a
Detrital spectral absorption: Laboratory studies of visible
light effects on phytodetritus absorption, bacterial spectral
signal, and comparison to field measurements
by James R. Nelson’ and Charles
Y. Robertson’
ABSTRACT
Visible light-dependent
changes in the visible and near-ultraviolet
spectral absorption
(250-750 nm) of phytodetritus
(killed and disrupted phytoplankton
and copepod fecal pellets)
were determined
in laboratory
experiments,
as were spectral changes due to the growth of
bacteria. Bacteria carbon and nitrogen
content were estimated.
Results of the laboratory
studies are compared to field measurements
of particulate
spectral absorption
in waters from
the South Atlantic Bight off the southeastern U.S. The correction for “pathlength
amplification”
of filtered particulate
samples (B) is also examined.
Light-dependent
changes in the spectral absorption
of phytodetritus
are principally
due to
the bleaching
of chlorophylls,
pheopigments,
and carotenoids.
With bleaching
of these
pigments
the residual
phytodetrital
absorption
is due to more light-stable
chromophores
which absorb in the violet to near-ultraviolet.
In the visible region, detrital-type
absorption
appears to represent the longer wavelength “tails” of absorption
spectra which reach maxima
in the ultraviolet.
Extension of the spectral measurements
to 250 nm revealed two general patterns. Biological
macromolecules
associated with phytoplankton,
fresh phytodetritus
and bacteria contribute to
a broad maximum
in absorption
centered around 260 nm (likely due to nucleic acids and the
aromatic amino acids of proteins). In the turbid near-shore waters of the South Atlantic Bight,
detrital-type
absorption
showed a rather featureless,
steady increase in absorption
with
decreasing
wavelength,
similar
to typical DOM
spectra, which may be due to relatively
refractory particulate
geomacromolecules
(i.e., complex organics such as fulvic acids). Some
near-surface field samples also indicated the presence of uv-photoprotective
pigments associated with the phytoplankton
community.
Detrital
type spectral absorption
in the visible may be contributed
by a variety of chromophores, some portion of which may be contained in heterotrophic
organisms. The nature of the
chromophores
and particle types which constitute detrital-type
absorption
may differ between
coastal and open ocean waters.
1. Introduction
Defining
organisms,
water
column
1. Skidaway
the contributions
of
and nonliving
particulate
is a fundamental
Institute
of Oceanography,
water,
dissolved
organic
material
to the optical
objective
of bio-optical
matter,
properties
oceanography
various
living
of the upper
(e.g.,
P.O. Box 13687, Savannah, Georgia 31416, U.S.A.
181
Smith
182
Journal of Marine Research
w, 1
and Baker, 1978; Prieur and Sathyendrenath, 1981; Kirk, 1983). Such information is
essential for relating remote and in situ optical measurements to the concentrations
and composition of particulate and dissolved materials.
In a number of studies, the composite property of particulate spectral absorption
has been partitioned into two components: the absorption by phytoplankton pigments, and the absorption by what has been operationally classified as “detritus,”
“tripton” or “nonalgal absorption” (e.g., Kiefer and SooHoo, 1982; Kishino et al.,
1984, 1985; Maske and Haardt, 1987; Bidigare et al., 1989; Morrow et al., 1989;
Roesler et al, 1989; Smith et al., 1989; Bricaud and Stramski, 1990). Partitioning of
particulate spectral absorption has been achieved by several approaches, including
solvent extraction of filtered samples (Kishino et al., 1984, 1985) extraction of
suspended particles (Maske and Haardt, 1987) statistically (Kiefer and SooHoo,
1982) by reconstruction of phytoplankton absorption from pigment measurements
(Bidigare et al., 1987) and using a combination of spectral measurements and
modeling (Roesler et al., 1989; Bricaud and Stramski, 1990). Microspectrophotometric techniques have also been used to characterize the spectral properties of
individual particles (e.g., Iturriaga and Siegel, 1989).
Spectrally, phytoplankton and detritus are quite distinct. Due to the chlorophylls,
carotenoids and biliproteins associated with their photosynthetic membranes, phytoplankton show pronounced absorption within certain wavebands, and differences in
pigment composition are reflected in the visible absorption spectra of major phytoplankton taxa (e.g., Prieur and Sathyendrenath, 1981; Bricaud et al., 1988). By
comparison, detrital absorption is low or negligible at longer wavelengths in the
visible with a steady, rather featureless increase in absorption from the blue into the
near-ultraviolet (e.g., Kishino et al., 1985; Iturriaga and Siegel, 1989).
Currently, the constituents of detrital absorption are not well-defined, either in
terms of particle types or chromophore composition (i.e., the molecules or portions
of molecules which absorb visible light). Material of detritus-like spectral properties
can dominate particulate absorption in the near-surface waters of diverse marine
systems, including estuarine and coastal environments (Kirk, 1983; Kishino et al.,
1985; Maske and Haardt, 1987) and at least some oceanic regions (Mitchell and
Kiefer, 1988b; Morrow et al., 1989; Smith et al., 1989). Detrital-type absorption
appears to be characteristic of particulate organic matter collected from deep ocean
waters (Yentsch, 1962; Kishino et al., 1985).
Phytoplankton are a likely source of organic detritus in the upper water column of
many coastal and oceanic regions. Phytodetritus could be produced by grazers (as
egested fecal material), or as the result of decomposition of senescent cells.
One objective of the present study is to investigate visible light-dependent changes
in the spectral characteristics of detritus derived from phytoplankton. In a related
investigation (Nelson, 1993) the chlorophylls, pheopigments and carotenoids contained in phytodetritus (dead phytoplankton cells and the fecal pellets of copepods)
19931
Nelson & Robertson: Detrital spectral absorption
183
bleached rapidly at light levels comparable to those found in the surface mixed layer
of well-illuminated
waters. Thus, in high light environments,
phytoplankton
pigments would be expected to be a minor component of detrital spectral absorption,
and the possible contribution
of phytodetritus
to the absorption of visible light would
depend upon the presence of other, more light-stable
chromophores.
We also
examine possible contribution
of heterotrophic
bacteria to detrital-type
absorption,
and the results of laboratory experiments are compared to selected field measurements made in waters of contrasting
optical character. The spectral absorption
measurements include the visible region and 250-400 nm in the ultraviolet
region.
Features of particulate absorption spectra in the near-ultraviolet
region are considered which may be diagnostic of different classes of chromophores,
and the types of
particles with which they are associated.
2. Methods
a. Biological material and experiments. Experimental
procedures
are described in
greater detail in a related paper (Nelson, 1993). As sources of phytodetritus
the
experiments used cultured phytoplankton
(the chlorophyte Dunaliella tertiolecta and
the diatom Skeletonema costatum) which had been harvested by centrifugation
and
disrupted by several freeze-thaw
cycles, and fecal pellets produced by copepods
(Eucalanus hyalinus) that were fed cultured diatoms (S. costatum and Thalassiosira
weissfrogii). Killed cells or fecal pellets were resuspended
in autoclaved, sterile
filtered seawater (in glass bottles) and illuminated
under fluorescent lighting (mixed
40 W “Cool-White”
and “Daylight”
lamps). Temperatures
for experiments ranged
between 19%22.2”C
with about a 1°C range of variation during individual
experiments. Suspensions were mixed with a magnetic stir bar and by bubbling with sterile
filtered air. Samples were collected at regular intervals for analyses of pigments and
particulate
spectral absorption.
Dark controls (foil-wrapped
bottles, stirred and
aerated) were run in parallel with the illuminated
treatments. For one experiment,
actively growing bacteria (1 pm filtrate from a contaminated
algal culture) were
added to the suspension of freeze-thawed
S. costatum cells and aged in darkness for
five days prior to exposure to fluorescent light. Samples for pigments, particulate
absorption, particulate carbon and nitrogen, and bacteria counts were collected at
daily intervals during the dark incubation
and hourly once the suspension was
exposed to light.
b. Field samples. Field samples for measurements of phytoplankton
pigments, particulate spectral absorption and particulate carbon and nitrogen were collected during
cruise “WINTER90”
of the R/V Cape Hatteras in January of 1990. Stations along a
transect line across the southeastern
U.S. continental
shelf north of Charleston,
South Carolina were occupied (Fig. 1). The inner end of the transect line lay in
moderately turbid coastal waters of 15 m depth. The outer end of the transect line
184
Journal of Marine Research
PL 1
Longitude
Figure 1. Locations of selected stations occupied during cruise “Winter90.”
was just beyond the shelf break in waters of > 200 m depth. Samples were collected
in Niskin bottles mounted on a CTD rosette, filtered onto glass fiber filters (Whatman GF/F, 25 mm for particulate absorption measurements, 47 mm for HPLC
pigment analyses), and stored frozen until analyzed in the shore laboratory (within
l-2 months).
c. Spectral absorption measurements and estimation of volume and pigment-specific
absorption coeficients. Spectral absorption measurements of particulate material
collected on glass fiber filters (25 mm GF/F) were made using a PC-controlled
Perkin-Elmer Lambda 6 UV-Vis spectrophotometer equipped with a scattered
transmittance accessory (end-on photomultiplier tube with the sample compartment
immediately in front of a diffusing plate entrance window). This accessory allows for
the efficient collection of forward scattered light passing through turbid suspensions
or through a highly scattering glass fiber filter. Damp sample and reference (blank)
filters were mounted in 1 cm quartz cuvettes and scanned from 750-350 nm (D.
tertiolecta phytodetritus) or from 750-250 nm (S. costatum phytodetritus, fecal pellets
and field samples).
Mitchell and Kiefer (1984; 1988a) described an approach for obtaining estimates
of volume absorption and pigment-specific absorption coefficients for particles
concentrated on a glass-fiber filter. This requires: (1) correction for the apparent
“pathlength amplification,” termed p, that results from the scatter of light by the
glass fiber filter; (2) determination of the possible dependence of p on the sample
optical density. For a particular instrument, p is obtained empirically from the ratio
of the optical density of a particle suspension to the optical density of an equivalent
pathlength of the suspension concentrated onto a glass fiber filter (e.g., for a 1 cm
19931
Nelson & Robertson: Detrital spectral absorption
185
pathlength
cuvette, a 1 cm thick layer of the particle suspension is filtered).
Discussion of the p correction is also found in Bricaud and Stramski (1990).
For determination
of B for our spectrophotometer,
phytoplankton
cultures
(Zsochysis galbana, D. tertiolecta) were grown in batch culture under continuous light
(47 FEin m-* s-l) at 18°C. Culture samples with optical densities <O.l cm-’ in the
visible were scanned in 1 cm quartz cuvettes from 750-250 nm with culture filtrate
(GF/F) in the reference cell. Bricaud, et al. (1983) calculated that for a spectrophotometer of optical configuration
similar to ours (i.e., cuvette positioned immediately
in front of an end-on photomultiplier
tube), scatter of light beyond the acceptance
angle of the photomultiplier
would be about 1% or less for cell suspensions where
absorbance is less than 0.1 cm-r. Thus, absorbance values for cell suspensions
measured by the spectrophotometer
will be considered to represent cellular absorbance. Resuspension
of cells in a minimally
scattering protein solution for the
estimation of B (Mitchell and Kiefer, 1988a) was not carried out.
The coefficients of variation
for absorbance values of triplicate cell suspension
measurements
were <2% throughout
the visible and near-uv region. The coefficients of variation for absorbance values of triplicate filtered samples were somewhat
higher, ranging from l-5% in the visible region, and up to 7-8% in the nearultraviolet
region (250-350 nm). The greater variability
in the near-ultraviolet
was
probably due to the lower transmittance
of light through the filter with decreasing
wavelength in this region. The optical density of a blank dampened filter (versus a
dry cuvette) was nearly constant in the visible region (with an absorbance of 0.780)
increased somewhat from 400 nm to 320 nm (to an absorbance of 0.910), and
increased sharply below 320 nm (to an absorbance of 1.625 at 250 nm). Instrument
drift for filtered samples was found to be significant below 350 nm. Consistent
replication for filtered samples required resetting the instrument blank (memorized
scan of dampened blank filters in both reference and sample compartments)
every
third or fourth sample run. Although the optical density of damp quartz fiber filters
(Whatman QMA) was lower (absorbance of 0.875 at 250 nm), trial sample analyses
with the quartz filters did not show any improvement
in spectral resolution, or
decrease in instrument noise or drift in the ultraviolet
region, hence the standard
glass fiber filters were employed.
Spectra of filtered samples were smoothed (using a routine that is included in the
spectophotometer
software package), and normalized to zero absorbance at 750 nm.
Although it is generally assumed that phytoplankton
show negligible absorption in
this region, Bricaud et al. (1988) concluded that some species did show a small
absorption
at 730 nm. Bricaud and Stramski (1990) noted that changes in the
dampness of reference and sample filters with time could contribute to variability in
the infrared absorbance of filtered samples.
186
Journal of Marine Research
[5L 1
spectral absorption. For some samples, the solvent extraction
technique of Kishino et al. (1984, 1985) was employed to distinguish between the
d. Partitionedparticulate
absorption contributed by methanol-soluble pigments (chlorophylls, pheopigments
and carotenoids) and methanol-insoluble chromophores. One of a duplicate pair of
samples filtered was scanned as described above. The second was extracted in 10 ml
methanol (several hours to overnight in the freezer). The extract solution was then
passed through the filter (to collect any particles dislodged during the extraction),
and the filter was rinsed with deionized water. The damp extracted filter was then
scanned with a damp blank filter in the reference cell. After smoothing spectra (as
above) the contribution of the extracted methanol-soluble pigments was estimated
by the difference between the total absorbance and the absorbance of methanolinsoluble chromophores.
e. Pigment analyses. Chlorophylls,
pheopigments and carotenoids were analyzed by
HPLC as described in the related study of pigment photooxidation (Nelson, 1993).
JT Bacteria counts. Bacteria counts were performed by epifluorescence microscopy.
Formalin-preserved samples were stained with the fluorochrome acridine orange
and filtered onto stained (Irgalan Black) 0.2 pm polycarbonate filters (Nuclepore)
and counted at x 1000 under oil immersion (Hobbie et al., 1977).
carbon and nitrogen. Filtered samples for particulate carbon and
nitrogen were freeze-dried and analyzed with a Perkin-Elmer Model 240C elemental
analyzer using acetanilide as a standard.
g. Particulate
3. Results
a. Determination
of thepathlength
amplification
factor for filtered samples. A quantita-
tive estimate of particulate volume absorption based on analyses of filtered samples
must compensate for the apparent amplification of the absorbance signal caused by
the scatter of light within the glass fiber filter. Several previous field studies of
particulate spectral absorption assumed a constant value for B to obtain estimates of
particulate volume absorption (e.g., Kiefer and SooHoo, 1982; Kishino et al., 1985;
Lewis et al., 1985). However, Mitchell and Kiefer (1988a) and Bricaud and Stramski
(1990) found that l3 varied with the optical density of the filtered sample determined
for their instruments. We found a similar dependence of B on the sample optical
density for our instrument (Fig. 2).
The relationship between l3 and sample optical density (o.d.(o) for our instrument
was determined by nonlinear least squares regression. A regression curve (Fig. 2) of
the form described by Mitchell and Kiefer (1988a) was fit to the combined data for
19933
Nelson & Robertson: Detrital spectral absorption
ti
0
0.05
0.1
0.15
Optical
0.2
Density
0.25
(filtered
0.3
0.35
187
0.4
0.45
sample)
Figure 2. Values of p (the ratio of absorbance, suspended:filtered, for samples of equivalent
cell density per unit area) versus absorbance for the filtered sample. The data points
(n = 1865) represent values at 1 nm intervals between 250-550 nm and 665-680 nm for 3
cell concentrations of both Zsochrysisgalbana and Dunaliella tertiolecta. Regions of very low
absorbance were omitted (see text). The solid line is the curve fit by nonlinear least squares
regression (see text) for the dependence of p on the o.d. of the filtered sample.
cultured I. galbana and D. tertiolecta (three cell concentrations
for each):
p = 1.0 + (0.46 x o.d.$“)
where p = the pathlength amplification
factor (ratio of absorbance for filtered to
suspended cells of equivalent
pathlength).
Regions of very low absorbance in the
phytoplankton
samples (550-665 nm, > 680 nm) were not included in the regression
analysis. The dependence of pathlength amplification
on sample optical density is
particularly pronounced for lower absorbance values, while at optical densities above
about 0.3 (filtered sample) the relationship
becomes nearly constant, as described
previously (Mitchell and Kiefer, 1988a; Bricaud and Stramski, 1990). As noted by
Mitchell and Kiefer (1988a), the p correction appears to be instrument-dependent;
the coefficients determined for our spectrophotometer
differ from those reported in
earlier studies (Mitchell and Kiefer, 1988a; Bricaud and Stramski, 1990).
The effect of pathlength amplification
is readily apparent in a comparison of the
absorbance spectra for suspended and filtered samples (equivalent cellular densities
per unit area) of 1. galbana and D. tertiolecta (Fig. 3a,b). The P-corrected spectra
(Fig. 3c,d) provide reasonable matches to the suspended cell spectra through most of
the visible region. Deviations between absorbance values for suspended and p-corrected filtered samples occur in the region of the chlorophyll a absorption maximum
188
Journal of Marine Research
[51,1
a lsochrysis
- -. FInered
- S”sp0”ded
b Dunaliella
- - Filtered
c lsochrysls
- - Filtered
Suspended
d Dunaliella
-- FInered
Suspended
400
WavelengCY(nm)
4w
550
Wavelength (nm)
Figure 3. Comparison of absorbance for suspended and filtered samples of Zsochrysis gdbana
and Dunaliellu tertiolectu (equivalent cellular densities per unit area) before (a, b) and after
(c, d) correction for pathlength amplification of filtered samples.
in the red (where p-corrected spectra are 15-18% lower), and in the near-uv region
(where B-corrected spectra can be S-9% lower). It does not appear that the
difference in red absorption was due to degradation of chlorophyll a upon filtration,
as has been reported by Stramski (1990) for certain centric diatoms. Pigment
analyses (HPLC) for filtered samples of the two species employed here showed only
trace amounts of pheopigments.
The looping
evident
in the plots of p versus filtered
sample optical densities (Fig. 2) has been described as an apparent “hysteresis
effect” by Bricaud and Stramski (1990; see their Fig. 1); that is, a possible wavelengthdependent variation of B with sample optical density.
b. Laboratory
experiments:
visible light-dependent
spectral changes in phytodettitus.
Rates of light-dependent degradation of chlorophylls, pheopigments and carotenoids in these experiments are reported in Nelson (1993). A steady decrease in
visible absorption occurred as the photosynthetic pigments bleached. Spectral
changes for D. tertiolecta phytodetritus (350-750 nm) after 3 and 6 hours of
illumination (3.5 and 7.0 Ein m-2 cumulative light exposure) illustrate the effect of
the bleaching of the photosynthetic pigments (Fig. 4a). The photosynthetic pigments
were reduced to less than 15% of their initial concentrations after 6 hours. Samples
from the dark control
bottles for this and the other experiments
showed little or no
change from the initial absorption values of treatments exposed to light (about 6
hour incubations in each case).
19931
Nelson & Robertson: De&al
spectral absorption
189
trj
-
16
I\
I
I
2.5
\
I
b
--
to
-
t6
13
0
.
350
.
.
.
.
450
.
.
.
.
.
550
Wavelength
.
..‘I..”
650
(nm)
Figure 4. Light-dependent changes in the spectral absorption of Dunaliella phytodetritus. (a)
Particulate volume absorption at the beginning of the experiment, and after 3 and 6 hours
exposure to visible light. (b) The same samples normalized to absorption at 675 nm.
Along with a decrease in total visible absorption, there was a shift in the spectral
character of absorption by the D. tertiolecta phytodetritus as the photosynthetic
pigments bleached. Normalized to absorption at 675 nm (the red maximum of
chlorophyll a in vivo), blue to near-uv absorption increased relative to red absorption
with the length of exposure to visible light (Fig. 4b).
Particulate absorption measurements made during the first experiment using S.
costatum phytodetritus were partitioned by solvent extraction. Figures 5a and Sb
illustrate initial and final solvent-partitioned absorption spectra from 350-750 nm.
The MeOH-insoluble chromophores (i.e., light-absorbing molecules other than the
MeOH-soluble chlorophylls, carotenoids and pheopigments) were stable under
visible light exposure. Bleaching of the MeOH-soluble phytoplankton pigments
dominated changes in total spectral absorption in the visible region. Extension of the
spectral measurements to 250 nm (Fig. 6a and 6b) shows that the visible absorption
of the MeOH-insoluble chromophores represents the longer wavelength tails of
absorption spectra which reach maxima in the region of 250-260 nm.
190
Pl, 1
Journal of Marine Research
--
\
Total
b
- . - Difference
,
550
Wavelength
(nm)
Figure 5. Solvent-partitioned
absorption of Skeletonema phytodetritus (350-750 nm). (a)
Initial sample. (b) after 6 hours exposure to visible light. The spectral curves show total
absorption, absorption due to MeOH insoluble chromophores, and the difference spectra
due to MeOH soluble pigments (chlorophylls, pheopigments, carotenoids).
The blue to near-ultraviolet absorbing material which remained after bleaching of
the photosynthetic pigments appears to have been predominantly contributed by
phytodetritus in the two experiments described above. During the 6 hour incubation
of the S. costatum phytodetritus, bacteria cell numbers were fairly low and did not
change significantly (Table 1). Viewed by epifluorescence microscopy (after staining
with acridine orange) bacteria present in the samples represented a very small
portion of the total field area relative to the killed phytoplankton cells. Under uv
excitation, the phytoplankton cellular material initially appeared bright red, due to
the fluorescence of chlorophyll. The red fluorescence gradually faded during the
course of the incubation in the illuminated treatment, leaving a much fainter orange
fluorescence in the final samples (6 h).
The copepod fecal pellets represented a biologically processed source of phytodetritus. The fecal pellets contained a mixture of intact chlorophylls and carotenoids,
and various pheopigment derivatives of the chlorophylls (pheophorbides, pheophytins, and pyropheophytins). Rates of bleaching for the chlorophylls, major carotenoids and pheopigments contained in the pellets were similar (Nelson, 1993). Over
the course of the 5.5 hour incubation (6.1 Ein m-* cumulative light exposure), these
19931
191
Nelson & Robertson: Detrital spectral absorption
6
a
,A
- - - Total
.. .. .. .. MeOH
-
250
400
. -
Difference
700
550
6
b
- - - Total
. .. .. .. MeOH
-
250
. -
Difference
550
400
Wavelength
700
(nm)
Figure 6. Solvent-partitioned
absorption of Skeletonema costatum phytodetritus with the
spectral measurements extended to 250 nm. (a) Initial sample. (b) after 6 hours exposure to
visible light. Spectral curves as in Figure 5.
pigments
were reduced
to about 40% of their initial
concentrations.
Chlorophyll
a-specific absorption increased with cumulative light exposure, roughly doubling in
the visible, and increasing by a factor of about 2.5 in the uv (Fig. 7a). This suggests
the greater stability under visible light of chromophores other than the photosynTable 1. Bacteria counts for incubation of Skeletonema costatum phytodetritus
autoclaved, 0.2 km filtered seawater.
Bacteria counts
(lo5 cells ml-l)
Hours
incubated
Light
exposure
(Ein m-*)
Mean
s.d.
0
3
6
6
0
2.8
5.6
0
2.56
2.52
2.86
2.84
0.42
0.47
0.78
1.22
(aerated) in
192
Journal of Marine Research
[5I, 1
---.
-
--
a
to
t3
15
16
b
to
13
-_.
-
250
400
Wavelength
550
15
16
700
(nm)
Figure 7. Visible and near-uv spectral absorption of copepod feces (produced by Eucalanus
hyalinus fed diatoms). (a) Visible light-dependent changes in chlorophyll u-specific absorption for initial sample and after 3, 5 and 6 hours exposure to visible light. (b) The same
samples normalized to absorption at 675 nm.
thetic pigments and their simple alteration
products (i.e., those detected by the
HPLC analyses). Spectra normalized
to absorption
at 675 nm (Fig. 7b) showed
higher relative absorption in the blue to near-uv with the length of exposure to visible
light, again indicating that the more light-stable chromophores
absorb primarily in
the blue to near-uv region.
In the second experiment
employing S. costatum phytodetritus,
bacteria were
deliberately introduced into the incubation bottles, and the aerated suspension was
kept in darkness for 5 days prior to exposure to light. Bacteria cell numbers increased
by more than a factor of 10 during the dark incubation,
while concentrations
of
particulate organic carbon and nitrogen increased by factors of about 1.6 and 2.0
(Table 2). The variability in bacteria counts reflects their uneven distribution
on the
microscope slides; patches of bacteria were associated with some of the phytodetritus
particles. In parallel with the increase in bacteria numbers and POC over the first 3
days of the incubation, the spectral absorption of the incubated particles showed a
large increase in the ultraviolet
region, centered around 260 nm (Fig. 8). At the end
24.8
45.5
72.0
94.0
120
125
126
0
Hours
incubated
3.60
0
0
0
0
0
0
0
Light
exposure
(Ein mV2)
PON
(ug 1-l)
269
432
592
559
551
558
545
-
POC
(kg 1-l)
1586
2475
2866
2674
2590
2690
2813
-
49.8
50.3
47.2
50.4
49.7
48.7
18.2
-
Chla
(kg 1-l)
6.89
6.74
5.66
5.58
5.48
5.62
6.02
-
C:N
(atom)
C:Chl a
(wt)
31.9
49.2
60.7
53.1
52.1
55.2
154.6
1.03
14.5
9.11
12.6
21.9
16.1
18.7
19.9
mean
0.69
1.03
2.22
0.42
7.73
4.0
6.11
-
s.d.
Bacteria numbers
(lo6 cells ml-l)
66.0
158.4
94.0
48.1
73.3
69.4
-
C (fg cell-‘)
2
sa
bF
R
=I
a
?Y
ST
3
a
s
2
3h’
-
6.4
4.6
4.4
4.2
4.5
5.2
-
-
12.1
40.0
25.1
13.5
19.2
15.6
-
;;I
C:N (atom)
$
E
g
s
%
3
3
N (fg cell-‘)
Estimated bacteria composition
Table 2. Skeletonema costatum phytodetritus aged in darkness in the presence of an active bacteria population. Changes in particulate
composition, bacteria numbers, and estimated bacteria C and N content. POC and PON values are averages of duplicate determinations,
chlorophyll a of triplicate determinations.
194
Journal of Marine Research
250
400
550
Wavelength
700
(nm)
Figure 8. Changesin spectral absorption of Skeletonema costatum phytodetritus aged 5 daysin
darkness (aerated) in the presence of an active, growing bacterial population. Spectral
curves are for the initial sample, after 2 days, after 3 days, and after 5 days incubation in
darkness.
of the 5 day dark incubation, absorption at 260 nm had decreased to about 74% of
the day 3 peak.
spectral absorption. The transect across the southeastern U.S. continental shelf provided considerable contrast in the particulate and
optical properties of near-surface waters. The shallow inshore waters were turbid.
On the outer part of the shelf, warm surface waters derived from a Gulf Stream
filament were encountered (L.P. Atkinson, unpublished data). The formation of
these features as the result of the passage of Gulf Stream frontal eddies along the
continental margin is discussed in Lee et al. (1981). Seaward of the 20 m isobath,
microscopic analyses indicated that the particulate matter in surface waters was
dominated by microorganisms (P.G. Verity, personal communication).
The spectral characteristics of particles and the magnitudes of volume absorption
showed considerable differences between the shallow inshore waters, and the
warmer waters at and beyond the shelf break. Near-surface particulate volume
absorption at three stations (15,75 and 300 m isobaths) is compared in Figure 9, and
Figure 10 compares particulate volume absorption at various depths at each station.
Between stations the greatest differences in volume absorption were in the blue to
near-uv region. Inshore, particulate absorption was dominated by detrital-type
chromophores, and the absorption due to phytoplankton pigments was superimposed on the pronounced detrital-type signal (Fig. 10a). At and beyond the shelf
break, upper water column particulate absorption in the visible was dominated by
c. Field samples of particulate
Nelson & Robertson: Detrital spectral absorption
19931
250
400
-
station
1
- -
station
12
-.-..
station
17
550
Wavelength
195
700
(nm)
Figure 9. Comparison of near-surface particulate volume absorption for samples collected at
three stations on a transect across the continental shelf north of Charleston, South Carolina
(29 January, 1990). Station locations are noted in Figure 1.
phytoplankton pigments (Fig. lob, 10~). Absorption maxima were present in the red
(675 nm) and blue (430 nm) due to chlorophyll a, and a broad shoulder between
450-475 nm was contributed by various accessory carotenoids and chlorophylls (e.g.,
for Sta 12 alloxanthin was the principal carotenoid at 3 m, fucoxanthin at 26 m and 37
m). The distinct absorption maximum at about 260 nm (Fig. lob, 10~) was also found
for cultured phytoplankton (Fig. 3 and unpublished data) and in the bacteriaphytodetritus incubation (Fig. 8). As noted above, the chromophores responsible for
the 260 nm absorption maximum are MeOH-insoluble (Figs. 5, 6); that is, these
among the “detrital” pigments distinguished by the solvent-partitioning technique of
Kishino et al. (1985). Particulate absorption for the deepest samples at the outer
stations (74 m and 88 m) was dominated by detrital-type chromophores (Figs. lob,
1Oc).
4. Discussion
a. “Detrital”absolption. As the term has been applied in bio-optical studies, “detritus”
is essentially an operational definition for particulate organic material characterized
by a featureless increase in absorption with decreasing wavelength from the blue into
the near-uv. Although the spectral character of detritus is quite distinct from that of
phytoplankton pigments, there is considerable overlap in absorption between the
two classes of chromophores. Resolving the spectral absorption of phytoplankton
from that of detritus (and possibly heterotrophic organisms) is critical for refining
remote
sensing algorithms
for phytoplankton
pigment
concentrations
in Case 2
waters (e.g., Gordon and Morel, 1983; Sathyendrenath et al., 1989) and for estimat-
196
Journal of Marine Research
[Cl
a Sta 1
-
3m
-.-
13m
700
b Sta 12
~
3m
-----*cm
- - ___-
37m
74m
cStal7
-
3m
--
36m
86m
400
Wavelength
550
(nm)
Figure 10. Particulate volume absorption for depths sampled at three stations on the
cross-shelf transect. (a) Station 01 (15 m isobath). (b) Station 12 (75 m isobath). Station 17
(approx. 300 m isobath). Note differences between ordinate scales.
ing the in situ quantum efficiency of photosynthesis
for natural populations
of
phytoplankton
(e.g., Cleveland et al., 1989). Furthermore,
due to their similarity in
absorption properties, estimates of DOM concentrations
based on remote sensing
(e.g., Carder et al., 1989) are likely to represent the combined effects of dissolved
compounds and particles with detrital-type
absorption.
In most studies of particulate
spectral absorption,
neither the chromophores
which constitute detrital-type
absorption
nor the particles with which they are
associated have been well characterized. Microspectrophotometric
techniques (Itturiaga et al., 1988; Itturiaga and Siegel, 1989) can provide direct measurement
of the
absorption of individual particles. Still, the nature of the detrital chromophores
is not
clearly established, and both detrital chromophores
and detrital particle type may
vary considerably between different environments.
19931
Nelson & Robertson: Detrital spectral absorption
197
Significant contributions to total particulate absorption by detritus have been
noted in previous studies of various coastal waters (Yentsch, 1962; Kishino et al.,
1985; Maske and Haardt, 1987). The inner shelf off the southeastern U.S. is another
system in which detritus dominates particulate spectral absorption (Fig. 10a). Within
the inshore waters of the South Atlantic Bight, strong tidal currents, augmented by
wind mixing, maintain a high suspended particle load. A pronounced cross-shelf
density gradient generally restricts the exchange of materials between the coastal
zone and the mid-shelf region (Blanton, 1981). Elemental analyses indicate the
presence of high concentrations of suspended clay particles in the inshore region
(e.g., Windom and Gross, 1989; Windom et al., 1989). Although microbial abundance
and chlorophyll concentrations can be high (chlorophyll a concentrations are often
2-5 kg I-*), experimental evidence and microscopic observations indicate that most
of the particulate organic carbon in the coastal boundary zone of the South Atlantic
Bight (depth < 10 m) is nonliving (Verity et al., 1993). Thus, in these waters, the
spectral signature of phytoplankton pigments is superimposed on the dominant
absorption of detrital-type chromophores (Fig. 10a).
Detrital-type chromophores can also make a significant contribution to nearsurface particulate spectral absorption in open ocean regions that are isolated from
terrigenous and sedimentary sources of particles. Detrital absorption equivalent to
or exceeding phytoplankton absorption in the blue has been observed in the eastern
North Pacific central gyre (Mitchell and Kiefer, 1988b) and the western Sargasso Sea
(Bidigare et aZ., 1989; Morrow et al., 1989; Smith et al., 1989; Cleveland et al., 1989).
Mitchell and Kiefer (1988b) noted a seasonal accumulation of “detritus” (spring to
fall) in the surface waters of the eastern North Pacific. They proposed that this could
reflect the trophic status of the microplankton community; the increasing detrital
component being due to the seasonal development of a microzooplanktondominated food web, and increasing production of fine biogenic detritus.
A variety of particle types and a variety of chromophores could contribute to
detrital-type absorption. In the coastal waters of the South Atlantic Bight potential
sources of detritus include material derived from the extensive salt marshes found
behind barrier islands, biogenic material produced within the water column, organic
coatings on suspended clay minerals, and resuspended sediments. Particulate geomacromolecules of humic-fulvic character (possibly formed by condensation of dissolved
organics contained in river waters) have also been observed in estuarine systems
(e.g., Noureddin and Courtot, 1989). In oceanic regions, a likely source of detrital
particles is the egested particulate wastes of microzooplankton grazers (e.g., Small et
al, 1979; Stoecker, 1984; Gowing and Silver, 1985). Small organic aggregates
collected in the Sargasso Sea (particles up to about 50 km length) were characterized
by detrital-type absorption spectra (Iturriaga and Siegel, 1989). Kiefer et al. (1990)
have also proposed that chromophores associated with living bacteria (the respiratory chain cytochromes in particular) may contribute to detrital-type absorption in
198
Journal of Marine Research
Pl, 1
oceanic waters; that is, chromophores of detritus-like absorption may be present in
living microorganisms. Several classes of biological molecules show significant absorption in the near-uv (discussed further below).
Regardless of their source, the accumulation of detrital-type chromophores in the
upper water column will depend upon both particle dynamics (production and
removal processes) and the extent to which the light-absorbing molecules are subject
to photochemical and microbially-mediated degradation. The experiments described
above were intended to examine the effects of visible light on the pigment content
and spectral absorption of phytodetritus.
changes in the spectral absorption of phytodettitus.
The
chlorophylls, pheopigments and carotenoids contained in phytodetritus bleach at
fairly rapid rates under visible light exposures found in the surface mixed layer of
well-illuminated waters (Nelson, 1993). The photooxidation of pigments led, as
would be expected, to a pronounced decrease in the visible absorption of the
phytodetritus (Figs. 4,5). Phytodetritus absorption spectra normalized to chlorophyll
a (Fig. 7a) and normalized to the red absorption maximum (Fig. 4b, 7b) emphasize
the detritus-like spectral character of the chromophores which showed greater
stability than the photosynthetic pigments under visible light exposure. Figure 7a
also illustrates the potential interference of detrital absorption in estimates of
pigment-specific absorption for field populations of phytoplankton (e.g., for evaluation pigment packaging and the effective absorption of visible light by phytoplankton). Where detritus constitutes a significant portion of total particulate absorption,
estimates of pigment-specific absorption based on measurements of total particulate
material can be substantially elevated over those of phytoplankton alone. Similarly,
the carbon to chlorophyll a ratio in phytodetritus can be considerably altered by
exposure to visible light. For the S. costatum phytodetritus aged 5 days in darkness in
the presence of an active bacterial population, the C:Chl a ratio increased from 32 to
55 due to the increase in bacterial carbon (Table 2). With exposure to visible light,
chlorophyll a was degraded and the C:Chl a ratio for the phytodetritus increased to
155 (Table 2).
The effects of uv light on the spectral absorption of phytodetritus were not
determined in the present study. Due to the strong uv absorption of detrital-type
chromophores, much of the photochemical activity associated with these compounds
would be expected to be uv-dependent. Such effects could be significant, particularly
in clearer oceanic waters where near-uv light can penetrate through the upper tens of
meters of the water column (Smith and Baker, 1981).
b. Visible light-dependent
Detritus-like spectral absorption in the visible region
appears to be characteristic of bacteria (Morel and Ahn, 1990) and heterotrophic
flagellates (Morel and Ahn, 1991). The increase in bacterial abundance during the 5
c. Bacterial spectral absorption.
19931
Nelson & Robertson: Detrital spectral absorption
199
day dark incubation of S. costatum phytodetritus was accompanied by an increase in
POC and PON, a pronounced increase in absorption in the uv, and a smaller increase
in absorption in the blue spectral region (Table 2, Fig. 8). The greatest increase in
particulate absorption, centered around 260 nm, is in the region of the expected
absorption maximum of nucleic acids (e.g., Lehninger, 1975). Other cellular constituents, notably the aromatic amino acids of proteins, could also contribute to
absorption between 260-280 nm (Hader and Tevini, 1987). A maximum in absorbance at about 260 nm is also characteristic of suspensions and filtered samples from
healthy phytoplankton cultures (e.g., Fig. 3).
Morel and Ahn (1990) noted a peak in absorption at 415 nm for heterotrophic
bacteria concentrated onto a GF/F filter, which they attributed to respiratory chain
cytochromes. A maximum in absorption at 415 nm was also noted for heterotrophic
protozoa (Morel and Ahn, 1991). Kiefer et al. (1990) proposed that bacterial
cytochromes may make an important contribution to detrital-type absorption in
oceanic regions (at least in the blue to violet spectral region). Reports indicating that
bacteria may, in fact, represent a considerable portion of the POC pool in oligotrophic regions (Fuhrman et al., 1987; Cho and Azam, 1990) suggest a potentially
significant role for bacteria in determining the optical properties of oceanic waters.
However, Morel and Ahn (1990) concluded that the dominant optical effect of
bacteria was in the scatter of light, and that they are rather weak absorbers of visible
light. Similarly, small heterotrophic flagellates appear to be efficient scatterers but
weak absorbers of visible light (Morel and Ahn, 1991).
In our experiment with aging phytodetritus, no distinct increase in absorption at
415 nm was noted during the period of increasing POC and increasing bacterial
abundance (Fig. 8). It is possible that changes in cytochrome absorption due to
increasing bacteria numbers were masked by the presence of pheopigments, chlorophylls and carotenoids associated with the phytodetritus. However, when considered
to 250 nm in the uv, it appears that much of the blue to violet absorption, for both
bacteria and phytodetritus, is contributed by the long wavelength “tails” of absorption spectra which reach maxima in the near-uv.
To attribute the changes in the near-uv absorption of aging phytodetritus (Fig. 8)
to bacterial growth requires that the bacterial cells were efficiently retained on the
GF/F filters used for particulate absorption (and POC) analyses. We do not have
measurements of bacteria cell dimensions during the dark incubations of phytodetritus. However, under epifluorescence microscopy the bacteria cells appeared to be
large compared to those found in typical field samples from the South Atlantic Bight.
An increase in the mean cell size of cultures of mixed heterotrophic bacteria has
been observed in the early growth stage of unamended and nutrient enriched
seawater cultures (e.g., Ammerman et al., 1984; Morel and Ahn, 1990). Growth
conditions for the bacterial innoculum prior to the experiment (a contaminated algal
mass culture) and in the initial days of the dark incubation (in the presence of
200
Journal of Marine Research
w,
1
freeze-ruptured diatoms) are likely to have been fairly enriched. Such an increase in
cell size would increase the likelihood of retention of bacteria on the GF/F filters.
An increase in total cellular nucleic acid content and in the ratio of RNA:DNA is
typical of the exponential phase of bacterial growth in batch culture (e.g., Pritchard
and Tempest, 1982). Thus, cell-specific absorption at 260 nm could be enhanced for a
rapidly growing bacterial population. The decrease in absorbance at 260 nm after day
3 of the dark incubation (Fig. 8) may reflect a decrease in cellular RNA content with
slowing growth rates. Alternatively, reduction in bacterial cell size with slowing
growth may have resulted in the passage of some cells through the GF/F filter.
However, total POC and PON did not decrease after day 3 (Table 2) suggesting that
much of the bacterial population was still retained on the glass fiber filters.
bacteria. An assessment of
the mean size of the bacteria cells during the aged phytodetritus experiment can be
obtained through estimation of their carbon content. Assuming that all bacteria were
retained on the GF/F filter over the first 3 days of the dark incubation, and that there
was no loss of phytodetritus particulate carbon (i.e., bacterial growth was supported
by dissolved compounds), we can calculate the average cellular carbon content for
the bacteria which grew during this period. In the first 3 days of the incubation,
bacteria numbers increased from 1.03 x lo6 to 9.11 x lo6 cells ml-‘, while carbon
increased from 1.6 to 2.9 p,g ml-’ (Table 2). If the change in particulate carbon
concentration was due only to the increase in bacteria cell numbers, a mean bacterial
carbon content of 158 fg C cell-’ is estimated. Similarly, bacterial nitrogen content is
estimated to be 40 fg N cell-l, and the molar C:N for the bacterial cells to be 4.6
(Table 2). If the estimated cellular carbon content of 158 fg cell-’ is valid for the
initial bacterial population, then less than 10% of the total POC at the beginning of
the experiment was bacterial carbon.
Lee and Fuhrman (1987) reported bacterial cellular carbon density to be 380 fg C
Fm-3 (or 380 kg C m-“). Applying this value for our highest estimate of bacterial cell
carbon content (158.4 fg cell-l), we obtain an estimate for the mean cellular volume
of 0.42 Frn3, which, for a spherical cell, would require a diameter of 0.93 pm. This is
similar to the mean volume of 0.36 Frn3 (equivalent spherical diameter of 0.88 pm)
reported by Alldredge and Youngbluth (1985) for bacteria attached to marine snow;
presumably an enriched microenvironment. By comparison, Alldredge and Youngbluth (1985) found the mean volume for free-living bacteria to be 0.19 km3
(equivalent spherical diameter of 0.72 Fm). For our lowest estimate of bacterial
cellular carbon content (48.1 fg cell-l) we obtain a cellular volume estimate of 0.13
Frn3 and a spherical diameter of 0.31 p,m. Other studies report even smaller cell
volumes ( < 0.1 pm3) for at least a portion of populations of free-living marine
bacteria (Ammerman et al., 1984; Simon and Azam, 1989).
On the basis of optical measurements and calculation of the refractive index of
d, Estimated carbon and nitrogen content of heterotrophic
19931
Nelson & Robertson: Detrital spectral absovtion
201
bacteria cellular material, Morel and Ahn (1990) concluded that Lee and Fuhrman’s
(1987) figure for bacteria cell carbon density was probably high. Morel and Ahn
(1990) calculated the carbon density of bacterial cells (grown in unenriched seawater
cultures) to be 228 fg krne3 (or 228 kg C m-3). If this lower intracellular carbon
concentration is appropriate, then estimates of cellular volume of 0.21-0.70 pm3 and
equivalent spherical diameters of 0.21-1.1 Frn are obtained for the range of bacterial
carbon contents we calculated (Table 2). Using the mean value of 84.7 fg C cell-l for
the bacterial carbon content from Table 2, estimates of cellular volume and spherical
diameter of 0.37 pm3 and 0.89 urn are obtained.
e. The near-w
chromophores.
spectral signatures of detrital particles and the possible nature of detrital
Since the objective of most previous studies of particulate spectral
absorption has been to determine the visible spectral properties of phytoplankton
and other particles, fewer measurements have been made below 350 nm. In coastal
waters, the attenuation of near-uv light is generally dominated by the absorption of
dissolved organic matter, while in clear oceanic waters, absorption by water is
considered to be the main source of near-uv attenuation (Kirk, 1983). Regardless of
the magnitude of uv spectral absorption by particles relative to water and dissolved
compounds, extending particulate spectral absorption measurements into the near-uv
may provide useful diagnostic information concerning the nature of detrital chromophores in different water types. Furthermore, better characterization of uv-absorbing
particulate chromophores will contribute to understanding the potential role of uv
radiation in biogeochemical processes in the upper ocean, a topic of recent interest
(Blough and Zepp, 1990).
When considered in the visible region (400-700 nm), the typical “detrital”
absorption spectrum consists of a featureless increase of absorption with decreasing
wavelength from the blue into the violet. Extended to 250 nm, two general patterns
emerge from the particulate absorption spectra determined in this study.
Living organisms and fresh biogenic material (such as the killed phytoplankton
used in the experiments described above) show a pronounced absorption maximum
at about 260 nm (Figs. 3,6a). The 260 nm maximum in absorption was also present in
samples from the mid to outer shelf of the South Atlantic Bight (Figs. 9, lob, 10~)
where microscopy indicated that the particulate organic material was dominated by
living organisms (P.G. Verity, personal communication). Clearly, uv absorption in
these samples is the composite signal of a number of chromophores. In addition to
nucleic acids and the aromatic amino acids of proteins, biogenic compounds which
absorb in the violet to near-uv include the unsaturated fatty acids of various lipids,
flavins, cytochromes and other heme derivatives, and possibly derivatives of the
chlorophylls, carotenoids and biliproteins. In some near-surface samples from the
mid and outer continental shelf of the South Atlantic Bight, a secondary maximum in
near-uv absorption was detected between 340-360 nm (Fig. 9, Fig. lob, and unpub-
202
Journal of Marine Research
[5L 1
lished data). This may be due to near-uv absorbing photoprotective pigments
(nonprotein amino acids) found in certain phytoplankton species (e.g., Yentsch and
Yentsch, 1982; Vernet et al., 1989; Carreto et al., 1990). The 340-360 nm absorption
maximum was most prominent where pigment analyses (Nelson, unpublished data)
and microscopy (Verity, unpublished data) indicated that the photosynthetic ciliate,
Mesodinium rubrum, was abundant (e.g., Fig. lob, 3 m sample from Sta 12).
We note that the sample containing M. rubrum (Fig. lob, 3 m) showed relatively
low absorption between 520-550 nm, a region of strong absorption by biliproteins.
The cryptophyte-type pigmentation of M. rubrum includes the biliprotein, phycoerythrin (Barber et al., 1969). The absence of a strong biliprotein signal is apparently an
artifact of filtration or storage of the frozen samples. Rupture of i&f. rubrum cells
upon filtration, and loss of the water-soluble phycoerythrin to the filtrate was
reported by Barber et al. (1969). Artifactual changes in phytoplankton absorption
upon filtration has also been observed by Stramski (1990) for certain centric diatoms.
Application of appropriate fixatives to samples prior to filtration may help stabilize
fragile cells for absorption measurements (Stramski, 1990).
In contrast to the samples dominated by fresh biological material, a different
pattern of near-uv particulate spectral absorption was found in the coastal zone of
the South Atlantic Bight and in the deepest samples collected at the outer shelf and
beyond the shelf break. In the inshore waters (Fig. lOa), the “biogenic” uv absorption maximum at 260 nm, and the visible absorption of phytoplankton pigments in
the blue region, appears to be superimposed upon a featureless increase in absorption to 250 nm, similar to the typical DOM absorbance spectrum. The deeper
samples from the outer shelf and shelf break (74 m and 88 m in Figs. lob, 10~) were
also characterized by DOM-like absorption (a featureless increase in absorption to
250 nm), with little apparent contribution from either phytoplankton pigments in the
visible or from biological macromolecules in the uv. The DOM-like absorption of
detritus in the near-uv could be due to geomacromolecules (i.e., complex organic
compounds), with the intensity of absorption being influenced by the molecular
environments of a variety of chromophoric groups, as is the case for humic and fulvic
acids (Schnitzer and Kahn, 1972).
The absorption of both DOM and detritus in the visible spectral region have been
modelled as exponential functions (Roesler et al., 1989, and references therein;
Bricaud and Stramski, 1990). Comparing samples from a variety of coastal and
oceanic locations, the range of variation in the coefficient which determines the
shape of the exponential curve appears to be somewhat greater for detritus than for
DOM (Roesler et al., 1989), suggesting greater compositional heterogeneity for
detritus. Different proportions of biological macromolecules and geomacromolecules between samples might account for some of the variability in detrital absorption curves in the visible.
As appears to be the case for the humic and fulvic constituents of the dissolved
organic pool, particulate geomacromolecules could be rather refractory. On the
19931
Nelson & Robertson: De&al
spectral absorption
203
other hand, with the exception of structural polymers associated with cell walls, much
of the macromolecular content of microorganisms, nucleic acids and proteins in
particular, might be considered substrates that would be readily utilized by heterotrophic organisms in the water column. Yet, despite the expected lability of most
biological macromolecules, there is evidence that significant detrital pools of these
compounds can accumulate within the water column.
Holm-Hansen et al. (1968) inferred that a substantial portion of the particulate
DNA they measured in the oceanic water column was associated with detrital
particles. Winn and Karl (1986) also presented evidence that 75-90% of the total
particulate DNA in waters from the oligotrophic Pacific was “nonreplicating”
(i.e.,
either detrital or contained in inactive organisms). Since the RNA content of living
organisms is generally 4 to 8 times that of DNA, detrital RNA could be a significant
portion of the total nucleic acid pool (dissolved and particulate). It has also been
demonstrated that clays and sands can tightly adsorb DNA, making it less accessible
to nuclease degradation (e.g., Lorenz et al., 1981; Aardema et al., 1983). Evidence for
detrital protein is somewhat conflicting (see Dortch and Packard, 1989), but, in
general, protein-N is the major constituent of PON (Packard and Dortch, 1975, and
references therein).
spectral absorption in near-surface waters is a
composite property that will be influenced by the sources and composition of
particles, particle dynamics, biological modifications of organic matter, and photochemical processes. The constituents of detrital absorption may differ considerably
between different optical water types, both in terms of particle types and their
chromophore composition. Detrital-type absorption in the visible region appears to
be characteristic of a variety of organic chromophores. The extent to which “detrital”
absorption is due to chromophores contained in heterotrophic microorganisms
remains an open question, especially in oligotrophic waters. A better definition of the
nature of detrital-type absorption will contribute to further development of biooptical models relating remote and in situ spectral measurements to the concentrations and composition of particulate organic matter in oceanic and coastal regions.
f
Concluding
remarks. Particulate
Acknowledgments.
We thank A. Boyette for graphics and D. Peterson for assistancein the
preparation of the manuscript. This work was supported by the National Science Foundation
(Grant OCE-8800399). Field work was also supported in part by the Department of Energy
(Grant DE-FG09-85ER-60354
to G.-A. Paffenhofer). We thank L. P. Atkinson, Chief
Scientist for “WINTER90,”
for providing the opportunity to participate in the cruise.
Instrumentation support provided by the Skidaway Institute of Oceanography is also acknowledged.
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