Theoretical Results Concerning the Optics of Phytoplankton, with Special Reference to Remote Sensing Applications

Summary

By using Mie theory, computations on particle scattering have been performed for both non-absorbing and weakly absorbing spheres of various sizes, and for various values of the real and imaginary parts of the refractive index. By combining these numerical data with different size distribution functions (assumed to be representative of phytoplankton populations and/or total seston populations), results are obtained concerning the efficiency factors for scattering and absorption, the backscattering efficiency and finally the spectral variations of these quantities in the vicinity of and inside absorption bands. If the anomalous scattering pattern, associated with anomalous dispersion, does not seem to occur for polydisperse systems (except for nanoplankton of very small size, say: dlt;5–8 µm), scattering is however clearly affected, i.e. depressed, by increasing absorption, in the size range of common algal cells.

The same efficiency factors are also obtained throughout the spectrum by means of an optical modelling of algal cells (spectral absorption, index, mean diameter, size distribution as independent variables). The results show that: i) the backscattering remains weak but is strongly influenced by absorption and ii) the specific absorption (per unit of pigment concentration in the water) varies with cell size as well as intracellular pigment concentration. Even if these possible variations lead to greater complexity, they have to be taken into account when modelling ocean colour.