183
(see e. g. Blank et aI., 1985), is perhaps significant in the backscattering process, even if their
optical signature is not detectable when dealing only with absorption or total scattering. Small
scales and unidentifiable biodetritic debris are potentially efficient contributors to the
backscattering process. It has recently been shown (Koike et at., 1990) that sub-micrometer
particles, mainly below 0.6/Lm, occur in the upper layers of the ocean in concentration of the
order of 1013 m- 3 • Most of them (- 95 %) are non-living, largely organic material, whereas
the remnant fraction is identifiable as bacterioplankton. To the extent that these results prove
to be ubiquitous, the open question of the backscattering coefficient formation begins to be
answered. This question is a paradoxical establishment of our ignorance, since we are able
even from space to detect the light the ocean returns, without having clearly identified the
responsible agent (beside the water molecules) for this backscattering phenomenon.
CONCLUSION
The theoretical frame for a comprehensive interpretation of the absorption, scattering, and
backscattering properties of various particles is well developeded. Variations in the optical
cross sections are predictable. When compared to experimental determinations, predictions are
to a large extent, and sometimes perfectly, verified within a wide domain of cell size, from
0.6 to about 25 /Lm, for both weakly absorbing heterotrophic organisms and variously
absorbing phototrophic algae. As a result of such comparisons, the bulk refractive index of
particles can be infered, under the proviso that the size or the size distribution is
independently determined. While being indirect, this way is perhaps the best to assess the real
part of the index of refraction, which is tightly related to the organic carbon content (except
in the case of calcite-bearing species).
A weakness of the theoretical approach, actually amenable to improvements, lies in the
simplifying assumptions of sphericity and homogeneity made when characterizing the particle.
If the spectrophotometric measurements are effected on a collection of particles (on
mono specific cultures, for instance), the departure from sphericity does not seem to
significantly affect the fit between predicted and experimental scattering properties, because
of the random orientation of the cells in suspension (very flat or elongated species admittedly
have not been studied). The heterogeneity is not easily detectable and, through the
(see e. g. Blank et aI., 1985), is perhaps significant in the backscattering process, even if their
optical signature is not detectable when dealing only with absorption or total scattering. Small
scales and unidentifiable biodetritic debris are potentially efficient contributors to the
backscattering process. It has recently been shown (Koike et at., 1990) that sub-micrometer
particles, mainly below 0.6/Lm, occur in the upper layers of the ocean in concentration of the
order of 1013 m- 3 • Most of them (- 95 %) are non-living, largely organic material, whereas
the remnant fraction is identifiable as bacterioplankton. To the extent that these results prove
to be ubiquitous, the open question of the backscattering coefficient formation begins to be
answered. This question is a paradoxical establishment of our ignorance, since we are able
even from space to detect the light the ocean returns, without having clearly identified the
responsible agent (beside the water molecules) for this backscattering phenomenon.
CONCLUSION
The theoretical frame for a comprehensive interpretation of the absorption, scattering, and
backscattering properties of various particles is well developeded. Variations in the optical
cross sections are predictable. When compared to experimental determinations, predictions are
to a large extent, and sometimes perfectly, verified within a wide domain of cell size, from
0.6 to about 25 /Lm, for both weakly absorbing heterotrophic organisms and variously
absorbing phototrophic algae. As a result of such comparisons, the bulk refractive index of
particles can be infered, under the proviso that the size or the size distribution is
independently determined. While being indirect, this way is perhaps the best to assess the real
part of the index of refraction, which is tightly related to the organic carbon content (except
in the case of calcite-bearing species).
A weakness of the theoretical approach, actually amenable to improvements, lies in the
simplifying assumptions of sphericity and homogeneity made when characterizing the particle.
If the spectrophotometric measurements are effected on a collection of particles (on
mono specific cultures, for instance), the departure from sphericity does not seem to
significantly affect the fit between predicted and experimental scattering properties, because
of the random orientation of the cells in suspension (very flat or elongated species admittedly
have not been studied). The heterogeneity is not easily detectable and, through the