The diurnal fluctuations in solar irradiance impose a fundamental frequency on ocean biogeochemistry. Observations of the ocean carbon cycle at these frequencies are rare, but could be considerably expanded by measuring and interpreting the inherent optical properties. A method is presented to analyze diel cycles in particulate beam-attenuation coefficient (c(p)) measured at multiple wavelengths. The method is based on fitting observations with a size-structured population model coupled to an optical model to infer the particle size distribution and physiologically relevant parameters of the cells responsible for the measured diel cycle in c(p). Results show that the information related to size and contained in the spectral data can be exploited to independently estimate growth and loss rates during the day and night. In addition, the model can characterize the population of particles affecting the diel variability in c(p). Application of this method to spectral c(p) measured at a station in the oligotrophic Mediterranean Sea suggests that most of the observed variations in c(p) can be ascribed to a synchronized population of cells with an equivalent spherical diameter around 4.6 +/- 1.5 mu m. The inferred carbon biomass of these cells was about 5.2 +/- 6.0 mgm(-3) and accounted for approximately 10% of the total particulate organic carbon. If successfully validated, this method may improve our in situ estimates of primary productivity.