Oceanic Primary Production: Comparison of Models

Abstract

Models that are available for computation of underwater primary production may be classified as available-light models, absorbed-light models, inherent-optical property models or growth models. A systematic comparison of these models reveals that it is simple to transform one type of model into another, and that all these models yield similar results when implemented with comparable parameters. The basic parameter set required to implement the models is small: given the initial slope of the photosynthesis-irradiance curve, the assimilation number, the specific absorption coefficient of phytoplankton and the carbon-to-chlorophyll ratio, one can implement any primary production model, since other parameters used in some of the models can be derived from this basic set. Residual uncertainty lies less with the models than with the parameters required for their implementation, when these models are used for remote sensing of primary production. Models may be classified also as spectral and non-spectral models. In both spectral and non-spectral models, the coupling between light and production occurs through the product of irradiance and the initial slope of the photosynthesis-irradiance curve. In non-spectral models one uses the product of the spectrally-averaged initial slope and total irradiance. In spectral models the spectral integral of the product of wavelength-resolved initial slope and irradiance are used. It can be shown that spectral and non-spectral models differ from each other to the extent that the covariance between spectrally-resolved initial slope and irradiance differs from zero. It can be shown through simple computations that the covariance term would go to zero only in exceptional cases. Given that the spectral dependence of photosynthesis and the spectral dependence of light penetration underwater are well-understood phenomena, there is little justification for not incorporating spectral effects into primary production models. Differences in spectral characteristics of absorption by different functional types of phytoplankton are small but significant. Coupled with the changes in the spectral quality of the underwater light field as one moves from oligotrophic to eutrophic waters, these differences could influence the types of phytoplankton that are likely to flourish in different types of waters.