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7. Modeling Atmosphere-Ocean Interactions and Primary Productivity
organic carbon is limited by low ambient Fe concentrations. Another assumption is that increased aeo lian Fe supply could relax this limitation and
allow excess surface nutrients to be utilized . Eviden ce supporting both of
these assumptions comes from rece nt laboratory and large scale field experiments (Coale et al. 199Gb; Martin et al. 1994; Martin et al. 1989; Morel,
Rueter & Price 1991b; Sunda & Huntsman 1997). Iron's importance in the
marine geoc hemistry of C and N stems from its central role in the physiology of marine primary produ cers.
Iron is an essential element for all living organisms (Williams 1981). Photoautotrophic physiology requires Fe for the production of chlorophyll, cytochromes and for the fixation and metabolism of nitrogen as a component
of specific metalloenzymes. Although Fe is required in trace quantities compared with other major mineral nutrients, it is largely insoluble in oxygenated seawater. Surface-water concentrations of soluble, biologically
available Fe are exceedingly low « 1 x 10- 9 mol L1
) in the open ocean
and depth profiles resemble those of other major plant nutrients (Bruland
1983; Martin & Gordon 1988; Martin et al. 1989). The transport of Fe into
the euphotic zone as estimated from simple advective/ diffusive models
alone appea rs to be insufficient to allow marine phototrophs to deplete
ambient macronutrients such as nitrate and phosph ate (Martin 1990).
Without additional sources of Fe, such as aeolian deposition, HNLC regions appea r to be Fe-limited ecosystems with low levels of phytoplankton
biomass, primary produ ctivity and export produ ction.
7.4. Atmosphere-Ocean Interaction Model
The model is a one-dimensional, time-depend ent box model that tracks the
movement of carbon, nitrogen (N0 3
- ) , and Fe through a conceptualized
ope n-ocea n ecosystem. With the exception of N0 3
- , nutrients are permitted
to cycle through four major reservoirs: the atmosphere, the surface ocean,
the deep ocean and the sediment. We assume that atmospheric inorganic
nitrogen deposition is insignificant compared with the flux of upwelled nitrogen to the surface ocea n reservoir. Vertical particulate fluxes from the
surface to deep ocean in the model result from the sinking of phytoplankton cells (primarily diatoms and calcite-forming organisms).
Before we discuss the structure and function of the model, it is important
to explain the biological and chemical processes that form the heart of the
model equations. Keep ing track of the various currencies in the model-C,
N and Fe- is made simple using STELLA and a Pc. However, important
concepts in carbonate chemistry and nutrient kinetics underlie the task of
acco unting for fluxes of C, N, and Fe, and the che mical changes they undergo. Clearly und erstanding the following principles will make applying
and interpreting the model easier .
7. Modeling Atmosphere-Ocean Interactions and Primary Productivity
organic carbon is limited by low ambient Fe concentrations. Another assumption is that increased aeo lian Fe supply could relax this limitation and
allow excess surface nutrients to be utilized . Eviden ce supporting both of
these assumptions comes from rece nt laboratory and large scale field experiments (Coale et al. 199Gb; Martin et al. 1994; Martin et al. 1989; Morel,
Rueter & Price 1991b; Sunda & Huntsman 1997). Iron's importance in the
marine geoc hemistry of C and N stems from its central role in the physiology of marine primary produ cers.
Iron is an essential element for all living organisms (Williams 1981). Photoautotrophic physiology requires Fe for the production of chlorophyll, cytochromes and for the fixation and metabolism of nitrogen as a component
of specific metalloenzymes. Although Fe is required in trace quantities compared with other major mineral nutrients, it is largely insoluble in oxygenated seawater. Surface-water concentrations of soluble, biologically
available Fe are exceedingly low « 1 x 10- 9 mol L1
) in the open ocean
and depth profiles resemble those of other major plant nutrients (Bruland
1983; Martin & Gordon 1988; Martin et al. 1989). The transport of Fe into
the euphotic zone as estimated from simple advective/ diffusive models
alone appea rs to be insufficient to allow marine phototrophs to deplete
ambient macronutrients such as nitrate and phosph ate (Martin 1990).
Without additional sources of Fe, such as aeolian deposition, HNLC regions appea r to be Fe-limited ecosystems with low levels of phytoplankton
biomass, primary produ ctivity and export produ ction.
7.4. Atmosphere-Ocean Interaction Model
The model is a one-dimensional, time-depend ent box model that tracks the
movement of carbon, nitrogen (N0 3
- ) , and Fe through a conceptualized
ope n-ocea n ecosystem. With the exception of N0 3
- , nutrients are permitted
to cycle through four major reservoirs: the atmosphere, the surface ocean,
the deep ocean and the sediment. We assume that atmospheric inorganic
nitrogen deposition is insignificant compared with the flux of upwelled nitrogen to the surface ocea n reservoir. Vertical particulate fluxes from the
surface to deep ocean in the model result from the sinking of phytoplankton cells (primarily diatoms and calcite-forming organisms).
Before we discuss the structure and function of the model, it is important
to explain the biological and chemical processes that form the heart of the
model equations. Keep ing track of the various currencies in the model-C,
N and Fe- is made simple using STELLA and a Pc. However, important
concepts in carbonate chemistry and nutrient kinetics underlie the task of
acco unting for fluxes of C, N, and Fe, and the che mical changes they undergo. Clearly und erstanding the following principles will make applying
and interpreting the model easier .