Journal of Oceanography, Vol. 54, pp. 517 to 526. 1998
Chlorophyll-Specific Absorption Coefficients and Pigments
of Phytoplankton off Sanriku, Northwestern North Pacific
KOJI SUZUKI1, MOTOAKI KISHINO2, KOUSEI SASAOKA3, SEI-ICHI SAITOH3 and TOSHIRO SAINO1
1Institute
for Hydrospheric-Atmospheric Sciences, Nagoya University,
Chikusa-ku, Nagoya 464-8601, Japan
2The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan
3Faculty of Fisheries, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
(Received 9 March 1998; in revised form 3 July 1998; accepted 4 July 1998)
The variety in shape and magnitude of the in vivo chlorophyll-specific absorption spectra
of phytoplankton was investigated in relation to differences in pigment composition off
Sanriku, northwestern North Pacific. Site-to-site variations of the absorption coefficients,
a*ph (λ), and pigment composition were clearly observed. At warm-streamer stations,
higher values of a*ph (440) and a*ph (650) were found with relatively high concentrations
of chlorophyll b (a green algae marker). At stations located in the Oyashio water (cold
streamer), a*ph (440) values were lower and fucoxanthin (a diatom marker) concentrations were higher, compared to the other stations. The peak in the absorption spectra at
the Oyashio stations was shifted toward shorter wavelengths, which was probably due to
the presence of phaeopigments. In a Kuroshio warm-core ring, the magnitude of a*ph
(440) was in between those at the warm-streamer and Oyashio stations, and the diagnostic
pigment was peridinin (a dinoflagellate marker). These findings indicated that major
differences in phytoplankton absorption spectra of each water mass were a result of
differences in the phytoplankton pigment composition of each water mass, which was
probably related to the phytoplankton community.
1. Introduction
Phytoplankton cells play a significant role in determining the optical properties of the ocean. The chlorophyllspecific absorption coefficient of phytoplankton, a*ph(λ), is
crucial for calculating the contribution of phytoplankton to
the absorption coefficient of seawater as well as for estimating
the amount of light absorbed by the phytoplankton in biooptical models of marine primary production (Sakshaug et
al., 1997). Such models using a*ph(λ), combined with satellite data, have been recently used to convert maps of
surface pigment concentration into maps of primary production at global or regional scales (e.g. Morel and André,
1991; Ishizaka, 1998).
The value of a*ph(λ) was previously considered to be
relatively constant, averaging approximately 0.016 m2
mg–1 (Bannister, 1974), and most bio-optical models for
estimating algal photosynthesis have often treated a*ph(λ) as
constant, using Bannister’s value or a typical value for a
given water type (e.g. Kiefer and Mithcell, 1983; Berthon
and Morel, 1992). However, it is now recognized that both
the magnitude and the spectral shape of a*ph(λ) vary considerably (e.g. Morel et al., 1993; Bricaud et al., 1995).
Bricaud et al. (1995), for instance, investigated the variability in a*ph(λ) using a data set that included 815 spectra
in different regions of the world ocean, covering chlorophyll
concentrations ranging from 0.02 to 25 mg m–3. The a*ph(λ)
values observed showed a decreasing trend from oligotrophic to eutrophic waters, spanning over more than one order
of magnitude (0.18 to 0.01 m2mg–1) at the blue absorption
maximum (ca. 440 nm). The variability in the magnitude
and spectral form of a*ph(λ) can be mainly attributed to two
factors: (1) package effect; i.e. pigments packed into chloroplasts are less efficient in absorbing light per unit pigment
mass, than when in an optically thin solution (Kirk, 1994),
and (2) pigment composition of phytoplankton cells (Bidigare
et al., 1990; Hoepffner and Sathyendranath, 1991). The
package effect has been examined intensively using theoretical formulae during the last decade, and it is now clear
that this effect depends both on the cell size and pigment
content of the cell for a given pigment composition (Kirk,
1994). In addition, the package effect is wavelength-dependent, being most pronounced where absorption is the
highest. As an instance, Bricaud et al. (1995) indicated that
the package effect induced a three- and a ten-fold reduction
in the height of the red and blue absorption band, respectively,
when chlorophyll concentration increased from 0.02 to 25
mg m–3. Although the relative contributions of the package
effect and the pigment composition to a*ph(λ) variation are
517
Copyright  The Oceanographic Society of Japan.
Keywords:
⋅ Phytoplankton
absorption,
⋅ phytoplankton
pigments,
⋅ off Sanriku,
⋅ North Pacific.
concomitant and cannot be simply partitioned from each
other, the effect of pigment composition in natural waters
has seldom been discussed due to the lack of detailed
pigment data measured by HPLC (high-performance liquid
chromatography). Hoepffner and Sathyendranath (1992),
Lazzara et al. (1996), and Lutz et al. (1996) investigated the
importance of the intracellular pigment composition in
explaining variability in a*ph(λ) in the North Atlantic Ocean,
but such studies have not been conducted in the Pacific, with
the exception of the work of Allali et al. (1997) in the
equatorial Pacific Ocean.
The northwestern North Pacific (off Sanriku) where
our study was conducted provided an excellent environment
for examining variability in the optical properties of phytoplankton due to the presence of several water masses of
different origins (i.e. coastal water, the Kuroshio water, the
Oyashio water and Kuroshio warm-core ring; Inagake and
Saitoh, this volume). Such contrasting environments usually
support a highly heterogeneous phytoplankton community
(e.g. Ishizaka et al., 1994). The objective of the present
study is to examine the variety in shape and magnitude of the
in vivo chlorophyll-specific absorption spectra of phytoplankton in relation to differences in pigment composition.
We were able to obtain a better understanding of regional
variations in the absorption coefficients of phytoplankton.
2. Materials and Methods
2.1 Sampling and hydrographical analysis
Samples were collected in the northwestern North
Pacific (off Sanriku) during KT-97-5 cruise (May 10–12,
1997) aboard the R/V Tansei-Maru (Fig. 1 and Table 1).
Seawater was taken from several depths (0–100 m) using a
12-bottle rosette sampler attached to a conductivity-temperature-depth (CTD) system. Surface water was sampled
with a plastic bucket. Nutrient concentrations were measured
with a BRAN+LUEBBE Autoanalyzer (TRAACS 2000).
Spectral downward irradiance including photosynthetically
available radiation (PAR) was measured by a
spectroradiometer (MER-2040, Biospherical Instruments
Inc.).
2.2 Pigment analysis
Chlorophyll a concentration was routinely determined
on board by fluorometry using N,N-dimethylformamide as
extraction solvent. These data were obtained from Saino and
Gomes (1997), and were used to calculate chlorophyllspecific absorption coefficient (a*ph(λ)) described below,
because HPLC pigment data were lacking at some stations
(Stns 3 and 6) in this study.
Water samples (1.5–3 liters) for HPLC pigment analysis
were filtered onto 47 mm Whatman GF/F filters. The filter
was folded once, blotted with a filter paper as much as
possible to reduce excess water content, and stored in liquid
518
K. Suzuki et al.
Fig. 1. Map of the sampling sites in this study.
nitrogen until analysis on land. The filter was cut into small
pieces, soaked in 5 ml 90% acetone, and then sonicated to
break cell walls. The suspension was sealed with Parafilm
(American National Can) to minimize solvent evaporation,
and extracted for ~4 hours in the dark at –20°C. The extract
was centrifuged for 2 min at 2000 r.p.m. to remove cellular
debris and glass fibers, and filtered through 0.45 µm PTFE
filters (GL Sciences) to remove fine particles. All procedures
for the extraction were conducted under subdued light in
order to prevent photodegradation of the pigments. After the
extraction, pigments were separated and quantified using
HPLC according to the procedure described by Suzuki et al.
(1997). Commercially available chlorophyll a, chlorophyll
b, lutein, α -carotene, β-carotene (Sigma), zeaxanthin
(Extrasynthese), and chlorophyll c1, chlorophyll c2, peridinin,
fucoxanthin, 19′-hexanoyloxyfucoxanthin, alloxanthin,
prasinoxanthin, diadinoxanthin (the International Agency
for 14C determination) were used as standards. Standards
were also obtained from extracts of algal cultures, purified
by a preparative-scale HPLC. Phaeophorbide a was prepared by the method of Roberts and Perkins (1962). The
HPLC method of Suzuki et al. (1997) does not resolve lutein
from zeaxanthin, and chlorophyll c1 from chlorophyll c2. The
zeaxanthin plus lutein peak was quantified in terms of
zeaxanthin equivalent. Similarly chlorophyll c1+2 and
phaeophorbide a-like pigments were quantified in terms of
chlorophyll c2 and phaeophorbide a equivalents, respectively.
In order to estimate the contribution of selected algal
classes to the total phytoplankton crop at the sampling
Table 1. Sampling record of this study.
Station
3
5
6
7
14
18
21
22
23
Date
Time
9 May 97
10 May 97
10 May 97
10 May 97
11 May 97
11 May 97
12 May 97
12 May 97
12 May 97
23:29–23:51
10:18–10:40
13:55–14:10
15:58–16:14
10:31–10:50
16:30–16:48
10:05–10:23
14:36–14:51
08:00–08:17
Latitude
Longitude
40°00 ′ N
40°00 ′ N
40°00 ′ N
40°00 ′ N
40°18 ′ N
40°36 ′ N
40°00 ′ N
39°30 ′ N
36°35 ′ N
143°00 ′ E
143°00 ′ E
143°01 ′ E
143°00 ′ E
143°35 ′ E
144°10 ′ E
143°00 ′ E
143°00 ′ E
141°30 ′ E
stations, a multiple regression analysis (e.g. Suzuki et al.,
1997; Laws, 1997) was performed using the integrated
concentrations (trapezoidal rule) of chlorophyll a and algal
chemotaxonomic pigments for each group above 80 m.
Chlorophyll a is universally used as an indicator of phytoplankton biomass and three major pigments were selected as
chemotaxonomic markers: chlorophyll b, fucoxanthin, and
peridinin, which were the prominent pigments in this study,
indicating the presence of green algae (prasinophytes and
chlorophytes; Jeffrey, 1974), diatoms (Jeffrey, 1974), and
dinoflagellates (Johansen et al., 1974), respectively. The
regression equation was initially set as follows:
tomultiplier tubes according to the glass-fiber filter technique of Kishino et al. (1985). A blank filter, wetted with
filtered seawater, was used as reference. Spectral values of
the absorption coefficient were recorded every 1 nm from
350 to 850 nm. All spectra were set to zero at 750 nm to
minimize differences between sample and reference filters,
assuming the lack of the absorption at 750 nm. Measured
ODf(λ) were corrected for the increase in pathlength caused
by multiple scattering in the glass-fiber filter using the
equation of Cleveland and Weidemann (1993):
[Chl a] = A[Chl b] + B[Fuco] + C[Peri] + D.
where ODs is the optical density of the particulate matter in
the suspension. The absorption coefficients of particulate
matter, ap(λ), were calculated as follows:
(1)
The [Chl a], [Chl b], [Fuco], and [Peri] in Eq. (1) indicate the
integrated concentrations of chlorophyll a, chlorophyll b,
fucoxanthin, and peridinin, respectively. The dimensionless
coefficients A, B and C, and the constant term D were
determined by multiple regression analysis using Microsoft
Excel. F-test and t-statistics were also conducted to check
the validation of the regression equation obtained, and to
determine whether each dimensionless coefficient was useful in estimating the assessed value of chlorophyll a, respectively. The portions of chlorophyll b-containing green
algae, fucoxanthin-containing diatoms, and peridinin-containing dinoflagellates, and the other algae to chlorophyll
biomass were determined by substituting the integrated
concentrations of chlorophyll b, fucoxanthin, and peridinin
in Eq. (1), and by dividing each term of the equation by the
calculated chlorophyll a.
ODs = 0.378ODf + 0.523ODf2
ap(λ) = 2.3ODs(λ)S/V
(2)
(3)
where S is the filter clearance area and V is the filtered
volume. After measurement, the filter was soaked in methanol
to extract phytoplankton pigments, and then the absorption
spectra of the extracted filter was once again measured to
obtain the optical densities of detritus. The absorption coefficient of detritus, ad (λ), was determined similarly using
Eqs. (2) and (3). Absorption of light by phytoplankton,
aph(λ), was obtained by subtracting ad(λ) from ap(λ) (Kishino
et al., 1985). Finally aph(λ) was converted to chlorophyll a
specific absorption coefficient (a*ph), i.e. divided by chlorophyll a concentration obtained by fluorometry.
3. Results and Discussion
2.3 Absorption measurement
Seawater samples between 200 and 800 ml in volume
were collected and filtered onto 25 mm Whatman GF/F
glass-fiber filters under low vacuum pressure (<100 mmHg).
Optical densities of particulate matter on the filter, ODf(λ),
were measured on board using a spectrophotometer (MPS2000, Shimadzu) equipped with matched end-on type pho-
3.1 Hydrography
Surface water temperature at Stns 3, 5, 6, 7 and 22,
which were in the warm-streamer of the Kuroshio water (see
Saitoh et al., 1998; Inagake and Saitoh, 1998) and hereafter
referred to the stations as warm-streamer stations, was
consistently around 11°C (Fig. 2). The depth of 1% PAR at
Chlorophyll-Specific Absorption Coefficients and Pigments of Phytoplankton
519
Fig. 2. Distributions of temperature (°C) and nitrate (µM) at each station. Broken line indicates the depth of 1% PAR.
the warm-streamer stations was between 45 and 55 m (Fig.
2). At Stns 14 and 21, the surface water mass was relatively
cold. This indicated the presence of Oyashio water, which
was also called a cold streamer (Saitoh et al., 1998; Inagake
and Saitoh, 1998). The euphotic zone at the Oyashio stations
was shallower than that at the other stations. It should be
noted that Stn 21 was also located at the same site as the
warm-streamer stations, and that observations at this station
were conducted 3 days subsequent to those at the warmstreamer stations (Table 1). Surface nitrate concentrations
(Fig. 2) at all warm-streamer stations were ca. 5 µM. A
prominent nitracline was located at about the 40 m layer at
Stn 14. Station 18 was located in a warm-core ring, where
temperature and nitrate concentration did not change remarkably in the water column above 100 m. According to
Saitoh et al. (1998), the warm-core ring was formed in
February, 1993, and was ca. 200 km in diameter. The south
station (Stn 23) was in the warm water area in the Kuroshio/
520
K. Suzuki et al.
Oyashio mixed water region (see Inagake and Saitoh, this
volume), where water temperature above 100 m was >10°C,
and hereafter referred to the Kuroshio-Oyashio station.
3.2 Distribution of phytoplankton pigments
Figure 3 shows the vertical distribution of the major
chemotaxonomic pigments detected by HPLC in this study.
The abundance and composition of the phytoplankton pigments were characteristic of each water mass (also see Table
2). In addition, the multiple regression analysis for estimating
the contribution of the selected algal classes to the total
phytoplankton crop revealed that dominant phytoplankton
group was different at each water mass (Table 3). On the
multiple regression equation, the dimensionless coefficients
of A, B, and C, constant D, the coefficient of determination
(r2), t- and F-values were shown in Table 4. The F value of
1260 was larger than the critical F value of 29.46 (significance level = 0.01), indicating that the multiple regression
Fig. 3. Profile of major chemotaxonomic pigments determined by HPLC at each station.
equation obtained was considered to be statistically significant. The t-values obtained were also greater than the tcritical value of 1.64 (single tail, significance level = 0.1).
This indicated that the concentrations of chlorophyll b, fucoxanthin, and peridinin were important variables when
estimating the assessed value of chlorophyll a. The magnitude of the t-values also indicated that fucoxanthin was the
most influential determinant of chlorophyll a concentration
in the study area.
At the warm-streamer stations (Stns 5, 7, and 22),
chlorophyll a concentrations were generally low, with values
<0.6 mg m–3 (Fig. 3 and Table 2). Chlorophyll b, which is a
marker for green algae, was a primary component of the
phytoplankton pigments, at the warm-streamer stations,
followed by chlorophyll a. Neoxanthin and prasinoxanthin,
which are also contained in green algae (Jeffrey, 1974; Foss
et al., 1984) were detected only at the stations (Table 2). The
multiple regression analysis indicated that chlorophyll bcontaining green algae were a primary or secondary constituent of the phytoplankton community above 80 m (Table
3). The contribution of the other phytoplankton group except green algae, diatoms and dinoflagellates to the chlorophyll biomass was relatively high at the warm-streamer
stations as estimated by the multiple regression equation.
The other phytoplankton group would include alloxanthincontaining cryptophytes (Pennington et al., 1985) and 19′hexanoyloxyfucoxanthin containing prymnesiophytes
(Wright and Jeffrey, 1987).
At the Oyashio stations (Stns 14 and 21), concentrations of chlorophyll a were higher, compared to those at the
other stations. Fucoxanthin, which is used as a chemotaxonomic marker for diatoms, was the most abundant carotenoid in the samples. The multiple regression analysis indicated fucoxanthin-containing diatoms contributed to >75%
Chlorophyll-Specific Absorption Coefficients and Pigments of Phytoplankton
521
Table 2. Integrated concentrations (mg m–2 ) of phytoplankton pigments determined by HPLC above 80 m depth.
Stn
Chl a
Chl b
Chl c1+2
Chl c3
Chillide a
Phaeo
Fuco
5
7
14
18
21
22
23
19.66
27.23
236.93
42.02
63.66
19.20
64.69
7.50
11.30
2.30
3.41
3.25
5.31
7.56
0.67
0.92
54.20
5.96
12.89
1.25
13.87
—
—
3.58
0.32
0.33
—
0.44
0.01
0.01
31.04
0.70
8.71
0.01
2.19
—
—
62.09
—
21.03
—
—
2.46
2.65
157.41
10.68
33.60
4.96
8.06
19HF
19BF
Peri
Allo
Diad
Neox
Pras
Zea/Lut
β -caro
4.92
6.24
2.02
8.19
2.81
4.42
9.94
1.02
1.67
—
3.25
1.38
1.76
2.77
0.08
0.78
5.19
12.53
2.02
0.77
40.87
1.17
1.58
4.16
4.12
1.66
1.28
2.14
0.66
0.87
19.24
4.39
4.35
1.26
9.43
2.82
4.02
—
—
—
2.42
—
0.69
1.82
—
—
—
0.71
—
0.14
0.19
0.23
0.16
0.19
0.21
0.60
0.05
0.07
1.55
0.03
0.22
0.04
0.61
Chl a, Chl b, Chl c1+2, Chl c 3, Chllide a, Phaeo, Fuco, 19HF, 19BF, Peri, Allo, Diad, Neox, Pras, Zea/Lut, and β-caro indicate
chlorophyll a, chlorophyll b, chlorophyll c1+2, chlorophyll c3, Chlorophyllide a, phaeopigments, fucoxanithin, 19′-hexanoloxyfucoxanthin,
19′-butanoyloxyfucoxanthin, peridinin, alloxanthin, diadinoxanthin, neoxanthin, prasinoxanthin, zeaxanthin/lutein, β-carotene, respectively.
—: not determined.
Table 3. Contributions of chlorophyll b-containing green algae, fucoxanthin-containing diatoms, peridinin-containing dinoflagellates
and the other phytoplankton groups to chlorophyll a biomass above 80 m at each station as revealed by the multiple regression
analysis.
Stn
Green algae (%)
Diatoms (%)
Dinoflagellates (%)
5
7
14
18
21
22
23
35
43
1
8
5
23
11
16
14
93
38
76
30
17
1
4
2
29
3
3
57
Others (%)
48
39
4
25
16
44
15
Table 4. The dimensionless coefficient (A, B, C), constant (D), determination coefficient (r2 ), t- and F-values of the multiple regression
analysis of pigments.
A
B
C
D
r2
t-value for Chl b
t-value for Fuco
t-value for Peri
F-value
0.971
1.40
0.897
10.0
0.999
2.01
51.1
10.5
1260
Chl b, Fuco and Peri indicate chlorophyll b, fucoxanithin and peridinin, respectively.
of the chlorophyll biomass above 80 m at the Oyashio
stations (Table 3). This indicates that diatoms probably
dominated in the phytoplankton communities. At approximately the same location in the spring of 1985, Odate and
Maita (1988/1989) measured chlorophyll a concentrations
greater than 1 mg m–3. Here the phytoplankton population
was mainly composed of diatoms and the >10 µm size
fraction contributed more than 80% of the total chlorophyll
a. Phaeopigments (phaeophorbide a and two phaeophorbide
a-like pigments) were detected only at the Oyashio stations
by HPLC, and its concentrations in surface waters were 1.63
mg m–3 at Stn 14 and 0.610 mg m–3 at Stn 21. This may
indicate vigorous grazing by zooplankton (Head and Harris,
1992).
522
K. Suzuki et al.
At the warm-core ring station (Stn 18), concentrations
of peridinin (a dinoflagellate marker; Johansen et al., 1974),
19′-hexanoyloxyfucoxanthin (a prymnesiophyte marker;
Wright and Jeffrey, 1987), and 19′-butanoyloxyfucoxanthin
(a pelagophyte marker; Andersen et al., 1993) were relatively high. In addition, the contribution of peridinin-containing dinoflagellates to the chlorophyll biomass above 80
m was 29% (Table 3). These data indicated that flagellates
were predominant at this station.
At Stn 23 situated in the mixing region of the Kuroshio
Extension and the Oyashio water, chlorophyll a concentrations in surface waters were higher than that at the warmstreamer and Kuroshio warm-core ring stations, but lower
than concentrations measured at stations in the Oyashio
water. Peridinin was the most abundant carotenoid and the
contribution of peridinin-containing dinoflagellates to the
chlorophyll biomass above 80 m was 57% at Stn 23. This
indicated that dinoflagellates dominated this phytoplankton
community.
3.3 Variability in chlorophyll-specific absorption coefficients
Chlorophyll-specific absorption coefficient at 440 nm,
a*ph(440), were usually high (mean ± standard dev. =
0.082 ± 0.022 m2mg–1) above 40 m layer at the warmstreamer stations (Fig. 4). These stations showed low fluorometric concentrations (0.615 ± 0.102 mg m–3) of chlorophyll a above 40 m. On the other hand, a*ph(440) values above
30 m at the Oyashio stations (Stns 14 and 21) were relatively
low (0.019 ± 0.009 m2mg–1) while the chlorophyll a concentrations above 30 m were high (5.78 ± 3.12 mg m–3). At
the warm-core ring station (Stn 18) and the KuroshioOyashio station (Stn 23), mean a*ph(440) values above 30 m
were 0.040 ± 0.003 and 0.054 ± 0.011, respectively, and the
mean chlorophyll a concentrations above 30 m were 1.12 ±
0.139 and 0.979 ± 0.318 mg m–3, respectively. The a*ph(440)
as well as chlorophyll a values at both these stations were
located between those measured at the warm-streamer and
the Oyashio stations. Such an inverse relationship between
a*ph(440) value and chlorophyll a concentration has also
been observed in previous studies (e.g. Bricaud Stramski,
Fig. 4. Spectra of chlorophyll-specific absorption coefficient of phytoplankton at each station.
Chlorophyll-Specific Absorption Coefficients and Pigments of Phytoplankton
523
1990), and is generally considered to be caused for two main
reasons: (1) an increasing package effect from oligotrophic
(low chlorophyll a concentration) to eutrophic (high chlorophyll a concentration) waters (e.g. Yentsch and Phinney,
1989) and (2) a possible inverse co-variation between the
relative abundance of accessory pigments and the chlorophyll a concentration level (i.e. the significant contribution
of non-photosynthetic pigments to a*ph) (e.g. Allali et al.,
1997).
It is known that the package effect increases when
either the average cell size or the absorption coefficient of
the cellular material increases, with the result of depressing
absorption at all wavelengths and flattening of the spectrum
(Morel and Bricaud, 1981). In this sense, at the Oyashio
stations, fucoxanthin-containing diatoms, which possibly
have large cell size, may cause an increase in the package
effect, i.e. decrease of the a*ph(440) value. In order to
quantify the package effect on the absorption samples of this
study, the values of the “package effect index” (Morel and
Bricaud, 1981), Q*a, were calculated. For a given sample
and a given wavelength, this index is defined as
Q*a = a*ph/a*sol
Fig. 5. Variations of the package effect index, Q*a(675), as a
function of chlorophyll a concentration determined by
fluoremetry at the warm-streamer stations (3, 5, 6, 7, and 22),
the Oyashio stations (14 and 21), the warm-core ring station
(18), and the Kuroshio-Oyashio station (23).
where a*sol is the specific absorption coefficient of the same
cellular matter ideally dispersed in a dissolved state. Q*a
varies from 1 (no package effect) to 0 (maximal package
effect). A minimal influence of accessory pigments is to be
expected with the red absorption band of in vivo chlorophyll
a (675 nm), except for the possible presence of chlorophyll
b and phaeopigments. Therefore, the enhancement due to
the presence of chlorophyll b by measuring the height of the
band above a baseline joining a*ph(660) to a*ph(700), was
corrected. However, the interference of phaeopigments could
not be removed, which probably leads to an overestimation
of the index value of the samples at the Oyashio stations. The
a*sol(675) coefficient of chlorophyll a was taken as 0.0206
Fig. 6. Examples of absorption spectra of phytoplankton normalized at 440 nm at each depth of the selected stations.
524
K. Suzuki et al.
(4)
m2mg–1 (Bricaud et al., 1983). The values of Q*a(675) with
fluorometrically determined chlorophyll a concentration at
the warm-streamer stations were higher than those at the
other stations (Fig. 5). This indicated that the package effect
at the warm-streamer stations was smaller than at other
stations, and this leads to a higher a*ph(440) at the warmstreamer stations. However, a large scatter was observed in
Q*a(675) at the warm-streamer stations and some values
exceeded the theoretical upper limit of 1. Bricaud et al. (1995)
suggested that this may be partly due to the uncertainty in the
pathlength amplification factor. This uncertainty increases
with decreasing optical density (Bricaud and Stramski,
1990).
The presence of non-photosynthetic pigments leads to
increasing a*ph(440) without a concomitant increase in the
amount of carbon fixed. As their abundance with respect to
photosynthetic pigments is variable, they could be responsible for part of the variability observed in natural waters for
the maximum quantum yield for carbon fixation (Babin et
al., 1996). Babin et al. (1996) and Allali et al. (1997) found
that non-photosynthetic pigments (especially zeaxanthin)
contributed substantially to a*ph(440) in oligotrophic waters. However, the contributions of non-photosynthetic
pigments (diadinoxanthin, neoxanthin, zeaxanthin/ lutein,
and β-carotene) to a*ph(440) in this study were certainly
minor, because the concentrations of these non-photosynthetic pigments were very low compared to the total amount
of other (photosynthetic) pigments at all the stations (Table
2). Therefore, the site-to-site variations of a*ph(440) in this
study were considered to be mostly due to the extent of the
package effect.
The variation in the magnitude of a*ph(λ) are accompanied by changes in the spectral shape. Hence variations in
the shape of the specific absorption spectrum of phytoplankton were analyzed after normalization of a*ph(440) (Fig.
6). The spectral shapes at each depth of the same station
were almost similar. At the warm-streamer stations (e.g. Stn
7), marked absorption peaks around 470 nm and 645 nm
were observed. This feature can be ascribed to the high ratio
(ca. 0.4) of chlorophyll b to chlorophyll a (Table 2). At the
Oyashio stations (e.g. Stn 14), the peak position shifted
toward shorter wavelengths as compared with observations
at the other stations. This may be due to the relatively high
concentrations of phaeopigments, whose blue maximum
peak is located at shorter wavelengths than that of chlorophyll a (Bricaud et al., 1995), at the stations. Otherwise
ad(λ) value obtained at the Oyashio stations may be underestimated, and might not be removed sufficiently from ap(λ)
value for estimating a*ph(λ) at the Oyashio stations. The
ad(λ) spectrum showed a consistent increase towards shorter
wavelengths in all the samples of this study (data not shown,
but see Kishino et al., 1985). The spectral shape observed
was similar at the warm-core ring station (Stn 18) and the
station located in the Kuroshio-Oyashio mixing region (Stn
23). This is likely due to the similar pigment composition at
the two stations. These data strongly suggest that major
differences in phytoplankton absorption spectra of each
water mass were a result of differences in the phytoplankton
pigment composition, which was probably related to the
phytoplankton community, of each water mass.
4. Conclusion
We examined the variety in shape and magnitude of the
in vivo chlorophyll-specific absorption spectra of phytoplankton in relation to differences in pigment composition
in the northwestern North Pacific (off Sanriku). Based on
our findings we believe that the absorption and pigments of
phytoplankton are characteristic of each water mass of the
study area. This result strongly support the idea that world
oceans may be portioned into various provinces based on
regional absorption characteristics, and such features could
be used to improve remote-sensing algorithms for each
province (Hoepffner and Sathyendranath, 1992; Lutz et al.,
1996).
Acknowledgements
We thank the captain and crew of the R/V Tansei-Maru
for their support during the KT-97-5 cruise. We also thank
National Space Development Agency of Japan for supporting
the ADEOS Field Campaign off Sanriku, organized by
Japan Marine Science Foundation. Drs. H. R. Gomes, J. I.
Goes and three anonymous reviewers are acknowledged for
helpful comments and suggestions on the manuscript. We
are grateful to Dr. S. Shang for help in preparing the map in
this manuscript. This study was partly supported by a grandin-aid for encouragement of young scientists to K. Suzuki
(09780492) of the Ministry of Education, Science, Sports
and Culture, Japan.
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