The value of adding optics to ecosystem models: a case study
Résumé
Many ecosystem models have been developed to study the ocean's biogeochemistry, but most of these models use simple formulations to describe light penetration and spectral quality. Given that processes such as photosynthesis and photo-oxidation are uniquely important for biogeochemical processes in the upper ocean, it is necessary to model light distribution accurately. In addition, the global scale observations of proxies of biogeochemical variables are based on the color of the ocean. The ability to simulate the color of the ocean provides the possibility of comparing model simulation with these observations. Here, an optical model is coupled with a previously published ecosystem model that explicitly represents two phytoplankton (picoplankton and diatoms) and two zooplankton functional groups, as well as multiple nutrients and detritus. Surface ocean color field and subsurface light field are calculated by coupling the ecosystem model with an optical model that relates biogeochemical standing stocks with inherent optical properties (absorption, scattering); this provides input to a commercially available radiative transfer model (Ecolight). We apply this bio-optical model to the equatorial Pacific upwelling region, and find the model to be capable of reproducing many measured optical properties and key biogeochemical processes in this region. Results include large contributions by non-algal particles to the total scattering or attenuation (>50% at 660 nm) and their small contribution to particulate absorption (<20% at 440 nm), and a remarkable contribution by picoplankton to total phytoplankton absorption (>95% at 440 nm). These results are consistent with the field observations. In order to achieve such good agreement between data and model results, however, key model parameters, for which no field data is available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals the significant role of colored dissolved organic matter to the modeled properties. Coupling explicit optics to an ecosystem model provides several advantages in generating: (1) a more accurate subsurface light-field, which is important for light sensitive biogeochemical processes such as photosynthesis and photo-oxidation, (2) added constraints on model parameters that help to reduce uncertainties in ecosystem model simulations, and (3) model output which is comparable to basic remotely-sensed properties. In addition, the coupling of biogeochemical models and optics paves the road for future assimilation of ocean color and in-situ measured optical properties into the models.
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