The optical properties of the oceanic cyanobacterium Synechococcus (clone WH8103) were examined in a nutrient-replete laboratory culture grown under a day-night cycle in natural irradiance. Measurements of the spectral absorption and beam attenuation coefficients, the size distribution of cells in suspension, and microscopic analysis of samples were made at intervals of 2--4 hours for 2 days. These measurements were used to calculate the optical properties at the level of a single ''mean'' cell representative of the actual population, specifically, the optical cross sections for spectral absorption &sgr;a(λ), scattering &sgr;b(λ), and attenuation &sgr;c(λ). In addition, concurrent determinations of chlorophyll a and particulate organic carbon allowed calculation of the Chl a- and C-specific optical coefficients. The refractive index of cells was derived from the observed data using a theory of light absorption and scattering by homogeneous spheres. Low irradiance because of cloudy skies resulted in slow division rate of cells in the culture. The percentage of dividing cells was unusually high (>30%) throughout the experiment. The optical cross sections varied greatly over a day-night cycle, with a minimum near dawn or midmorning and maximum near dusk.

During daylight hours, &sgr;b and &sgr;c can increase more than twofold and &sgr;a by as much as 45%. The real part of the refractive index n increased during the day; changes in n had equal or greater effect than the varying size distribution on changes in &sgr;c and &sgr;b. The contribution of changes in n to the increase of &sgr;c(660) during daylight hours was 65.7% and 45.1% on day 1 and 2, respectively. During the dark period, when &sgr;c(660) decreased by a factor of 2.9, the effect of decreasing n was dominant (86.3%). With the exception of a few hours during the second light period, the imaginary part of the refractive index n' showed little variation over a day-night cycle, and &sgr;a was largely controlled by variations in cell size. The real part of the refractive index at λ=660 nm was correlated with the intracellular C concentration and the imaginary part at λ=678 nm with the intracellular Chl a concentration. The C-specific attenuation coefficient showed significant diel variability, which has implications for the estimation of oceanic primary production from measurements of diel variability in beam attenuation. This study provides strong evidence that diel variability is an important component of the optical characterization of marine phytoplankton. ¿ American Geophysical Union 1995