Limnol.
Oceanogr., 34(8), 1989, 1694-1705
0 1989, by the American
Society of Limnology
and Oceanography,
Inc.
A bridge between ocean optics and microbial ecology
Charles S. Yentsch
Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine 04575, and
Center for Remote Sensing, Boston University, Boston, Massachusetts 02215
David A. Phinney
Bigelow Laboratory for Ocean Sciences
Abstract
Phytoplankton cell size is believed to be closely regulated by the nutrient regime of water masses
in the western North Atlantic Ocean. Since particle size affects attenuation of light in ocean water,
we argue that a bridge between classical ocean optics and microbial ecology has formed whereby
the physics of ocean color must include consideration of ecological factors important to the diversity
of phytoplankton species.
Observations of the specific absorption coefficient, a*, have been made during 10 oceanographic
cruises in the western North Atlantic. The range in Chl a concentration for ocean samples from
the various water masses in this region was 0.1-8.0 pg liter-l. Diffuse attenuation spectra of particles
measured by a glass-fiber filter technique show strong correlations between extracted chlorophyll
and blue (440 nm) or red (670 nm) absorption. Of the two, a*440 is the most variable. Average
specific absorption coefficients are calculated to be 0.049 m2 mg-’ at 440 nm and 0.028 at 670
nm, but both relationships are seen to be nonlinear. The nonlinearity observed is attributed to
differences in cell size “packaging” within natural phytoplankton populations. Variation in cell
size and a* are primarily related to the availability of nitrate-N. In addition, other factors contribute
to the variability in a*440, such as detritus and short wavelength UV-absorbing pigments.
Particles that scatter and absorb light in
the sea come from various sources; however, a major source in the open ocean is
primary production. Changes in optical
characteristics of water masses are now related to biochemical processes (Pak et al.
1988; Mitchell and Kiefer 1988a, b; Morel
and Prieur 1977; Smith and Baker 1978;
Yentsch 1962) and have given rise to a need
for better understanding of the causes of
ocean color (Bricaud et al. 1983). Thus, the
need to interpret ocean color as measured
by satellite calorimetry has moved classical
optics into the arena of marine biology. It
is clear that a bridge has formed between
the two disciplines presenting challenges to
relate ocean optics and microbial ecology.
For satellite calorimetry to be an accurate
measure of phytoplankton chlorophyll, one
must expect a linear relationship between
Acknowledgments
Research supported by NASA, NSF, ONR, and the
W. M. Keck Foundation.
We acknowledge helpful discussions with R. W. Eppley and C. M. Yentsch. S. Sathyendranath and an unidentified reviewer provided many suggestions. Graphics were provided by James Rollins and manuscript
preparation by Frances Scannell and Margaret Colby.
light attenuation and chlorophyll concentration. The correlation between upwelled
radiance and chlorophyll concentration has
been shown to be reasonably strong (Gordon and Clark 1980), but other observations show considerable nonlinearity in the
relationship between attenuation and phytoplankton
chlorophyll
concentration
(Smith and Baker 1978). This nonlinear relationship is significant and associated with
the diversity in optical properties (size, refractive index, shape) of biogenic particles
(Morel and Bricaud 198 1; Sathyendranath
et al. 1987; Spinrad and Brown 1986).
In this paper, we explore the relationship
between particulate light absorption and total amount of Chl a in the water masses of
the western North Atlantic. We will attempt
to demonstrate that the specific absorption
coefficient for natural phytoplankton populations (a”), known to be a function of many
factors, is related to cell size. We propose
that the change in particle size spectrum is
related to the nutrient regimes of a water
mass.
Methods
Samples for analysis were collected with
water bottles or by submersible pump from
1694
1695
Ocean optics and microbial ecology
39
Fig. 1. Station locations.
depths within the euphotic zone. Generally
speaking, our observational program concentrated on regions of the western North
Atlantic where sharp baroclinicity occurred,
i.e. the cold wall of the Gulf Stream, warm
core rings, and tidal fronts in the Gulf of
Maine. The data presented do not attempt
to delineate spatial or temporal differences
(Fig. 1). The treatment is to relate a specific
optical parameter to phytoplankton population biomass expressed as chlorophyll
concentration. The range of observations is
set high and low by chlorophyll concentrations in coastal and oligotrophic waters.
The measurement of diffuse attenuation
by particles has been previously described
(Yentsch 1962; Mitchell and Kiefer 1984;
Phinney and Yentsch 1986; Yentsch and
Phinney 1988). For sea observations, 0.52 liters of sample are filtered through a 2.5cm Whatman GF/F glass-fiber filter. The
wet filter with the particulate matter is held
upright in a Lucite holder in the spectrophotometer cuvette compartment. An identical wet blank filter is used in the reference
light path (Fig. 2A). The diffuse attenuation
spectrum as a function of wavelength is
scanned between 750 and 350 nm with a
Bausch and Lomb Spectronic 2000 dualbeam spectrophotometer.
These absorbance values (AX) measured by the glassfiber filter technique are empirically related
to diffuse attenuation spectra for suspensions of phytoplankton cultures obtained by
a modified “opal glass” technique (Shibata
1958). A wetted, blank, glass-fiber filter was
placed over the end of two quartz lo-cm
C
A
hv
Sa
I
+-----L------4
I
350
I
450
I
550
I
550
0
750
l(nm)
Fig. 2. Cuvette calibration for the filter technique. A. Filter technique used at sea. B. Cells in a 1O-cm cuvette,
1.9-cm i.d., with blank filters (GFF) for diffusers. C. Cells on filter at exit of cuvette; blank filter is diffuser
reference. D. Absorption spectra of panels A-C for Phaeodactylum tricornutum. Filtered media used in reference
cuvette.
1696
Yentsch and Phinney
625 nm is discussed later. The cuvette provided the measure of optical light path (L
= 10 cm) for a sample volume of 25 ml.
To calculate the specific absorption coefficient, a*& from the absorbance measured
by the glass-fiber filter technique, AX, we
assume that all particles in an ocean sample
(0.5-2.0 liters) are concentrated into the cuvette volume (25 ml) such that the absorbance values have units of 10 cm-l :
0.6
0.5
0.4
s
3
0.3
0.2
Chl ( micrograms / liter )
0.6
1
a*X [m- ‘(pg Chl a)-‘]
[2.3xAX (10 cm-‘) x 10
X cuvette volume (ml)]
=
[sample volume (ml)
x sample Chl (pg liter-l)]
(1)
where chlorophyll concentration is measured on 85% acetone extracts of the filter
with the method of Yentsch and Menzel
(1963). Pure Chl a from spinach (Sigma
Chem. Co.) is used as a standard. A factor
of 2.3 is used to convert log base- 10 values
to natural logarithms.
Flow cytometry measurements at seawere
made
with a Becton-Dickinson FACS ana4
6
0
2
6
lyzer. This instrument measures chloroChl ( micrograms / liter )
phyll fluorescence (68 5 nm) and Coulter
Fig. 3. Absorbance (A440 and A670) vs. chloro- impedance volume on single cells in a flow
phyll concentration.
stream. Hence the size distribution of the
phytoplankton population is measured. The
optical cuvettes. Volumes of mixed phyto- instrument was configured as follows: 76plankton cultures were added to the sample pm orifice, 436-nm excitation wavelength,
cuvette while filtered growth medium oc- Corning CS2-64 emission filter for chlorocupied the reference cuvette (Fig. 2B). The phyll fluorescence. Impedance volume curdiffuse attenuation spectrum of the suspcn- rent equaled 0.7 1 mA such that a range of
sion between 750 and 350 nm was recorded. particle diameters between 2.5 and 35 pm
The sample cuvette contents were filtered were measured. Four milliliters of sample
onto a glass-fiber filter, the spectrum was were prescreened through 53-grn Nitex netmeasured with filtered growth medium in ting and placed in a vial. After 10,000 t,otal
the sample cuvette and the sample filter events (particles) had been analyzed by flow
placed on the end of the cuvette (Fig. 2C). cytometry, the volume of sample remaining
Finally, the cuvettes were removed and the in the vial was measured with a micropipet
spectrum recorded in the manner of mea- to calculate the sample volume analyzed.
surements made at sea (Fig. 2A). Because Size spectra as equivalent spherical diamthe internal diameter (d) of the cuvette and eter of chlorophyll-containing particles repthe area of the filter with particles is con- resent a subset of the total particles anastant and care is taken to ensure even dis- lyzed.
tribution of particulates on the filter, the
three spectra measured in this manner are Results
a*440, a*670-The range in chlorophyll
identical for samples with high absorption
(Fig. 2D). The departure of the curves in concentration is representative of the water
the low absorption region between 525 and masses of the western North Atlantic; high
1697
Ocean optics and microbial ecology
Table 1. Calculated means and SE, 8 = chlorophyll, ? = x4670.
Chl range
N
8
SE2
5 0.06800 0.00735
0.05-0.09
0.10-0.40 12 1 0.25099 0.00799
0.41-0.50 24 0.43917 0.00496
0.51-1.0 104 0.76903 0.01807
66 1.50348 0.035 11
1.1-2.0
29 2.48068 0.04330
2.1-3.0
8 3.40625 0.06614
3.1-4.0
4.1-7.0
4 4.39500 0.11510
>7.0
1 7.73000 0.0
P
SE P
0.00920
0.02102
0.03583
0.05414
0.09489
0.13548
0.17125
0.21200
0.29900
0.00153
0.00084
0.00231
0.00157
0.00267
0.00427
0.01027
0.00745
0.0
concentrations are observed in coastal and
slope waters and low values are typical of
the central gyre. The variability in blue light
attenuation by particles measured as A440
(Fig. 3) ranges between 0.02 and >0.50. In
this plot a deviation from linearity occurs
when chlorophyll values are < 0.5 pg liter-‘,
which is consistent with the observations of
Smith and Baker (1978). The mean a*440
for the entire data set is 0.049 m2 mg-l.
Between 0.2 and 0.5 ,ug liter-‘, a*440 appears to be more variable than at concentrations above or below these values.
Figure 3 shows a strong linear correlation
(r2 = 0.88 1) between chlorophyll absorption
at 670 nm and extracted chlorophyll. (The
attenuation maxima for the red band of
chlorophyll ranges between 670 and 675 nm.
For convenience we have labeled this band
670 nm.) The data fitted with a straight line
gives an average value for a*670 of 0.028
m2 mg-l. As in the case of the blue attenuation, an inflection exists (deviation from
linearity) around 0.5 pg liter-l of chlorophyll. It is apparent that the variability in
the data increases with increasing concentration of chlorophyll, at least in the case of
the region of l-4 hg liter-l chlorophyll.
Even though the data shown in Fig. 3 are
linearly correlated, there are distinctive features in the plot that suggest that it is not
linear (nonrandom distribution of errors) as
previously mentioned. The slope appears to
change in the region indicated by the arrow
on the plot. In an attempt to uncover the
degree of nonlinearity, we have divided
chlorophyll and absorbance into nine ranges
and computed the means and standard
errors (Table 1). When these mean values
are plotted (Fig. 4), the degree of nonlinearity is accentuated. For chlorophyll con-
.25 -
-0
1
2
3
4
5
6
7
6
Chl ( micrograms / liter )
Fig. 4. Power curve_(Y = aP) fitted to the mean
values (Chl = X, A = Y) shown in Table 1.
centrations < 1.Opg liter-‘, the slope (A670 :
Chl) is steeper than at chlorophyll concentrations > 1.O pg liter-‘. A power curve provides a best-fit of the means and yields the
relationship
A670 = 0.0667 Chl (pg liter-1)0.758.
A relationship between chlorophyll,
a*670, and cell size would be predicted for
natural phytoplankton populations from
previous theoretical and experimental work
(Duysens 1956; Kirk 1975; Morel and Bricaud 198 1; Bricaud et al. 1983). We believe
that the scatter and deviation from linearity
shown in our data (Fig. 3) are largely due
to differences in the mean cell size of phytoplankton species. Therefore, we view the
range of observed chlorophyll (O-8 pg liter-l) not only as an index of a range in
phytoplankton biomass but also as an index
of mean cell size (i.e. low chlorophyll concentration corresponds to small cells and
high chlorophyll concentration to the presence of large cells in addition to small ones).
The data set, as a whole, shows a linear
relationship with a high correlation coefficient suggesting that, in a statistical sense,
light attenuation of natural populations
tends to obey Beer’s law. However, by dividing the chlorophyll data into concentration ranges and obtaining mean values, we
observe a nonlinear relationship that can be
fitted by a power curve. The exponent (b)
we obtain by y = aP suggeststhat spherical
Yentsch and Phinney
1698
Sargasso Sea
Gulf of Maine
2m
Chl = 0.1
2.5
5
10
DIAMETER
15 20
2.5
( pm )
5
DIAMETER
10
15 20
( pm )
18m
Chl = 0.90
2.5
10
15 20
DIAMETER ( pm )
2.5
10
DIAMETER
24 m
Chl = 2.55
Chl =0.50
2.5
5
10
DIAMETER
15 20
( pm )
15 20
( pm )
2.5
5
DIAMETER
10
15 20
( pm )
Fig. 5. Size spectra of phytoplankton (cellular fluorescence) for the Sargasso Sea (4 July 1985) and Gulf of
Maine (20 July 1985) obtained by flow cytometry.
Ocean optics and microbial ecology
1699
0.5
volume is the important factor in regulating
7
cellular absorption of light.
Phytoplankton cell size spectra -We have
much more synoptic data on a* than phytoplankton sizing. Even with the added utility of flow cytometers, measurement of size
spectra of phytoplankton at natural concentrations present in oligotrophic waters is time
consuming. In Fig. 5 we show what we believe are two extremes in size spectra that
lend support to the interaction of optics and
V.” I
1’
2’
0
i
;
Iii
il
cell size. At one extreme are the phytoplankChlorophyll
ton populations of the oligotrophic waters
0.4
of the north central Sargasso Sea. At the
1
other extreme are the populations from the
1
chlorophyll-rich waters of the Gulf of Maine.
In both cases, there is a subsurface chlorophyll maximum: at 58 m in the Sargasso
Sea and at 24 m in the Gulf of Maine. The
maximum cell size (mean spherical diameter) for the SargassoSeapopulation is < 5.O
.
w
.*a . .
.
.
'.
0.1 .*..':
* .
.
pm, centered at about 3.0 pm. In contrast,
. .
.
-I
the Gulf of Maine population exhibits three
prominent maxima in cell size: 3.0, 5.0, and,
in the case of the surface, 10.0 pm. The
0
1
2
3
4
5
6
comparison of the two regions demonNOB”
strates that with increasing richness (oligo1.0
trophy to eutrophy) the size spectrum be. *. .
.
. ’
c
comes more skewed due to the appearance
of large cell sizes. It is important to recognize that this is not because of removal of
small cells but rather to the addition of large
0 0.6
.cells (Yentsch and Spinrad 1987).
Relationship between cell size a* and the
5
I 0.4
availability of new nitrogen -The primary
x,
departure from linearity in the plot ofA
vs. Chl (arrow in Fig. 3) occurs with chlorophyll concentrations of ~0.5 pg liter-l. In
Fig. 6 we directly compare a* with chlorophyll and nitrate concentrations measured at the same locations. Also included
is the relationship between thef-ratio (NH3 :
Fig. 6. Relationship between chlorophyll (pg liN03-) and nitrate concentration as obter-l), nitrate &g-atom liter *), and a*440. Also shown
served by Platt and Harrison (1985). This is
the relationship betweenf-ratio and nitrate concencurve is characteristic of observations made tration (Platt and Harrison 1985).
in the mid-Atlantic Bight (Harrison et al.
1987). We believe that a* is closely coupled
to the availability of nitrate-N. When this a* more than double. At low levels of nitrate
nutrient is < 1.0 pg-atom liter-l (f-ratios
the higher efficiency for nutrient absorption
< 0.8), the principal source of nitrogen would
by small cells, which have a large ratio of
be in the form of ammonia, presumably
surface area to volume favoring survival, is
supplied by grazing herbivores. As nitrate
consistent with the idea that a* is a function
levels and f-ratios decrease, the values for of cell size.
l
0.01
l
1700
Yentsch and Phinney
a suspension of absorbing particles which
Duysens devised as
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Transmission
Fig. 7. Graphical representation of change in absorbance (A) with change in particle transmission (T,).
With changing concentration of absorbing particles in
suspension, absorbance equals (1 -- T,) instead of Beer’s
law, ln( 1IT,).
Discussion
Cell size and packaging-The
problem
concerning the measurement of light absorption in suspensions of cells surfaced in
the-early plant physiology literature. It was
recognized
that Beer’s law was not always
obeyed by light-absorbing particles in suspensions. The obvious reasons are that some
of the light passesunobstructed, that wavelengths that are weakly absorbed have longer light paths (detours) due to scattering,
and that there is self-shading if particles are
strongly absorbing.
The geometric factors that influence particle absorption are not obvious. The example used by Duysens (1956) to demonstrate the geometry where particle
absorption obeys Beer’s law consisted of a
solid layer of particles illuminated by an
optical cross-section. This is analogous to a
molecular solution of pigment such as chlorophyll. In this case,Beer’s law can be useda change in concentration changes absorptionA = n X ln(l/T,)
where A is absorbance, n the number of
particles, and TP the transmittance of particles. This equation contrasts with that for
A = n x (1 - TP).
In the latter case, where the transmission of
the particles is zero, absorbance will be 1.O.
Changes in phytoplankton concentrations obey Beer’s law when particles (cells)
are nearly nonabsorbing or when the suspension is extremely dense, but a marked
deviation from Beer’s law [ln( 1/TP)] occurs
between these extremes (Fig. 7). For example, if the transmission of the particles
changes from 0.6 to 0.2, and Beer’s law is
obeyed, the absorbance ln( 1/TP) change will
be 0.48. The measured absorbance (1 - TP)
of the suspension, however, is lower, changing to -0.4. When making spectral measurements of suspensions (i.e. changes from
highly absorbed wavelengths to weakly absorbed wavelengths and vice versa), the
problem is the same. Duysens called this
spectral “flattening.” None of the techniques (diffusers, integrating spheres) designed to remove optical artifacts is perfect
(Maske and Haardt 1987). The filter technique does come close to Duysens’ standard
particle package: where all particles are distributed into a thin layer, absorption is hence
equivalent to ln( 1/TP). There are some differences between spectra measured by diffusers and filters (see Fig. 2). These differences are at wavelengths where pigment
absorption is weak; we suspect that these
diflerences represent the longer light path in
cuvettes due to scattering. Thus, agreement
with Beer’s law occurs at high and low values of particle absorption, whereas deviations from Beer’s law occur between the extremes.
Other causes for change in a*440-The
attenuation of blue light by oceanic particles
is much higher than can be accounted for
simply by an “average” attenuation by chlorophyll and accessory pigments of phytoplankton. Some workers, by the use of difference spectra (phytoplankton-ocean
particle spectra), have generated so-called
detrital spectra. These are characterized by
a monotonous increase in attenuation from
red to blue regions of the spectrum (see
Yentsch 1962). These spectra are recognized as being similar to that for dissolved
1701
Ocean optics and microbial ecology
yellow substances, which in turn has reinforced speculation as to the nature of this
material. The problem with the so-called
detritus decomposition interpretation is that
many substances, detrital and nondetrital,
attenuate light in this region and there is no
obvious optical way of identifying detritus
alone.
Second, there are large variations in the
attenuation spectra for phytoplankton
(Sathyendranath et al. 1987; Bricaud et al.
1988). Most of the variation concerns the
blue region of the spectrum. The magnitude
of these differences is seen in the spectra of
Aureococcus and Skeletonema (Fig. 8) normalized at 670 nm. Aureococcus attenuates
blue light 1.5 times greater than Skeletonema at 450 nm. One is tempted to assign
such differences to optical geometry: Aureococcus is 2.0 pm in diameter, Skeletonema
is 25 pm, yet there is a major difference in
’ accessory pigmentation. Both contain fucoxanthin, but Aureococcus has in addition
19’butanoyloxyfucoxanthin
(Hooks et al.
1988), which suggestsreal differences in absorption between these two organisms.
Finally, a group of compounds known to
be in sizable abundance in some algae are
the mycosporinelike UV-absorbing pigments (Chalker and Dunlap 1982). These
pigments are believed to be UV blockers
(i.e. DNA protectors) in phytoplankton.
They are easily detected in the in vivo absorption of some species (Yentsch and
Yentsch 1982) and can be characterized and
estimated by extractions with aqueous
methanol. In natural populations from specific areas, we have observed large increases
in the short wavelength attenuation due to
the presence of these pigments (Fig. 9). The
environmental conditions in the surface
waters of the two areas that are compared
are quite different: in the Gulf of California,
the water mass was vertically stable and
bathed in sunlight from a cloudless sky; the
North Atlantic population was in a water
column where the depth of the mixed layer
was 50 m. Solar radiation was moderate due
to low sun angle and clouds. At the time of
this writing it is uncertain how much of these
blocking pigments occur in natural populations of phytoplankton, or whether the capacity to produce this pigment is confined
-
B
.2 .l 400
500
600
700
h(nm)
Fig. 8. Absorbance spectra for Aureococcu.ssp. and
Skeletonema costatum.
to certain groups or to all algae. One thing
is clear-when these pigments are present,
they can be a major source of the high attenuation of near-UV and blue light in the
in vivo spectra of oceanic phytoplankton.
Cell size and nutrient availability-Earlier we noted that a bridge had formed between classical ocean optics and marine microbial ecology. The substance of this bridge
concerns nutrient enrichment of the upper
layers of the oceans. We are accustomed to
the idea that the overall biomass of phytoplankton populations is regulated by the
level of nutrients in a water mass; however,
the role nutrients play in determining species
diversity is less clear (Banse 1976; Malone
1980). We have argued that changes in cell
size and optical features such as a* are regulated by nutrient availability. It can now
be asked, are these concepts borne out by
measurements of the size spectra of marine
phytoplankton?
It is not apparent how the “availability”
of nutrient influences size spectra; however,
some clues come from study of the shape
of these spectra. The cell size distributions
generated by the flow cytometer suggest the
existence of a log-normal spectrum (Co) (Fig.
lo), which is common (basic) to all size
1702
Yentsch and Phinney
Station 5 19 Mar 88
Gulf of California
2m Chl = 1.70
Station 53 14 Ott 88
Gulf Stream System
2m Chl = 0.55
in vivo
95 % MeOH
95% MeOH
h (nm)
h (nm)
Fig. 9. Absorbance spectra and spectra of 95% methanol extracts for natural populations in the surface waters
of the Gulf of California and the North Atlantic.
spectra. We argue that the major differences
in the spectra arise from “nutrient availability,” which is manifested by an increase
of cells of larger sizes C, and C, (Fig. 10).
Furthermore, we believe that it is these additions to the basic spectrum (C,) that cause
the variability in a* observed with increasing concentrations of chlorophyll (Table 1).
Thus, optical diversity in particles results
from species diversity, and changes in both
time and space are lodged in the ecological
theory of phytoplankton diversity (Hutchinson 1967; Kilham and Kilham 1980).
The neo-Darwinian explanation of removal by natural selection does not seem
appropriate. What does seem appropriate is
the concept of opportunism, i.e. opportunistic growth. The important point is the
surface-area-to-volume relationship and its
role in nutrient uptake (see Smayda 1980;
Kilham and Kilham 1980; Malone 1980).
We propose a simple resource limitation
model (Yentsch 1974; Tilman 1977) in the
manner shown in Fig. 10. Assuming that
the seasonal regime of the upper layers of
the oceans offers a range of nitrate (O-5 j&M),
one can visualize how size spectra could
change in time and space. Inherent to this
concept is that being large has advantages,
yet most of the experimental data show small
cell sizes out-competing large cells under
any oceanic conditions (Geider et al. 1986).
Such data are in contrast to field observations, where nutrient-rich and highly productive regions are characterized by large
species (Davis 1982). This apparent contradiction has led others to postulate complex
interactions of growth and grazing by herbivores (Sommer 1988; Walsh 1976).
We believe that in the course of time algal
species evolved into large, more complex
cells, which provided cellular systems more
flexibility or adaptability, such as storage,
buoyancy regulation, resting stages, etc. All
these, as well as those not stated (e.g. see
Smayda 1980), become necessary for the
1703
Ocean optics and microbial ecology
survival of the species, especially in coastal
and shelf ecosystems where rapid environmental change is often the case. The relationship between turbulence and cell size
(Margalef 1978; Legendre and LeFevre in
press) appears to be one where most agree:
turbulence convects nutrient which overcomes the uptake problems associated with
large surface-area-to-volume cells.
To say that information on evolutionary
theory is required to explain the workings
of particle optics may sound a bit much.
However, those familiar with the difficulty
of determining the causes of biogeographic
boundaries recognize these formidable
problems. Problems in identifying the cause
of temporal and spatial change in particle
optics (i.e. ocean optical geography) are just
as severe. In the case of the biogeographer,
the major tools are taxonomic, whereas the
optical oceanographer uses ataxonomic
tools. It remains to be seen whether these
two disciplines progress in parallel, converge, or totally diverge.
Conclusions
In the western North Atlantic, the attenuation of light by oceanic particles is dominated by the absorption characteristics of
natural populations of phytoplankton.
Cross-sectional area values for a*440 and
a*670 are strongly correlated with measured chlorophyll in vitro. Both a*440 and
a*670 show that the relationship between
measured absorbance and chlorophyll is
nonlinear: for chlorophyll concentrations
< 1.Opg liter- l the absorbance per unit chlorophyll is higher than for chlorophyll concentrations > 1.O pg liter-‘.
Absorbance at 670 nm and chlorophyll
are fitted with a power curve y = axb. The
value for the exponent (0.75) suggests that
the nonlinearity observed is the result of
packaging due to differences in cell size. We
did not try to analyze variability in a*440
because of the contribution by other colored
substances in unknown concentrations
which, in addition to the geometrical problems of packaging, increase variability.
Comparison of cell size spectra, measured
by flow cytometry, shows that spectra from
extremes (oligotrophy or eutrophy) are
dominated by a log-normal curve which has
a maximum at -3.0 pm. However, for re-
0
I
I
I
I
I
I
I
I
I
I
2
4
6
8
10
12
14
16
18
20
Diameter ( microns )
-
GOM
Fig. 10. Concept of“bio-particle”
relationships.
>
size and nutrient
gions where nutrients are plentiful, this
spectrum is modified by including larger cell
sizes-without any apparent loss of the small
sizes. We believe that variability due to cell
size is a consequence of differences in nutrient availability among water masses. For
example, in oligotrophic water masseswhere
nitrogen is recycled as ammonia, the populations are primarily small cells. In water
masses where nitrate-N is plentiful, large
cells prevail. We conclude that the factors
regulating variability in species diversity
apply to particle optics. Any analysis of
changes in time or space requires knowledge
of oceanographic criteria and ecological theoryReferences
BANSE, K.
1976. Rates of growth, respiration and
photosynthesis of unicellular algae as related to
size-a review. J. Phycol. 12: 135-140.
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