Ian J. Totterdell
James Rennell Centre for Ocean Circulation, NERC, Gamma House
Chilworth Research Centre
Chilworth, Southampton, SO16 7NS, United Kingdom
The impetus to incorporate biological models in OGCMs has come so far from studies of the global carbon cycle, which are primarily interested in the ecosystems in general terms, such as primary production and export flux, rather than in the details of the ecosystems themselves. However, if the biological interactions within the ecosystems are to be modeled and understood more detailed models will be needed. They will also give more confidence that the current extremely simple, carbon-cycle-oriented models are producing good simulations for the right reasons. The most urgently needed improvements in the models, and the measurements that will be required to provide validations and parameter values, are discussed in Section 3.
In the original paper this ecosystem existed in a slab mixed layer, the thickness of which was specified over an annual cycle, and biological quantities could be lost from this layer by turbulent mixing, detrainment (in the spring) and sinking (detritus only), while nitrate was resupplied by entrainment (from the end of summer) and by turbulent mixing throughout the year. The zooplankton variable is not specified to be any particular species, and in fact combines aspects of both micro- and mesozooplankton in that it both grazes bacteria (as well as phytoplankton and detritus) and produces fecal pellets (or detrital particles) which have a significant sinking rate. The zooplankton graze on each of their three types of prey with a preference governed by that prey's availability; hence a 'switching' behavior is produced.
The model was tested initially against data from Bermuda Station 'S', and later (Fasham 1993) against data from OWS 'India' (59°N, 19°W). The comparison of the _-dimensional model at OWS 'India' showed poor agreement unless the function describing zooplankton mortality (including predation by higher organisms) was altered so that there was a lower specific rate in winter. This change enabled more zooplankton to survive the winter and so provided a larger initial population which was able to respond in time to limit the spring bloom of phytoplankton before the nutrients were depleted.
FDM90 has been embedded in the Princeton OGCM and results published for simulations of the North Atlantic (Sarmiento et al. 1993; Fasham et al. 1993). The model output was compared both with satellite CZCS data and with in situ observations at Bermuda Station 'S' and OWS 'India'. In both of the latter comparisons the biological model over-estimates the phytoplankton concentrations in the spring bloom and under-predicts the summer populations. Also the predicted zooplankton biomass is higher than that observed. These problems relate to the balance between the rates of primary production and grazing, and will be considered again later.
A version of FDM90, with carbon flows coupled to the biology (Anderson 1993) is also being run in the Princeton OGCM, with global coverage, but as yet no results are available. Drange (1994) has independently coupled carbon flows to FDM90, and embedded it in an isopycnic-coordinate OGCM of the North Atlantic. Both this implementation and that in the Princeton OGCM restrict the active biology to the upper levels of the water-column, 180 m and 123 m respectively. Another interesting study involved embedding FDM90 in a quasi-geostrophic eddy-resolving model (Burren 1993).
A more complex version of FDM90 has been developed (Ducklow & Fasham 1992). In this model (DF92), the phytoplankton compartment of FDM90 has been split into separate picoplankton and net phytoplankton compartments, while the original zooplankton compartment has become separate protozoa and mesozooplankton compartments. These additions enabled some representation to be made of the microbial loop, and indeed were made so that the role of bacteria in recycling carbon and nutrients could be examined.
Finally, and with particular interest for this meeting, some work was undertaken to embed FDM90 in a model of the Southern Ocean. The model was a version of the Fine Resolution Antarctic Model (FRAM), but with coarser resolution (1x2° rather than _ x _; hence 'CRAM'). The coarser resolution was required to reduce the computer storage and cpu-time to realistic levels. However, problems were encountered with the climatological forcing used for CRAM (and FRAM) which involved annual-mean quantities for many of the variables. In fact, the only seasonality was provided by the variation of the incident solar radiation. Also the lack of a suitable ice-model, the coarse vertical resolution (20 m) in the euphotic zone and the lack of a wind-mixed-layer meant that the project was abandoned after just a few trial runs.
A simpler biological model has been developed by Kurz (1993) for inclusion in the Hamburg HAMMOC3 ocean model. The limiting nutrient is phosphate, and there are also compartments for phytoplankton and zooplankton. The biological processes included are primary production, grazing, natural mortality of phytoplankton and loss from the zooplankton to sinking detritus (due to fecal pellet production and/or higher predation). The biological model is restricted to the mixed-layer, which in the Hamburg model has no seasonal variation. This model has the advantages that it is simple to understand and runs exceedingly fast (the biological time step is one week; the physical time step is one month). It is also an improvement on the previous implicit biology of the Hamburg model. However, particularly in its current implementation, this model is most suitable for long climate runs rather than studying the dynamics of ecosystems.
Taylor, Harbour, Harris, Burkill and Edwards (1993) describe a model (hereafter THHBE93) of the ecosystem in the North Atlantic, based on work by Azam et al. (1983) and Taylor & Joint (1990). This nitrogen-based model includes bacteria, picophytoplankton, heteroflagellates, phytoflagellates, micrograzers (or ciliates), net diatoms and net phytoplankton, as well as silicate, nitrate, ammonium, DOC and detritus. With so many compartments it is obviously impractical to list every biological interaction, but in general the modeled processes are photosynthesis (including uptake of nutrients), grazing (that by mesozooplankton is imposed from measured rates), excretion and remineralization, as well as sinking (diatoms and detritus only). The detailed inclusion of the smaller organisms and their interactions allows a good description of the microbial loop, while having diatoms as a separate variable allows the silicate-limitation of their growth to be included.
The model is compared to the (relatively-detailed) observations made during the North Atlantic Bloom Experiment (NABE) in 1989 and on cruises during the following two years. The area of study covers both areas that exhibit nutrient depletion in the summer (e.g. 47°N, 20°W) and those which remain nutrient-replete (e.g. 60°N, 20°W), and THHBE93 successfully simulates both types of annual cycle. This model has so far only been run in half-dimensional mode (confined to a mixed layer).
The Hadley Centre for Climate Prediction and Research, part of the U.K. Meteorological Office, have developed a coupled ocean-atmosphere GCM, which can also be run (with suitable climatological forcings) in ocean-only or atmosphere-only mode. Work is nearing completion on a model of the global carbon cycle to be embedded in this coupled OAGCM, and ocean-only runs have been performed with a sub-model describing the marine biology. This model is nitrogen-based (with coupled carbon flows) and has nutrient, phytoplankton, zooplankton and detritus as the variables. The phytoplankton photosynthesize (taking up nutrient) and suffer mortality (becoming detritus); the zooplankton graze on both phytoplankton and detritus, produce fecal pellets (detritus) and suffer mortality due to predation (mainly becoming detritus); while the detritus sinks and is remineralized.
In the current implementation the biology is not restricted to the top couple of hundred meters, though the rate of photosynthesis is set to zero below 180 m. A fraction of the detritus can be found sinking at all depths, and zooplankton acting as detritivores can be found to below 400 m in the aftermath of a North Atlantic bloom. This model can, with a single parameter set, simulate the low-chlorophyll, high-nutrient ecosystem observed at OWS 'Papa' and in many parts of the Southern Ocean, and also the nutrient-depleting blooms of the North Atlantic, given suitable climatological forcings. This model is particularly constrained by computer storage and cpu-time requirements, as the Hadley Centre coupled OAGCM cannot be run in an 'off-line' mode. Having only one compartment for nutrients, it cannot distinguish new and regenerated production.
There are other models that deserve mention. Hofmann & Ambler (1988) produced a biological model for the US continental shelf featuring detritus, two size-classes of phytoplankton and five stages of the copepod life-cycle (including the egg and the adult stages). This was then embedded in a 2-D physical model (Hofmann 1988). There are also reports that an ecosystem model developed jointly at the Marine Research Centre in Helsinki, Finland and the Marine Research Institute in Tallinn, Estonia has been experimentally incorporated in the OPYC isopycnic code in Hamburg (OPYC circular no. 46, 22nd Dec. '93).
Also, while the large-scale models developed to date have mainly been intended - at least as incorporated in the OGCMs - to examine the role played by the ocean biology in the global carbon cycle, that will not always be the case. The marine ecosystems are interesting in themselves, and to simulate them in any detail, and understand the interactions, more complex models with better parameterized processes will have to be developed.
I see it as crucial, therefore, that extensive measurements are made of microzooplankton biomass and grazing rates in any field program in the Southern Ocean. As yet there are no automated techniques, but that should not be allowed to prevent the measurements being made.
The experimental equipment for taking long, continuous and automated readings of chlorophyll in the top 500 m of the water column already exists. Laboratories in the U.K., particularly IOSDL and PML, have made extensive use of the SeaSoar for this purpose, so it should be possible to get detailed information on the spatial variability of phytoplankton.
The lateral heterogeneity of the mesozooplankton populations is probably even more important to measure. This should be especially true in the Southern Ocean where krill swarms can devastate regions of high productivity. Indeed, it may not be possible to model this trophic level accurately by an Eulerian model. Acoustic techniques for measuring large populations of mesozooplankton are now becoming available, and should be used in any Southern Ocean survey.
Lastly, the Bellingshausen Sea surveys along 85°W in the austral summer 1992-3 discovered a 'jet' of extremely high chlorophyll (D. Turner, pers. comm.). It was at about 67°S-69°S, and the chlorophyll values were as high as 5-6 mg-Chl a/m3. Further west (upstream), the values were 7 mg-Chl a/m3, and it was believed that the jet was carrying biological material advected from a zone of very high productivity further west, possibly near Peter the First Island. The ecosystem to the south of this jet was totally dominated by material detrained from it, at least at the time of the survey, which was over a month after the retreat of the ice. Should we aim to be able to model such locally-important features?
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