Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 5 2007 10.5194/bg-4-817-2007 http://www.biogeosciences.net/4/817/2007/ http://www.biogeosciences.net/4/817/2007/bg-4-817-2007.html http://www.biogeosciences.net/4/817/2007/bg-4-817-2007.pdf 817 835 2007-10-11 The value of adding optics to ecosystem models: a case study M. Fujii mfujii@sgp.hokudai.ac.jp E. Boss F. Chai Sustainability Governance Project, Creative Research Initiative "Sousei", Hokkaido University, N9W8, Kita-ku, Sapporo, Hokkaido 060-0809, Japan School of Marine Sciences, 5706 Aubert Hall, University of Maine, Orono, ME 04469-5706, USA Many ecosystem models have been developed to study the ocean's biogeochemical properties, but most of these models use simple formulations to describe light penetration and spectral quality. 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 fields and subsurface light fields 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. Our model results suggest that non-algal particles largely contribute to the total scattering or attenuation (>50% at 660 nm) but have a much smaller contribution to particulate absorption (<20% at 440 nm), while picoplankton dominate the 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 are available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals a significant role played by colored dissolved organic matter through its influence on the quantity and quality of the ambient light. 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