180
empirical bulk b-values, at least when they are at their minimum. In spite of their somewhat
arbitrary character, the b* values adopted are believed to be reasonable and lead to the simple
conclusion that algae appear to contribute to at least one half of the mean scattering
coefficients in oceanic waters.
The bacterial contribution, b m , can be tentatively estimated by using Cole's expression
(Eq. 12) for the "typical" marine bacterium (with a size of 0.55 ",m and a relative refractive
index of 1.05). With these values, the computed ~ value is 0.089 and the scattering cross
section Sb is 0.021 10- 12 m 2 (cf. Eq. 11 '). After combining with Eq. 19, it results that
b -0019o.s2
m .
(25)
also represented on Fig. 10. The contribution of microbes to the formation of b turns out to
be rather small. This conclusion is only a qualitative one because of the sensitivity of b m to
the bacterial size. Indeed, the scattering cross section Sg Qb varies as the fourth power of the
size, and b m should be doubled if the typical size is increased from 0.55 up to 0.65 ",m. In
addition, the b m value depends directly on the validity of Eq. 12. It can safely be concluded,
however, that microbes are much less efficient than algae in forming the scattering coefficient.
This disproportion tends to reduce only in oligotrophic situations.
With the same assumption concerning the size distribution previously used to predict
absorption, the contribution of bigger heterotrophs to the formation of the scattering
coefficient can be estimated (Eq. 24, where Qb is computed through Eq. 11 as a function of
the size for non-absorbing particles with n = 1.05). The result, very different from that
obtained in the case of absorption, shows that the main part of scattering originates from
particles with "intermediate" sizes (Fig.9). The hypothesis of a conservative biomass
(Sheldon's rule and Junge exponentj = 4) states that the contribution of all heterotrophs with
sizes above that of microbes (assumed < 0.7 ",m) is about 7 times that of microbial organisms.
The contribution of particles with size above 10 ",m is always negligible. If the living biomass
in this range of intermediate sizes is approximately equally shared by both heterotrophs and
phototrophs (Azam et al., 1983), their respective contributions would be of the same
magnitude. When added, they account for the mean b values observed in oceanic waters (Eq.
23a).
empirical bulk b-values, at least when they are at their minimum. In spite of their somewhat
arbitrary character, the b* values adopted are believed to be reasonable and lead to the simple
conclusion that algae appear to contribute to at least one half of the mean scattering
coefficients in oceanic waters.
The bacterial contribution, b m , can be tentatively estimated by using Cole's expression
(Eq. 12) for the "typical" marine bacterium (with a size of 0.55 ",m and a relative refractive
index of 1.05). With these values, the computed ~ value is 0.089 and the scattering cross
section Sb is 0.021 10- 12 m 2 (cf. Eq. 11 '). After combining with Eq. 19, it results that
b -0019
m .
(25)
also represented on Fig. 10. The contribution of microbes to the formation of b turns out to
be rather small. This conclusion is only a qualitative one because of the sensitivity of b m to
the bacterial size. Indeed, the scattering cross section Sg Qb varies as the fourth power of the
size, and b m should be doubled if the typical size is increased from 0.55 up to 0.65 ",m. In
addition, the b m value depends directly on the validity of Eq. 12. It can safely be concluded,
however, that microbes are much less efficient than algae in forming the scattering coefficient.
This disproportion tends to reduce only in oligotrophic situations.
With the same assumption concerning the size distribution previously used to predict
absorption, the contribution of bigger heterotrophs to the formation of the scattering
coefficient can be estimated (Eq. 24, where Qb is computed through Eq. 11 as a function of
the size for non-absorbing particles with n = 1.05). The result, very different from that
obtained in the case of absorption, shows that the main part of scattering originates from
particles with "intermediate" sizes (Fig.9). The hypothesis of a conservative biomass
(Sheldon's rule and Junge exponentj = 4) states that the contribution of all heterotrophs with
sizes above that of microbes (assumed < 0.7 ",m) is about 7 times that of microbial organisms.
The contribution of particles with size above 10 ",m is always negligible. If the living biomass
in this range of intermediate sizes is approximately equally shared by both heterotrophs and
phototrophs (Azam et al., 1983), their respective contributions would be of the same
magnitude. When added, they account for the mean b values observed in oceanic waters (Eq.
23a).