Geoscience Reference
In-Depth Information
The smaller this ratio is, the better the capacity of the ocean for
absorbing CO
2
. Ocean acidification increases the Revelle factor,
which leads to a drop in the ability of seawater to absorb CO
2
.
Consequently, the oceanic sink for CO
2
should decrease in the future;
this is a positive feedback that increases the atmospheric concentration
in CO
2
. Furthermore, the solubility of CO
2
and the chemical balance
between the different types of dissolved inorganic carbon depend on
the temperature and salinity (see the Appendix, section 5.8). Both will
be modified in the coming decades leading to a decrease in the
solubility of CO
2
[ORR 11] as well as an increase in stratification and
a slowing down of the formation of deep water [SAR 98].
With regard to the biological pump, it was mentioned in the previous
section that photosynthesis can benefit the increase in CO
2
. It is
important to note that an increase in photosynthesis does not
automatically mean an increase in the biological pump. What is
important in this case is the net community production, in other words
the net balance between photosynthesis and the mineralization of
fixed carbon (respiration). It is this organic surplus of carbon that is
exported to depths, generating a net flux of atmospheric CO
2
in the
ocean. As mentioned earlier, conflicting results have been obtained
depending on the phytoplanktonic species considered and not much
data are available for communities and ecosystems. Nitrogen-fixing
cyanobacteria is the group which
has the most consistent data. They
could provide an increased amount of nitrogen for primary production,
but current data, carried out mostly on one species, do not allow
extrapolation to the world ocean. It has already been mentioned that
multiple mesocosm experiments have highlighted a change in the C:N
ratio of the organic matter produced [RIE 07]. The global impact of
this stochiometric change has been tested by Oschiles
et al.
[OSC 08]
who concluded that the resulting negative feedback will be relatively
limited. The lack of data for planktonic communities makes it
impossible to currently estimate the sensitivity of the biological pump
to ocean acidification and does not enable us to estimate the amplitude
of the possible feedback to the climate.
Given that the precipitation of CaCO
3
increases CO
2
partial
pressure, a decrease in calcification in response to ocean acidification
