91
detrital material in shallow seas such as the North Sea. In open-ocean waters, detrital presence
is certainly less, but also variable, as correlations between chI a and Secchi disc visibility are
normally low even here (Megard and Berman, 1989). Indeed, detritus, not chI a and
associated algal pigments, contributes most to absorption of blue light, the spectral window
through which the Secchi disc is seen.
The contribution of algal pigments and detritus to total particulate light absorption in marine
waters should be mapped systematically on a global scale, now that oceanographic expeditions
are not normally made in the framework of international joint efforts such as 1.G.O.F.S. and
W.O.C.E. Bidigare et al. (1989a,b) and Bricaud and Stramski (1990) have started this project
by describing methods that are either based on spectral criteria and theoretical considerations
(Bricaud and Stramski) or combine HPLC analysis of pigments in samples with a record of
in situ particulate light absorption using a simple procedure. Bidigare used the concentrations
of the various pigments to reconstruct the specific in situ light absorption by the whole set of
pigments present in the sample, taking into consideration the shifts in maximum absorption
of the pigments. These shifts are smallest in the red part of the spectrum (in vivo chI a peaks
at 673 nm, in organic solvents at 660-665 nm) and greater in the blue part. Bidigare et al.
(1989b) summarize existing knowledge of in vivo pigment absorption maxima in their Table
1. Pigment-specific absorption coefficients vary with the degree of pigment packaging, but this
effect was found to be of little importance in natural mixed plankton populations by Smith
et al. (1989). With Bidigare's method, it is possible to obtain the specific detritus absorption
at all wavelengths by subtraction of the reconstructed in vivo pigment absorption from total
in vivo particle absorption as measured on glass fiber filters.
I have designed a simpler method, resembling that of Kishino et al. (1985), to distinguish the
contribution of phytoplankton from that of detritus in total particulate light absorption. The
method was conceived on the basis of the experience that in vivo light absorption spectra of
mixed and unialgal cultures have the same shape as spectra made of acetone extracts - except
of course that (see above) the in vivo spectrum is always flatter (Duysens, 1956), and its peaks
are shifted to longer wavelengths (Fig. 16). Concentration changes are reflected proportionally
at all in vivo-recorded wavelengths, and wavelength-specific absorptions in the acetone
extracts are significantly correlated with absorptions in the in vivo spectra when a wavelength
shift is taken into consideration (in the red around 10 nm, in the blue up to 40 nm). Normally,
an acetone spectrum is composed of the individual absorption spectra of the various pigments
as measured by HPLC; these always account for> 80% of the total spectrum. An acetone
extract of any natural population is easily made, and its absorption spectrum can then be
converted to a reconstructed in vivo absorption spectrum by using the conversion of acetone
detrital material in shallow seas such as the North Sea. In open-ocean waters, detrital presence
is certainly less, but also variable, as correlations between chI a and Secchi disc visibility are
normally low even here (Megard and Berman, 1989). Indeed, detritus, not chI a and
associated algal pigments, contributes most to absorption of blue light, the spectral window
through which the Secchi disc is seen.
The contribution of algal pigments and detritus to total particulate light absorption in marine
waters should be mapped systematically on a global scale, now that oceanographic expeditions
are not normally made in the framework of international joint efforts such as 1.G.O.F.S. and
W.O.C.E. Bidigare et al. (1989a,b) and Bricaud and Stramski (1990) have started this project
by describing methods that are either based on spectral criteria and theoretical considerations
(Bricaud and Stramski) or combine HPLC analysis of pigments in samples with a record of
in situ particulate light absorption using a simple procedure. Bidigare used the concentrations
of the various pigments to reconstruct the specific in situ light absorption by the whole set of
pigments present in the sample, taking into consideration the shifts in maximum absorption
of the pigments. These shifts are smallest in the red part of the spectrum (in vivo chI a peaks
at 673 nm, in organic solvents at 660-665 nm) and greater in the blue part. Bidigare et al.
(1989b) summarize existing knowledge of in vivo pigment absorption maxima in their Table
1. Pigment-specific absorption coefficients vary with the degree of pigment packaging, but this
effect was found to be of little importance in natural mixed plankton populations by Smith
et al. (1989). With Bidigare's method, it is possible to obtain the specific detritus absorption
at all wavelengths by subtraction of the reconstructed in vivo pigment absorption from total
in vivo particle absorption as measured on glass fiber filters.
I have designed a simpler method, resembling that of Kishino et al. (1985), to distinguish the
contribution of phytoplankton from that of detritus in total particulate light absorption. The
method was conceived on the basis of the experience that in vivo light absorption spectra of
mixed and unialgal cultures have the same shape as spectra made of acetone extracts - except
of course that (see above) the in vivo spectrum is always flatter (Duysens, 1956), and its peaks
are shifted to longer wavelengths (Fig. 16). Concentration changes are reflected proportionally
at all in vivo-recorded wavelengths, and wavelength-specific absorptions in the acetone
extracts are significantly correlated with absorptions in the in vivo spectra when a wavelength
shift is taken into consideration (in the red around 10 nm, in the blue up to 40 nm). Normally,
an acetone spectrum is composed of the individual absorption spectra of the various pigments
as measured by HPLC; these always account for> 80% of the total spectrum. An acetone
extract of any natural population is easily made, and its absorption spectrum can then be
converted to a reconstructed in vivo absorption spectrum by using the conversion of acetone