166
visible toward UV, with a rather constant coefficient (see Equation 17). In oceanic waters,
far from terrigenous influence, the endogenous yellow substance has a weak, albeit not
negligible influence (Bricaud et aI., 1981). Some questions remain about its true abundance,
because its optical influence is easily blended with that of filterable organisms (small bacteria
or viruses, for instance, or tiny "colored" debris also absorbing in the blue part of the
spectrum). Notwithstanding the restriction, arising from the imprecise limit between dissolved
and particulate matter, the optical influence of all other commensurable particles is to be
studied. Their role in contributing to the absorption and scattering properties of oceanic waters
will be successively examined below; for the scattering and backscattering coefficients,
conclusions will be summarized, since details can be found elsewhere (Morel and Ahn, 1991).
Absorption coefficient of oceanic waters
Analytical approach
Among the colored particles, there is no doubt that phytoplanktonic cells play the major role.
This is evident by the green shift of oceanic waters with increasing algal biomass, finally
resulting in the dark green color in eutrophic situations contrasting with the deep blue color
of oligotrophic waters. The general shape of absorption spectra by phytoplanktonic cells
accounts for this shift. The complexity of the real-world, however, renders any precise
prediction of the actual absorption by mixtures of various algae occuring in the natural
environment difficult.
The inter- or even the intraspecific variability of algal absorption is the first difficulty to cope
with when trying to predict the influence of this compartment. The adoption of an "average"
absorption spectrum obviously cannot account for all populations, which have various pigment
compositions and various package effects. Another cause of uncertainty is from that algae in
sea water are inseparable from detritus, more or less colored, and from heterotrophic protists
bearing respiratory pigments also absorbing in the violet part of the spectrum. Because of their
relative abundance, bacteria are likely the more important absorbers among these organisms.
In oceanic waters, the absorption coefficient due to all these particulate materials could ideally
be written as
visible toward UV, with a rather constant coefficient (see Equation 17). In oceanic waters,
far from terrigenous influence, the endogenous yellow substance has a weak, albeit not
negligible influence (Bricaud et aI., 1981). Some questions remain about its true abundance,
because its optical influence is easily blended with that of filterable organisms (small bacteria
or viruses, for instance, or tiny "colored" debris also absorbing in the blue part of the
spectrum). Notwithstanding the restriction, arising from the imprecise limit between dissolved
and particulate matter, the optical influence of all other commensurable particles is to be
studied. Their role in contributing to the absorption and scattering properties of oceanic waters
will be successively examined below; for the scattering and backscattering coefficients,
conclusions will be summarized, since details can be found elsewhere (Morel and Ahn, 1991).
Absorption coefficient of oceanic waters
Analytical approach
Among the colored particles, there is no doubt that phytoplanktonic cells play the major role.
This is evident by the green shift of oceanic waters with increasing algal biomass, finally
resulting in the dark green color in eutrophic situations contrasting with the deep blue color
of oligotrophic waters. The general shape of absorption spectra by phytoplanktonic cells
accounts for this shift. The complexity of the real-world, however, renders any precise
prediction of the actual absorption by mixtures of various algae occuring in the natural
environment difficult.
The inter- or even the intraspecific variability of algal absorption is the first difficulty to cope
with when trying to predict the influence of this compartment. The adoption of an "average"
absorption spectrum obviously cannot account for all populations, which have various pigment
compositions and various package effects. Another cause of uncertainty is from that algae in
sea water are inseparable from detritus, more or less colored, and from heterotrophic protists
bearing respiratory pigments also absorbing in the violet part of the spectrum. Because of their
relative abundance, bacteria are likely the more important absorbers among these organisms.
In oceanic waters, the absorption coefficient due to all these particulate materials could ideally
be written as