Upper Ocean 1-D Seasonal Models
File last modified 16 November 1998
19.4a Putting Biological Productivity in the Model
What we are going to do now is to take our best-guess model system, i.e. the model parameters which best simulate the temperature, mixed layer depth, and dissolved gas histories, and apply a variety of oxygen productivity scenarios. Up to this point, our modeling efforts have been relatively firmly constrained by some straight-forward and conceptually simple processes. What we mean is that the forms of our model processes are well understood. For example
Some of the processes are less well formulated, but are well constrained by noble gas observations:
but given enough observations, we can constrain these processes to satisfactorily handle gases in the model runs. This at least means we can handle the physicochemical behavior of oxygen.
Once we step into the realm of biological "forcing" of the model, however, anything is possible and nothing is truly credible. But will we stop here? Noooooooo, not being fearless geochemists. We shall push back the bounds of reality further, and venture into fantasyland once again.
What don't we know? Well, we don't know the depth distribution of oxygen production, and we don't know the timing. We do have the observed seasonal cycle of oxygen. Now the depth distribution of oxygen production is important. That oxygen produced well below the mixed layer will tend to be retained, whereas oxygen produced very near the surface, and especially in the mixed layer, will be very quickly lost to the atmosphere. Turn the problem around the other way, if we want to explain an observed supersaturation of oxygen (over and above what is expected from physical processes, i.e. over and above the argon excesses) we will have to work harder if it is produced near the surface rather than deeper down. The same argument applies for the timing of production: if productivity occurs before the onset of seasonal stratification, the oxygen produced is readily lost to the atmosphere by gas exchange. If it is produced after seasonal stratification, it will tend to be trapped.
While we don't have absolute, a priori knowledge of the vertical structure and timing of oxygen production, we do have some basic ideas of what it may look like. So we could take our "best guess" profiles/time-histories, and obtain a "best guess" oxygen production scenario. No, not very convincing, but a start. What we can further do is to take "extreme" cases to calculate lower and upper bounds on the rates of oxygen production. This, at least, will give us the envelope of probable productivities.
Now we have a no-see-um type situation here. Any production which occurs at the same time that vigorous convection and gas exchange takes place will tend not to be recorded in the gas cycles. However, we can use sediment trap data to give us an approximate idea of the timing of oxygen production. The rationale is that oxygen production is actually export primary production in the sense that it must be associated with the removal of carbon from the "field of play", because if the carbon were allowed to remain in the system, it would be recycled (by bacterial oxidation or zooplankton respiration) and consume the very oxygen created. Since the ratio of O2:C is approximately the same in both photosynthesis and remineralization/respiration, this would be a zero-sum game.
Now people have slandered sediment traps very badly, because it has been argued that they can greatly over- or under-trap the actual particle flux for hydrodynamic reasons. We will argue here that to a first approximation that if the traps are deep enough, this bias will approximately be season-independent (although that too may be arguable, since the bias certainly depends on particle size distributions, which are in turn likely seasonally modulated) Below is a plot of seasonally averaged organic carbon flux determinations from Deuser et al., (1990) for a 3200 m sediment trap, and surface pigment
This gives us the timing of things: correcting for the 6 week lag between the surface pigment signal and the 3200m trap carbon flux response. We make the questionable assumption that the fraction of export production reaching the trap is constant throughout the year, so that we now have construct the simplest form of oxygen production curve:
i.e., assume kind of "variables separable". Now there is not fundamental basis on which we expect this to be true, because the depth distribution of production will be driven by nutrient availability, which will change in depth structure throughout the year. Clearly this is an interim choice, and one which needs to be reconsidered if one were to do fully prognostic modeling of the system (i.e. make a predictive model of biological production driven by nutrient transport, light profiles, species succession and grazing). However, for now, we'll use a simple sinusoidal to match the character of the above curves, with the appropriate phase.s
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