Geoscience Reference
In-Depth Information
CO
2
fertilization depends on the physiological char-
acteristics of individual phytoplankton groups,
favouring in particular organisms with a compara-
ble inefi cient carbon acquisition pathway. Chapter
7 gives a detailed discussion of the effects of CO
2
fertilization on phytoplankton. Enhanced photo-
synthesis
per se
will not cause a change in the net
air-sea balance of CO
2
. This requires a net increase
in net community production, i.e. the net balance
between gross CO
2
i xation and respiration, as it is
net community production that provides the organic
carbon that can be exported to depth. Furthermore,
marine primary production is limited by irradiance
and nutrients, and the total biomass is kept in check
by grazing by zooplankton. Thus, only if those lim-
iting factors can be overcome by CO
2
fertilization,
causing an additional export of organic carbon by
the soft-tissue pump from the surface, will ocean
acidii cation-induced changes in primary produc-
tion cause earth system feedbacks.
is of similar magnitude to the negative feedbacks
estimated by modelling studies for CO
2
-sensitive
calcii cation rates (Section 12.2.2.4). A dominant
'side' effect of the enhanced C:N ratios identii ed by
Oschlies
et al.
(2008) is the enhanced oxygen con-
sumption associated with the respiration of carbon-
rich organic matter at depth. In their model, this
leads to a 50% expansion of oceanic suboxic regions,
with direct consequences for the amount of nitro-
gen loss by denitrii cation and anaerobic ammo-
nium oxidation (anammox), and hence for the
oceanic inventory of i xed nitrogen.
The negative feedback of enhanced inorganic
carbon-to-nitrogen consumption depends on the
export of the additional organic carbon produced
out of the surface mixed layer. For a pelagic Arctic
ecosystem, Thingstad
et al.
( 2008 ) showed that this
depends on the presence or absence of growth-lim-
iting nutrients for both autotrophic and hetero-
trophic processes. Depending on the nutrient status,
enhanced production of organic carbon can even
lead to reduced phytoplankton biomass as a result
of stimulated bacterial competition for nutrients.
Ecological impacts can also be induced by changes
in temperature, which may result in shifts among
autotrophic and heterotrophic processes (Wohlers
et al.
2009). Future studies are needed to examine
the combined effects of elevated CO
2
and higher
temperatures.
12.2.3.2 Export
A powerful way for the soft-tissue pump to over-
come the stringent control of nutrients on primary
production and export is to alter the stoichiometric
nutrient to carbon ratio of the organic matter pro-
duced and exported. Mesocosm experiments with
natural plankton communities have indeed reported
enhanced carbon drawdown under elevated CO
2
( Riebesell
et al.
2007). In these experiments, the stoi-
chiometry of the carbon-to-nitrogen drawdown
increased from 6.0 at 350 μatm to 8.0 at about 1050
μatm. While the signii cance of this excess carbon
drawdown is not fully established yet and the
mechanisms not fully understood, a possible route
is via enhanced carbon i xation by the phytoplank-
ton at higher CO
2
levels, exudation of carbon-rich
dissolved organic matter, and its subsequent export
in form of aggregates (Arrigo 2007). To estimate the
potential global impact of such a CO
2
-sensitive stoi-
chiometry of C:N drawdown and, possibly, export,
Oschlies
et al.
( 2008 ) extrapolated the mesocosm
results to the global ocean by means of a simple
ecosystem-circulation model. They found that
enhanced C:N ratios could accomplish a negative
feedback on atmospheric carbon levels for a busi-
ness-as-usual scenario (SRES A2; see Chapter 15)
amounting to 34 Gt C by the end of the century. This
12.2.3.3 Remineralization
Most of the exported organic carbon is remineral-
ized in the upper 1000 m, but about 10% escapes to
the deep ocean, where it is remineralized or buried
in sediments and sequestered from the atmosphere
on geological timescales. Therefore, changes in the
efi ciency with which the organic carbon is trans-
ported to depth provide a powerful means to alter
the overall efi ciency of the soft-tissue pump and
thereby alter the air-sea balance of carbon, giving
rise to an indirect group 1 type feedback.
The analysis of deep l uxes (water depth > 1000
m) of particulate inorganic and organic carbon sug-
gests a close association of both phases (Armstrong
et al.
2002 ; Klaas and Archer 2002 ). While the
exact mechanism behind this observation awaits
further elucidation (Passow and De La Rocha 2006),
Armstrong
et al.
( 2002 ) proposed that CaCO
3
acts as
