Geoscience Reference
In-Depth Information
pool. All these transformations have been shown to
be potentially sensitive to CO
2
. The production of
POC through pelagic photosynthesis is the i rst step
in the biological carbon cycle, providing organic
carbon that is either consumed by heterotrophic
organisms in surface waters or exported into the
deep sea. When exported to depth it is slowly rem-
ineralized and remains out of contact with the
atmosphere over timescales of hundreds to thou-
sands of years or is buried in sediments over much
longer timescales. Results of laboratory and i eld
studies suggest that primary production is likely to
increase slightly in response to increased CO
2
, and
all else being equal, this should act to enhance the
biological carbon pump and oceanic CO
2
sequestra-
tion—providing a negative climate feedback. This
potential CO
2
-dependent increase in primary pro-
ductivity could, however, be mitigated by changes
in ocean nutrient supply related to changes in
surface-water stratii cation (Boyd and Doney 2003).
Moreover, changes in the composition of phyto-
plankton assemblages could either amplify or
diminish the impacts of altered photosynthetic rates
on carbon export (see below).
The extent to which primary production is con-
sumed versus exported depends on what fraction
of the inorganic carbon is converted into dissolved
organic and particulate inorganic forms (DOC and
PIC, respectively). The presence of both DOC and
PIC can enhance the downward transport of POC
into the deep sea, but the formation of these two
carbon pools appears to have different CO
2
sensi-
tivities. Decreased pelagic calcii cation and the
resulting decline in the strength of the carbonate
pump lower the drawdown of total alkalinity in the
surface layer, thereby increasing the uptake capac-
ity for atmospheric CO
2
in the surface ocean.
Assuming a rate of CaCO
3
export of 1 Gt C yr
-1
(Milliman 1993), the capacity of this negative feed-
back is relatively small ( Chapter 12 ; Table 6.6 ). The
sensitivity of this response in coccolithophores
appears to be species-dependent (Langer
et al.
2006 ),
permitting changes in species composition to
dampen or eliminate the response at the commu-
nity level. Shifting species composition and the
potential for adaptation (currently unknown) could
make this a transient feedback. In view of the
scarcity of information on the pH sensitivity of
foraminifera and pteropods it is premature to spec-
ulate on the signii cance of this feedback process.
CO
2
-dependent changes in calcii cation can also
have an impact on marine biogeochemical cycles
by inl uencing POC l uxes, since CaCO
3
may act as
ballast in particle aggregates, accelerating the l ux
of particulate material to depth (Armstrong
et al.
2002 ; Klaas and Archer 2002 ; but see also Passow
2004 ). Reduced CaCO
3
production could therefore
slow down the vertical l ux of biogenic matter to
depth, shoaling the remineralization depth of
organic carbon and decreasing carbon sequestra-
tion (positive feedback; see Chapter 12). The capac-
ity of this feedback depends on the pH sensitivity
of pelagic calcii ers and the quantitative impor-
tance of CaCO
3
(versus opaline silicate) as ballast
for particle export, both of which are poorly under-
stood. If CaCO
3
is a prerequisite for deep transport
of POM and if pelagic calcii cation remains sensi-
tive to high CO
2
, this feedback could have a high
capacity and extended duration (Table 6.6). A
decrease in the efi ciency of the biological pump
would lead to a change in upper-ocean nutrient
and oxygen status with likely impacts on the
pelagic community structure.
Potential changes in CaCO
3
ballasting could be
offset by CO
2
-dependent changes in DOC produc-
tion. Increased production of extracellular organic
matter under high CO
2
levels ( Engel 2002 ) may
enhance the formation of particle aggregates (Engel
et al.
2004 ; Schartau
et al.
2007 ) and thereby increase
the vertical l ux of organic matter (negative feed-
back; Table 6.6 ; see also Chapter 5 ). This response
may in fact have been responsible for the increase in
carbon drawdown observed at elevated
p
CO
2
in
some mesocosm experiments with natural phyto-
plankton assemblages (Arrigo 2007). Provided that
the response is of a general nature in bloom-form-
ing phytoplankton, this process may have signii -
cant biogeochemical implications by inl uencing
the remineralization depth for organic carbon.
Again, not knowing whether evolutionary adapta-
tion will select against this response prevents us
judging the longevity of this process.
Substantial changes in marine primary produc-
tion and nutrient cycling must be expected if ocean
acidii cation causes a systematic shift in phyto-
plankton stoichiometry. No clear trend with
p
CO
2
is
