171
A population of microbes with a numerical abundance NIV (cell per m- 3 ) will give rise to an
absorption coefficient am = (NIV) Sa- Typical marine bacteria have sizes between OA and 0.7
ILm (for the present computations, a mean size of 0.55 ILm is adopted), and the absorption
coefficient of the cellular material, as (at" = 415 nm) is about OA5 lcP m- I (Morel and Ahn,
1990). The absorption cross section of such a bacterium amounts to 3.9 10- 15 m 2 at its
maximum, 415 nm.
In the photic zone, bacterial numbers (m- 3 ) and Chi a concentrations (in mg m- 3 ) seem to be
significantly correlated through a non linear expression proposed by Cole et al. (1988) and
adopted here, written as
(NjV)-0.91 10 12 (Chi) 0.52
(19)
Therefore, the partial absorption due to the presence of bacteria can be established as a
function of Chi a concentration (Fig. 8a) according to am = (NIV) Sa, leading to
a -3.57 10- 3 (Chl)O.52
m
To go further and account for bigger protists, some hypotheses are needed. The absorption
cross sections for heterotrophic flagellates (mean size 3A ILm) and naked ciliates (mean size
13.5 ILm) have been determined (Morel and Ahn, 1991), and the absorption coefficients of
their cellular material (at" = 415 nm) have been deduced. They do not markedly differ from
that of bacteria and the above as value can be kept for further computations. The contribution
of these protists to absorption can be estimated, provided that their numerical abundance is
known or assumed. Abundances of flagellates and microbes have been studied in the field
(Sieburth, 1983; Geider, 1988); flagellate and aloricate ciliate biomasses have also been
compared (Sherr et aI., 1986). This information, however, remains relatively scarce and
generalization is presently risky. A working hypothesis (and in effect an upper limit) could
consist of considering that, within the chain bacteria - flagellates - ciliates, equal bio-volumes
occur in logarithmically equal size intervals (as in Sheldon et al., 1972) and thus that the
whole heterotrophic population is continuously distributed according to a Junge power function
(with j = 4; see Eq. 5). Very likely such a hypothesis will lead to an overestimate of the
heterotrophic compartment. Even if the Sheldon rule is verified between bacterioplankton,
nanoplankton and then microplankton, the later categories include not only heterotrophic but
A population of microbes with a numerical abundance NIV (cell per m- 3 ) will give rise to an
absorption coefficient am = (NIV) Sa- Typical marine bacteria have sizes between OA and 0.7
ILm (for the present computations, a mean size of 0.55 ILm is adopted), and the absorption
coefficient of the cellular material, as (at" = 415 nm) is about OA5 lcP m- I (Morel and Ahn,
1990). The absorption cross section of such a bacterium amounts to 3.9 10- 15 m 2 at its
maximum, 415 nm.
In the photic zone, bacterial numbers (m- 3 ) and Chi a concentrations (in mg m- 3 ) seem to be
significantly correlated through a non linear expression proposed by Cole et al. (1988) and
adopted here, written as
(NjV)-0.91 10 12 (Chi) 0.52
(19)
Therefore, the partial absorption due to the presence of bacteria can be established as a
function of Chi a concentration (Fig. 8a) according to am = (NIV) Sa, leading to
a -3.57 10- 3 (Chl)O.52
m
To go further and account for bigger protists, some hypotheses are needed. The absorption
cross sections for heterotrophic flagellates (mean size 3A ILm) and naked ciliates (mean size
13.5 ILm) have been determined (Morel and Ahn, 1991), and the absorption coefficients of
their cellular material (at" = 415 nm) have been deduced. They do not markedly differ from
that of bacteria and the above as value can be kept for further computations. The contribution
of these protists to absorption can be estimated, provided that their numerical abundance is
known or assumed. Abundances of flagellates and microbes have been studied in the field
(Sieburth, 1983; Geider, 1988); flagellate and aloricate ciliate biomasses have also been
compared (Sherr et aI., 1986). This information, however, remains relatively scarce and
generalization is presently risky. A working hypothesis (and in effect an upper limit) could
consist of considering that, within the chain bacteria - flagellates - ciliates, equal bio-volumes
occur in logarithmically equal size intervals (as in Sheldon et al., 1972) and thus that the
whole heterotrophic population is continuously distributed according to a Junge power function
(with j = 4; see Eq. 5). Very likely such a hypothesis will lead to an overestimate of the
heterotrophic compartment. Even if the Sheldon rule is verified between bacterioplankton,
nanoplankton and then microplankton, the later categories include not only heterotrophic but