Author(s): Andr Morel, Annick Bricaud
The radiation absorption within a medium supposedly non-absorbing in itself but containing an absorbing substance as particles is examined theoretically. Absorption by such particles can be treated independently of (Mie) scattering under an assumption concerning the complex refractive index that is reasonable in case of algal cells (index close to that of water and weakly absorbing substance). The effect of discreteness on absorption properties is ruled by a function of the dimensionless parameter that combines through their product the size (d) and the absorption coefficient of the cell material (acm). The limiting value of this function (d → 0) describes the case of a true solution of the same material, whereas its variations with d and acm imply that the specific absorption coefficient, for a given substance, is variable in magnitude and in spectral behaviour. Consequently, Beer's law, which rests on the existence of a constant specific coefficient, generally cannot apply when canopy changes intervene in an algal population. Various cases of such changes are studied along with their consequences on absorptive properties.The theoretical conclusions are exemplified by some experimental results concerning algal cultures. The absorption properties of the cell material for each species can be extrapolated from the actual absorption spectra of intact cells.Problems that originate from the non-constancy of the specific absorption coefficient (spectral values or mean value) are examined in view of two kinds of applications: the photosynthetic (quantum) yield evaluation and the algal biomass assessment by remote sensing, for which the constancy is generally postulated.