteropod Limacina helicina can represent
as much as 60% of the prey of the juvenile pink
salmon in the northern Gulf of Alaska (Armstrong
et al. 2005). Due to their key role as a prey, a decline
in pteropod populations has the potential to gener-
ate signii cant changes at higher levels of the food
web. However, if pteropods show a considerable
decrease in abundance, it is certain that the ecologi-
cal niche they occupy will not remain empty.
Whether predators will be able to use the species
that will replace pteropods and whether the new
species will have a similar nutritional value is com-
pletely unknown.
Finally, it is important to note that the direct bio-
logical effects of ocean acidii cation operate at the
cellular level but that it is the expression of these
effects in populations and ecosystems that are of
societal concern. Scaling from physiological data to
the levels of populations and ecosystems remains
to be done (Section 15.4.6).
15.2.2.7 Ocean acidii cation will impact upon food
webs and higher trophic levels
The evidence that ocean acidii cation will have an
effect on food webs and higher trophic levels is lim-
ited. There is a lot more information on the effects of
15.2.2.8 Ocean acidii cation will have biogeochemical
consequences at the global scale
Ocean acidii cation interacts with the major ocean
biogeochemical cycles by modifying the rates of
 
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