Geoscience Reference
In-Depth Information
case, for example, of estuaries where pH gradients often exist between
the river and river mouth. Discriminating the effects of the changes in
pH from those of other environmental parameters, such as salinity or
the concentration in oxygen, can be particularly arduous. Observations
carried out on greater spatial scales can also be considered, for
example, the effects of latitudinal variation of the saturation in CaCO
3
on calcareous shells. Changes in weight, thickness and the makeup of
the shells of tropical and polar invertebrates have been attributed to
the decrease in the saturation in CaCO
3
at higher latitudes [WAT 12].
Even in this case, it is difficult to distinguish the effects of
acidification from those of other parameters such as temperature and
salinity, which can also exhibit pronounced latitudinal changes. Well-
known examples of environments with a strong pCO
2
gradient are
zones where volcanic activity releases CO
2
. CO
2
is sometimes the
major component in the emitted gas, generating a large pH gradient
away from the CO
2
vents. This is the case, using the example of a site
located in the Gulf of Naples (Italy), where the composition of benthic
communities varies across the pCo
2
gradient, with calcifying
organisms progressively disappearing when we approach the site
where the gas bubbles are released and where the marine phanerogam
Posidonia oceanica
benefits from the high availability of CO
2
(Figure 5.10, [HAL 08]). These environments are naturally acidified,
which allow us to take into account the interactions between
organisms over long periods of time. They, therefore, provide crucial
information on the effects of the acidification of oceans on
communities and on the potential for adaptation of other organisms.
However, spatial and temporal changes in pH are sometimes
very pronounced, which complicate the interpretation of the data and
the extrapolation of results to other sites. Finally, CO
2
vents
are interesting for studying organisms and benthic communities;
however, it is not of much interest for planktonic organisms and
communities.
With regard to plankton, multiple perturbation experiments at a
community level have been carried out in recent years, with some on
large-sized mesocosms [ENG 05, RIE 13] (Figure 5.11).
