DIEL VARIATIONS IN THE OPTICAL-PROPERTIES OF A MARINE DIATOM

Diel variations in the optical properties of the marine diatom Thalassiosira pseudonana were examined in a nutrient-replete laboratory culture exposed to a day-night cycle in natural irradiance which simulated the light regime in upper layers of the euphotic zone (approximately 10 m). Measurements of the beam attenuation coefficient, absorption coefficient, and size distribution of cells in suspension were used to determine optical cross-sections and refractive index of cells. The optical cross-sections varied greatly over a diel cycle; the absorption cross-section (sigma(a)BAR) increased by as much as 35% and the attenuation (sigma(c)BAR) and scattering (sigma(b)BAR) cross-sections by 80% between minima near sunrise and maxima near sunset. These changes were associated with significant variations in refractive index and cell size. Diel changes in the imaginary part of the refractive index (n'), driven by variation in intracellular pigment concentration, accounted for 58-72% of the difference between the daily maxima and minima in sigma(a)BAR. Variations in the real part of the refractive index (n) contributed substantially (up to 85%) to the daily range in sigma(c)BAR, and sigma(b)BAR. Increases in cell size were important during the morning, increasing the attenuation cross-section to a similar extent as the refractive index. Variability in n correlated strongly with changes in intracellular C concentration. The normalization of attenuation and scattering cross-sections to cellular C content resulted in a threefold reduction in the magnitude of diel variations. If such patterns are general features of planktonic microorganisms in the natural environment, it would suggest that varying particle concentration need not be the only (or even major) factor responsible for diel variability in the bulk optical properties of seawater. As a consequence, estimating particle production rates from diel variability in the beam attenuation coefficient may not be justified in some oceanic situations; however, using this variability to estimate production in terms of particulate organic C may be appropriate.