175
Analytical approach
The scattering cross sections of various living organisms (photo- and heterotrophic) have been
determined. Examples already provided (Fig. 2, 3) demonstrate that Qb may experience wide
inter-specific variations leading to still wider variations in the scattering cross section, Sg Qb
(from about 0.2 for bacteria to more than 1000 /-tm2 for diatoms). The highly variable
character of the "chlorophyll-specific" coefficient b' is more important when calculating Eq.
22 (see e.g. Bricaud et al., 1988). In the spectral domain 570-590 /-tm (where b' is less
affected by the absorption bands), algal cells tend to exhibit lower b' values « 0.2 m 2 mg
Chl· l ) when they are big (> 5-10 /-tm), whereas higher b' values (> 0.2 and up to 0.6 m 2
mg ChI-I) would occur in the size range 1 to 5 /-tm (Morel, 1987). Note that a rigorous sizedependency could be predicted if the refractive index increment, (n-l), was sufficiently
constant and if CiChl> the intracellular chlorophyll concentration, was also constant. The second
condition, in particular, is far from being satisfied, as C iChl may exhibit large inter- and even
intra-specific changes, from 1 to 15 Kg m- 3 (see Table 1 in Morel and Bricaud, 1986).
Therefore, the general trends for b', as described above (and adopted later on), are to be
considered only as possible hypotheses.
The weakly absorbing heterotrophic organisms act mainly as scattering objects with
predictable properties under the proviso that a value can be assigned to the refractive index
of the cellular material. The term b h can thereafter be computed if the numerical abundance
and typical sizes of heterotrophs are known (or assumed). In summary, in spite of an
increasing knowledge relative to the scattering properties at the level of individual cells
(phototrophs and heterotrophs), analytical methods are still tentative because field data about
cell densities are rather scarce.
Empirical approach
The scattering coefficient b (at A = 550 nm) has been measured within the upper layers of
the open ocean and directly related to C, the Chlorophyll + Pheophytin concentration, without
any further identification or analysis of the particles contributing to its formation. The
Analytical approach
The scattering cross sections of various living organisms (photo- and heterotrophic) have been
determined. Examples already provided (Fig. 2, 3) demonstrate that Qb may experience wide
inter-specific variations leading to still wider variations in the scattering cross section, Sg Qb
(from about 0.2 for bacteria to more than 1000 /-tm2 for diatoms). The highly variable
character of the "chlorophyll-specific" coefficient b' is more important when calculating Eq.
22 (see e.g. Bricaud et al., 1988). In the spectral domain 570-590 /-tm (where b' is less
affected by the absorption bands), algal cells tend to exhibit lower b' values « 0.2 m 2 mg
Chl· l ) when they are big (> 5-10 /-tm), whereas higher b' values (> 0.2 and up to 0.6 m 2
mg ChI-I) would occur in the size range 1 to 5 /-tm (Morel, 1987). Note that a rigorous sizedependency could be predicted if the refractive index increment, (n-l), was sufficiently
constant and if CiChl> the intracellular chlorophyll concentration, was also constant. The second
condition, in particular, is far from being satisfied, as C iChl may exhibit large inter- and even
intra-specific changes, from 1 to 15 Kg m- 3 (see Table 1 in Morel and Bricaud, 1986).
Therefore, the general trends for b', as described above (and adopted later on), are to be
considered only as possible hypotheses.
The weakly absorbing heterotrophic organisms act mainly as scattering objects with
predictable properties under the proviso that a value can be assigned to the refractive index
of the cellular material. The term b h can thereafter be computed if the numerical abundance
and typical sizes of heterotrophs are known (or assumed). In summary, in spite of an
increasing knowledge relative to the scattering properties at the level of individual cells
(phototrophs and heterotrophs), analytical methods are still tentative because field data about
cell densities are rather scarce.
Empirical approach
The scattering coefficient b (at A = 550 nm) has been measured within the upper layers of
the open ocean and directly related to C, the Chlorophyll + Pheophytin concentration, without
any further identification or analysis of the particles contributing to its formation. The