DIEL VARIATIONS IN THE OPTICAL-PROPERTIES OF A MARINE DIATOM

Diel variations in the optical properties of the marine diatom Thalass iosira pseudonana were examined in a nutrient-replete laboratory cultu re exposed to a day-night cycle in natural irradiance which simulated the light regime in upper layers of the euphotic zone (approximately 1 0 m). Measurements of the beam attenuation coefficient, absorption coe fficient, and size distribution of cells in suspension were used to de termine optical cross-sections and refractive index of cells. The opti cal cross-sections varied greatly over a diel cycle; the absorption cr oss-section (sigma(a)BAR) increased by as much as 35% and the attenuat ion (sigma(c)BAR) and scattering (sigma(b)BAR) cross-sections by 80% b etween minima near sunrise and maxima near sunset. These changes were associated with significant variations in refractive index and cell si ze. Diel changes in the imaginary part of the refractive index (n'), d riven by variation in intracellular pigment concentration, accounted f or 58-72% of the difference between the daily maxima and minima in sig ma(a)BAR. Variations in the real part of the refractive index (n) cont ributed substantially (up to 85%) to the daily range in sigma(c)BAR, a nd sigma(b)BAR. Increases in cell size were important during the morni ng, increasing the attenuation cross-section to a similar extent as th e 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 threefo ld reduction in the magnitude of diel variations. If such patterns are general features of planktonic microorganisms in the natural environm ent, it would suggest that varying particle concentration need not be the only (or even major) factor responsible for diel variability in th e bulk optical properties of seawater. As a consequence, estimating pa rticle 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.