168
spectrum by the whole algal population (Bidigare et al., 1987). The important unknown in
such a reconstruction is the actual packaging which affects these pigments when they are
within the cells. To the extent that ChI a fluorescence (at 685 nm) and absorption cross
section (within the A - domain corresponding to excitation) are tightly linked (Perry and
Porter, 1989; Sosik, 1988; Sosik et al., 1989), the flow cytometric technique, via the
fluorescence signal, could also provide valuable information about the algal absorption
(accounting for the packaging effect). The advantage of such a method lies in that a great
number of cells can be analyzed and thus a significant figure could be obtained.
Indirect methods, as alternatives to experimental ones, have also been recently proposed
(Morrow et al., 1989; Bidigare et al., 1989; Bricaud and Stramski, 1990). Though they are
based upon different assumptions, they essentially rest on spectral criteria and typical slopes
of the absorption spectrum. The actual absorption spectra determined for the total particulate
matter collected on a cleared filter can be numerically decomposed and divided into the
contribution by living algal cells and by the non-algal materials (probably mainly detrital).
These methods (also that of Kishino et al., 1985) have a flaw which affects the absolute
values of the absorption coefficient when measured with the cleared filter technique: the
pathlength amplification of the measured absorption (the "(3 factor", Butler, 1962) is poorly
understood. Its prediction remains questionable, as the configuration of joined and
multilayered particles settled on a filter itself acts as a diffusing plate.
In spite of drawbacks in any of the above methods, substantial improvements on the
knowledge concerning algae and detritus are to be expected from their systematic use and their
mutual complementarity. Concerning the heterotrophic organisms, recent determinations of
their absorbing capabilities (Morel and Ahn, 1990; 1991; Stramski, 1990) open the possibility
of assessing their optical role in oceanic waters.
Empirical approach
Mainly for the purpose of modeling (see Gordon, 1987 or Morel, 1991), empirical and global
equations (derived from field data) can be adopted to predict the absorption coefficient of
oceanic waters in the upper layers as a function of a single variable, the Chlorophyll a +
spectrum by the whole algal population (Bidigare et al., 1987). The important unknown in
such a reconstruction is the actual packaging which affects these pigments when they are
within the cells. To the extent that ChI a fluorescence (at 685 nm) and absorption cross
section (within the A - domain corresponding to excitation) are tightly linked (Perry and
Porter, 1989; Sosik, 1988; Sosik et al., 1989), the flow cytometric technique, via the
fluorescence signal, could also provide valuable information about the algal absorption
(accounting for the packaging effect). The advantage of such a method lies in that a great
number of cells can be analyzed and thus a significant figure could be obtained.
Indirect methods, as alternatives to experimental ones, have also been recently proposed
(Morrow et al., 1989; Bidigare et al., 1989; Bricaud and Stramski, 1990). Though they are
based upon different assumptions, they essentially rest on spectral criteria and typical slopes
of the absorption spectrum. The actual absorption spectra determined for the total particulate
matter collected on a cleared filter can be numerically decomposed and divided into the
contribution by living algal cells and by the non-algal materials (probably mainly detrital).
These methods (also that of Kishino et al., 1985) have a flaw which affects the absolute
values of the absorption coefficient when measured with the cleared filter technique: the
pathlength amplification of the measured absorption (the "(3 factor", Butler, 1962) is poorly
understood. Its prediction remains questionable, as the configuration of joined and
multilayered particles settled on a filter itself acts as a diffusing plate.
In spite of drawbacks in any of the above methods, substantial improvements on the
knowledge concerning algae and detritus are to be expected from their systematic use and their
mutual complementarity. Concerning the heterotrophic organisms, recent determinations of
their absorbing capabilities (Morel and Ahn, 1990; 1991; Stramski, 1990) open the possibility
of assessing their optical role in oceanic waters.
Empirical approach
Mainly for the purpose of modeling (see Gordon, 1987 or Morel, 1991), empirical and global
equations (derived from field data) can be adopted to predict the absorption coefficient of
oceanic waters in the upper layers as a function of a single variable, the Chlorophyll a +