167
limiting conditions (Neori etal.,1982; Mitchell and Kiefer,1983). Changes were seen within one day, a time period too short for significant
taxonomic changes to occur. Also, the light was attenuated with neutral
density filters, indicating that light intensity, and not spectral composition is the relevant parameter. We have seen similar patterns in nutrient rich Antarctic waters as well as the oligotrophic Pacific gyre where
nutrients were not measurable in the euphotic zone. Thus, nutrient availability is not a cause of the spectral shifts.
Although it is probably valid to argue that the fluorescence spectra
are due to photosynthetically viable pigments, the absorption spectra
include detrital pigments as well as any non-photosynthetic pigments which
may be produced by the phytoplankton.
Interpretation of these spectra
is therefore more complicated.
In figure 7, the samples below the mixed
layer clearly show changes which one could argue are photoadaptive.
However, the mixed layer shows the highest overall chlorophyll specific
volume absorption coefficient, while the spectrum is dominated by short
wavelength absorption, compared to the deeper samples. This high absorption clearly does not transfer energy efficiently to chlorophyll ~, as
evidenced by the fluorescence excitation and efficiency spectra (Figs.
8,9).
It is reasonable to propose that these features are associated
with photo-protective pigments, since the mixed layer has high irradiance
levels, and photoinhibition is known to occur at high light levels
(Jassby and Platt,1976). An alternative hypothesis is that there is a
higher accumulation of detrital particulates in the mixed layer. Yentsch
(1962) proposed that the high value in the ratio of blue to the red absorption peak in field samples, as compared to cultures, might be due to
the presence of detrital particles.
He noted for very deep samples the
absorption is dominated by short wavelength absorption and the red peak
disappears. We have found for absorption spectra of mixed layer samples
from the central Pacific gyre, the ratio of blue absorption to the red
peak is greater than the example presented here. Furthermore, the water
column in the gyre is characterized by having the highest primary production in the mixed layer, long term stability in vertical profiles of
particle and chlorophyll concentrations, and nitrogenous nutrients are
undetectable in the euphotic zone. The observed photosynthesis in the
mixed layer must be balanced by grazing for long term stability to exist,
and is in turn maintained by recycled nutrients (Eppley etal., 1973).
Such a mechanism could result in a higher proportion of detrital particles
in the mixed layer compared to the lower euphotic zone, provided grazing
is carried out predominantly by micro zooplankton producing non-sinking
fecal material.
limiting conditions (Neori etal.,1982; Mitchell and Kiefer,1983). Changes were seen within one day, a time period too short for significant
taxonomic changes to occur. Also, the light was attenuated with neutral
density filters, indicating that light intensity, and not spectral composition is the relevant parameter. We have seen similar patterns in nutrient rich Antarctic waters as well as the oligotrophic Pacific gyre where
nutrients were not measurable in the euphotic zone. Thus, nutrient availability is not a cause of the spectral shifts.
Although it is probably valid to argue that the fluorescence spectra
are due to photosynthetically viable pigments, the absorption spectra
include detrital pigments as well as any non-photosynthetic pigments which
may be produced by the phytoplankton.
Interpretation of these spectra
is therefore more complicated.
In figure 7, the samples below the mixed
layer clearly show changes which one could argue are photoadaptive.
However, the mixed layer shows the highest overall chlorophyll specific
volume absorption coefficient, while the spectrum is dominated by short
wavelength absorption, compared to the deeper samples. This high absorption clearly does not transfer energy efficiently to chlorophyll ~, as
evidenced by the fluorescence excitation and efficiency spectra (Figs.
8,9).
It is reasonable to propose that these features are associated
with photo-protective pigments, since the mixed layer has high irradiance
levels, and photoinhibition is known to occur at high light levels
(Jassby and Platt,1976). An alternative hypothesis is that there is a
higher accumulation of detrital particulates in the mixed layer. Yentsch
(1962) proposed that the high value in the ratio of blue to the red absorption peak in field samples, as compared to cultures, might be due to
the presence of detrital particles.
He noted for very deep samples the
absorption is dominated by short wavelength absorption and the red peak
disappears. We have found for absorption spectra of mixed layer samples
from the central Pacific gyre, the ratio of blue absorption to the red
peak is greater than the example presented here. Furthermore, the water
column in the gyre is characterized by having the highest primary production in the mixed layer, long term stability in vertical profiles of
particle and chlorophyll concentrations, and nitrogenous nutrients are
undetectable in the euphotic zone. The observed photosynthesis in the
mixed layer must be balanced by grazing for long term stability to exist,
and is in turn maintained by recycled nutrients (Eppley etal., 1973).
Such a mechanism could result in a higher proportion of detrital particles
in the mixed layer compared to the lower euphotic zone, provided grazing
is carried out predominantly by micro zooplankton producing non-sinking
fecal material.