138
7. Modeling Atmosphere-Ocean Interactions and Primary Productivit y
system. The model shows that under the initial conditions the supply of
N0 3
- / Fe to the surface ocea n is significantly higher compared with the
Redfield ratios of pa rticulate matter, indicating iron limitation (Figure 7.5).
The amount of Fe input to the surface can only suppo rt a downward flux of
particulate carbon that equals 5% of the potential given N0 3
- inputs. Under
these conditions the ocea n is a source of CO 2 to the atmosphere. This
ph en omenon is typical of warm water upwelling systems where high-Ct,
nutrient-rich bottom water reaches the surface, is warmed , decreasing the
solub ility of CO 2 in water, and diffuses into the atmosp here (Coope r et al.
1996).
7.6. Conclusions
This model repre sen ts physical processes very simply: heat exchange is not
repr esented, mixing rates are constant, and in the one-dimensional model
no lateral advection occurs. The temperature of surface waters has a profound effect on the solubility of CO 2 and the resulting equilibrium concentration of dissolved CO 2 (Millero 1995). Because cold water holds much
more CO 2
, the high-latitude HNLC regions (Subarctic Pacific and Southe rn
Ocean) are more likely to act as sinks for atmospheric CO 2 despite inefficient surface-nutrient utilization. In addition, the Southe rn Ocean is an important area of deep water formation where cold surface waters and inorganic and organic carbon effectively and rap idly communicate with the
ocea n interior. Regionally specific models that inco rpora te temp eraturedep endent eq uilibrium constants for the carbonate system may allow for
more accura te calculation of CO 2 fluxes. We expect that the equilibrium
constants used for this model (20°C) lead to a conservative estimate of airsea CO 2 exchange. Including cold surface waters with increased produ ction
in response to Fe additions should lead to a much greater effect on the atmospheric p C0 2
.
Othe r simplifications in this mod el that would othe rwise contribute to
the remin eralization of nutrient s and expo rt flux in marine ecosystems are
the exclusion of higher trophic-level grazers and the microbial loop.
These trophic levels effect the recycling of nutrients in surface waters, and
therefore the amount of expo rt production . However, we assume that
the overall production in oligotrophic waters (and hen ce, the maximal
drawdown of CO 2
) is regulated by the upwelling flux of N0 3
- and, more
impo rtantly in the HNLC, the combined upwelling and aeo lian inputs of
Fe. The refore, the effects of these annual and inter-annu al processes are
minimal compa red with the a priori knowledge of iron limitation in this
system.
Because this model is in steady-state, the sources and sink terms are con-
7. Modeling Atmosphere-Ocean Interactions and Primary Productivit y
system. The model shows that under the initial conditions the supply of
N0 3
- / Fe to the surface ocea n is significantly higher compared with the
Redfield ratios of pa rticulate matter, indicating iron limitation (Figure 7.5).
The amount of Fe input to the surface can only suppo rt a downward flux of
particulate carbon that equals 5% of the potential given N0 3
- inputs. Under
these conditions the ocea n is a source of CO 2 to the atmosphere. This
ph en omenon is typical of warm water upwelling systems where high-Ct,
nutrient-rich bottom water reaches the surface, is warmed , decreasing the
solub ility of CO 2 in water, and diffuses into the atmosp here (Coope r et al.
1996).
7.6. Conclusions
This model repre sen ts physical processes very simply: heat exchange is not
repr esented, mixing rates are constant, and in the one-dimensional model
no lateral advection occurs. The temperature of surface waters has a profound effect on the solubility of CO 2 and the resulting equilibrium concentration of dissolved CO 2 (Millero 1995). Because cold water holds much
more CO 2
, the high-latitude HNLC regions (Subarctic Pacific and Southe rn
Ocean) are more likely to act as sinks for atmospheric CO 2 despite inefficient surface-nutrient utilization. In addition, the Southe rn Ocean is an important area of deep water formation where cold surface waters and inorganic and organic carbon effectively and rap idly communicate with the
ocea n interior. Regionally specific models that inco rpora te temp eraturedep endent eq uilibrium constants for the carbonate system may allow for
more accura te calculation of CO 2 fluxes. We expect that the equilibrium
constants used for this model (20°C) lead to a conservative estimate of airsea CO 2 exchange. Including cold surface waters with increased produ ction
in response to Fe additions should lead to a much greater effect on the atmospheric p C0 2
.
Othe r simplifications in this mod el that would othe rwise contribute to
the remin eralization of nutrient s and expo rt flux in marine ecosystems are
the exclusion of higher trophic-level grazers and the microbial loop.
These trophic levels effect the recycling of nutrients in surface waters, and
therefore the amount of expo rt production . However, we assume that
the overall production in oligotrophic waters (and hen ce, the maximal
drawdown of CO 2
) is regulated by the upwelling flux of N0 3
- and, more
impo rtantly in the HNLC, the combined upwelling and aeo lian inputs of
Fe. The refore, the effects of these annual and inter-annu al processes are
minimal compa red with the a priori knowledge of iron limitation in this
system.
Because this model is in steady-state, the sources and sink terms are con-