0 1993, by the American Society of Limnology and Oceanography, Inc Diel variations inthe optical properties of a marine diatom

Diel variations in the optical properties ofthe marine diatom Thalassiosira Bseudonana were examined in a nutrient-replete laboratory culture xposed to a day-night cycle in natural irradiance which simulated the light regime in upper layers of the euphotic zone (- 10 m). Measurements of the beam attenuation coefficient, absorption coefficient, and size distribution f 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 (5,) increased by as much as 35 % and the attenuation (5,) and scattering (ch) 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 5,. Variations in the real part of the refractive index (n) contributed substantially (up to 85%) to the daily range in cc and 5,. Increases in cell size were important during the morning, increasing the attenuation cross-section toa similar extent as the refractive index. Variability inn correlated strongly with changes in intracellular C concentration. The normalization of attenuation and scattering cross-sections to cellular C content resulted in a threefol