Geoscience Reference
In-Depth Information
occurred in the past decades. When the rate of envi-
ronmental change in the geological record is fast,
the probability of extinction is increased, suggest-
ing that adaptation to slower changes is possible.
Our understanding of the sensitivity of marine
organisms to ocean acidii cation is almost entirely
based on short-term perturbation experiments, last-
ing between a few hours and a few weeks. The
results of such studies provide valuable insights
into the phenotypic response of test organisms and
communities to ocean acidii cation. However, such
studies are generally too short to allow for evolu-
tionary adaptation.
Ocean acidii cation occurs gradually over times-
cales of decades to centuries. With generation times
of hours to days, unicellular algae and bacteria will
go through tens of thousands of generations as
p
CO
2
increases to projected maximum levels, which
may be sufi cient for adaptive processes to become
relevant. Moreover, most studies so far have focused
only on selected phases of test organisms' life cycles.
An individual may experience very different envi-
ronmental conditions at different stages during its
life cycle and its potential to adapt to stresses is
likely to vary over different developmental phases.
The potential for adaptation can be investigated
experimentally in studies using long-term exposure
to elevated CO
2
. This approach is most promising for
organisms with short generation times, such as auto-
and heterotrophic microorganisms. Adaptation
potential can also be deduced from
in situ
observa-
tions at natural CO
2
venting sites, where benthic
marine communities have experienced high
p
CO
2
and low pH conditions for several centuries or mil-
lennia. While this approach provides some valuable
information on the potential range of adaptive
responses at the organism, community, and eco-
system levels, the extrapolation of the observed
responses to future ocean acidii cation is compli-
cated for a number of reasons. The fact that
p
CO
2
and pH levels at these sites strongly vary over time
and space (e.g. depending on water currents) and
often exceed values projected for the next centuries
makes it impossible to determine a dose-response
relationship. As many benthic organisms are motile
or have pelagic life stages, it is also difi cult to dis-
tinguish between the local community, which has
gone through multiple generations at the venting
site, and those individuals which have recently
invaded the area. Apart from CO
2
venting sites,
areas regularly experiencing upwelling of CO
2
-
enriched deep waters provide another natural labo-
ratory for studying adaptation potential. While
studies at these sites are facing some of the same
difi culties as those at CO
2
venting sites (e.g. con-
tinuous invasion of non-exposed individuals), the
range of
p
CO
2
is usually smaller, with less uncer-
tainty regarding the dose-response relationship.
Adaptation potential, or the lack thereof, can also be
deduced from species extinction during high-CO
2
events during earth's history. A major difi culty lies
in the uncertainty about whether these events pro-
vide a reasonable analogue for ocean acidii cation
in the Anthropocene (see Chapters 2 and 4). In sum-
mary, the level of evidence that some species will
adapt is limited. Due to the important role that
adaptive processes may have on the response of the
biota to ocean acidii cation, it is crucial to make the
best use of all available approaches for addressing
this critical issue.
15.2.2.6 Ocean acidii cation will change the
composition of communities
Ecosystems are structured by the physico-chemical
environment, including light, temperature, oxygen
conditions, availability of nutrients and food, energy
input to the mixed layer or, for benthic systems, sub-
strate stability and heterogeneity. Ocean acidii ca-
tion directly modii es the chemical boundary
conditions of marine communities and, in synergy
with climate change, might result in the reorganiza-
tion of certain marine ecosystems. For example, the
poleward migration of species (Fields e
t al.
1993)
could be impaired because ocean acidii cation will
be particularly strong at high latitudes. The differen-
tial response of organisms to changes in carbonate
chemistry is likely to modify the composition of
communities through species loss and migration,
changes in species succession, or, more generally,
through altered competition between species.
Evidence for changes in community composition
in response to ocean acidii cation is still scarce.
However, comparative analysis of shallow-water
benthic communities in the vicinity of volcanic CO
2
vents in the Mediterranean Sea demonstrates a 30%
decrease in the overall diversity as well as a decrease
