A model was developed to examine the influence of phytoplankton community structure on the relationship between diffuse attenuation and ratios of upwelling radiance. Shifts in phytoplankton communities were represented by changing mean optical properties as a function of chlorophyll (C, mg m-3), consistent with large data sets from the field and laboratory. The product of cell size and internal pigment concentration, dci, governs pigment packaging, which alters the specific absorption coefficients of phytoplankton (aph*, m2 mgChl-1). Pigment packaging was parameterized as a function of C by combining the relationship between dci and aph* from phytoplankton cultures with that between aph* and C from the field, using data for 675 nm, where absorption by accessory pigments is low. Changes in accessory pigmentation were approximated by quantifying residual variability in aph* at other wavelengths, as functions of C, once the variability with dci was taken into account. Absorption by colored dissolved organic matter (CDOM), detrital absorption, and scattering by particles were also parameterized as functions of C, so that bio-optical relationships could be modeled as functions of trophic status. The model thus reconciled recognized relationships between optical properties and C with ecologically interpretable shifts in phytoplankton communities. Empirical relationships between diffuse attenuation and ocean color were well reproduced at low (0.5 mg m-3) to medium (10 mg m-3) C. Analysis of variability imposed by a range of dci suggests that it may be possible to recognize phytoplankton communities with cell sizes and intracellular pigment concentration different from the central tendency, given a set of wavelengths which minimizes the influence of CDOM and detrital absorption. ¿ 1999 American Geophysical Union