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Theoretical scattering computations are performed on a phytoplankton model consisting of three concentric spheres. The outer sphere represents the wall, membrane, or frustule and is assigned a relatively high index of refraction with no absorption. The middle layer represents the chloroplasts and is assigned a relatively low index of refraction with absorption. The core represents the balance of the cytoplasm and is given a low index of refraction with no absorption. The model is applied to an observed size distribution from the Pacific central gyre. This model produced significantly more backscattering and attenuation than a low-index, homogeneous-sphere model. Changing the distribution of cell-wall thickness as a function of particle size produces much variation in the shape of the volume scattering function.

Optical oceanography models of attenuation and scattering properties often contain simple spectral relationships. Electromagnetic theory, however, predicts fluctuations in the spectra of the attenuation coefficients and scattering properties of substances at wavelengths near an absorption peak. We have modeled these effects for phytoplankton using homogeneous, two‐layered, and three‐layered sphere models of cell structure and using a wide range of plausible particle size distributions. The magnitude of the scattering in backward directions is affected the most. The effect on the beam attenuation spectra is relatively small compared with the effect on the absorption and scattering coefficients. The backscattering coefficient shows large variability, varying by almost a factor of 3 for some models. The results suggest that beam attenuation at any wavelength in the red shorter than the wavelength of the chlorophyll absorption peak will be insensitive to the chlorophyll content of the particles. Increases in the pigment content per unit volume of phytoplankton will increase the index of refraction in the infrared and therefore increase the attenuation and scattering coefficients there.

The relationships between beam attenuation, absorption, suspended particle concentration, size distribution and pigment content are examined for a region where the particle concentration and pigment maxima are widely separated. Mie scattering analyses are performed on this data to predict the profiles of backscattering. It was found that absorption and scattering are not and have no theoretical reason to be well correlated in a vertical section even if the particle concentration is perfectly correlated with phytoplankton biomass. Due to the rapid change in irradiance with depth, pigment concentration and therefore paniculate absorption are not expected to be correlated with biomass or productivity as calculated from published laboratory results and theoretical models of phytoplankton pigment content. The beam attenuation is a good indicator of particle concentration and its vertical distribution relative to the pigment distribution is consistent with models of phytoplankton growth. An examination of data from a meridional transect of the North Pacific and from the Northeast Pacific coastal upwelling region suggested the universality of these findings. The remotesensing algorithms that predict backscattering from chlorophyll concentration were developed using near‐surface data. Use of these algorithms for the prediction of vertical structure of backscattering is discouraged.

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