春季ブルームから夏季にかけて北方沿岸水域においてサイズ分画したクロロフィルα濃度及びそれらの細胞密度から見

積った植物プランクトン炭素の現存量

誌名誌名 日本プランクトン学会報

ISSNISSN 03878961

巻/号巻/号 392

掲載ページ掲載ページ p. 127-144

発行年月発行年月 1993年2月

農林水産省 農林水産技術会議事務局筑波産学連携支援センターTsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research CouncilSecretariat

Bulletin of Plankton Society of Japan Vol. 39 No. 2, pp. 127-144. 1993

Phytoplankton Carbon Biomass Estimated from the Size-Fractionated Chlorophyll a Concentration and Cell density in the Northern Coastal Waters from

Spring Bloom to Summer1),z),

3)

TsuNEo 0DATE4i,si, MrTsURu YANADA4l,

HrROYUKI MrzuT A 4l AND Y OSHIAKI MAITA

4>

Research Institute of North Pacific Fisheries,

Faculty of Fisheries, Hokkaido University,

3-1-1 Minato-cha, Hakodate 041

Abstract

Phytoplankton carbon biomass (PC) was estimated from size-fractionated concentration of chlorophyll a (CHL) and cell density in Funka Bay, Hokkaido, from February to August 1989, using the conversion equations from cellular CHL content to cell volume, cell volume to cellular PC, and the cellular PC conversion factors taken from literatures. The temporal variations of phyto­plankton biomass and size composition based on the measured CHL were compared those based on the estimated PC, and then discrepancies arising from differences of size composition and of cyanobacterial abundance in estimating PC from CHL data were recognized. The phytoplankton biomass in terms of CHL was predominated by the > 10 µm size fraction during a spring phytoplankton bloom and by the 2-0. 2 µm size fraction after the bloom. Cell density of cyanobacteria largely increased in the thermally stratified water column from June to July and reached the maximum in August. The total CHL during the spring bloom was 37-fold higher than after the spring bloom. The former total PC was 61-fold higher than the latter one. After the bloom the abundance of the 2-0. 2 µm size fraction was 61 % of the total CHL while it was 35 % in the PC based comparison. On the other hand, the total CHL in summer was 80 % of that observed after the bloom. If the comparison was conducted based on the PC, the abundance of total phytoplankton in summer was 25-fold larger than that o bservered after the bloom. The predominance of the 2-0. 2 µm size fraction based on PC in summer was further emphasized (98 %) compared to 60 % based on the CHL. It is concluded that high occurrence of cyanobacteria

1) Accepted 14 December 1992 2) w:$f!v-1'7'.PG~:$v::::ipV:f-Cil}J/B~7JcW!U::::;f;l•-C-tt17-:')}[!!l]Lt~::7oo711va1ll!t:&.U.:Z:-hGO)r,ffi)J?l

Wi!tiP G JVfl1t-:i t~W>1m f 7 :,; ::7 ~ :,; ,l:R*O)ffl.t'fI 3) Contribution No .260 from the Research Institute of North Pacific Fisheries, Faculty of Fisheries,

Hokkaido University 4) 1hitl[4:: · YlEE3 fililJ • 7.KEE3ffi:Z • *EB~~B il#ii~::k+7.Kmf+ttG 1U$7.Kmft!Jf9'EQ01:§'it 5) ffl.tE : ~li::k+f:t~$tlJJJ(+ffG

Present address : Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu, Mie 514

128

with high PC:CHL creates a larger discrepancy than a dominance of small-sized eukaryotic cells with low PC: CHL when PC is estimated from CHL in this bay.

Keywords: phytoplankton, size-fractionation, carbon, chlorophyll a, cell density, Funk a Bay

There is no convenient method to directly measure phytoplankton carbon (PC) with field samples, because of difficulty in separating phytoplankton from detritus, bacteria, and microzooplankton (BANSE 1977, EPPLEY et al. 1977, GEIDER 1987). One of the most frequently employed techniques in assessing phytoplankton abundance is an evaluation of cell pigment, usually chlorophyll a (CHL) (CULLEN 1982, GmsKES 1991), since this is universally present in marine algae (RAYMONT 1980). The merit of the CHL measurement is that all phytoplankton in a sample are dealt with rapidly and easily. In most cases, the CHL can be determined regardless of phytoplankton cell size or taxon.

The CHL values are often used to determine phytoplankton carbon (PC) biomass assuming a fixed carbon to chlorophyll a ratio (PC: CHL), although a wide variation of PC: CHL is known, depending upon nutrient availability (ANTIA et al. 1963, BANNISTER & LAWS 1980, CAPERON & MEYER 1972, EPPLEY 1968' Lr et al. 1992' RIEMANN et al. 1989' STRICKLAND 1960)' light and temperature conditions ( GEIDER 1987) , relative growth rate (GOLDMAN 1980, LAWS et al. 1983), species (CHAN 1980, GmsKES 1991), and size (MALONE 1980) . Several different approaches have been adopted to estimate PC: CHL in nature (EPPLEY 1968, EPPLEY et al. 1977, LORENZEN 1968, REDALJE & LAWS 1981, WELSCHMEYER & LORENZEN 1985) . EPPLEY et al. (1977) have regarded a comparison of a chemically determined CHL with PC from microscopic observation as the most reliable. This approach, however, had involved an uncertainty of completely counting of small sized cells ( GEIDER 1987) .

In temperate and subarctic neritic waters, measurements of CHL with size-fractionation have revealed that phytoplankton abundance is predominated by a large size algae, mostly diatoms, during a spring bloom of phytoplankton, while a dominant size class of the summer phytoplankton community is less than several micrometer (CAMPBELL & CARPENTER 1987, DlJRBIN et al. 1975, LARSSON & HAGSTROM 1982, MAITA & 0DATE 1988, MALONE 1980, SHAPIRO & HAUGEN 1988). Recent observations conducted with an epifluorescence microscope show that coccoid cyanobacteria are dominant in the summer phytoplankton community (CAMPBELL & CARPENTER 1987' JOINT et al. 1986' MrYAZONO et al. 1992, 0DATE 1989, SHAPIRO & HAUGEN 1988, WATERBURY et al. 1986).

Despite of many problems in assessing phytoplankton biomass from CHL, the following two aspects will be focused in this study. 1) PC:CHL of smaller cell-sized eukaryotic phytoplankters is generally lower than larger ones (FURUYA 1990). 2) Cyanobacteria have high PC: CHL, although their cell size is small (FURUYA 1990) . Dominance of small-sized eukaryotic cells or abundant cyanobacteria suggest to apply different PC: CHL ratios to total phytoplankton community.

If a concentration of CHL and a cell density of phytoplankton in a field

0DATE et al.: Phytoplankton Carbon and Chlorophyll a 129

sample are known, an approximate of PC can be calculated using the conversion equations between CHL per cell and cell volume (FURUYA 1990, MALONE 1980) and between cell volume and cellular PC (STRATHMANN 1967) . In practice, the size-fractionated CHL and cell density would provide the accurate estimation of PC for small-sized cells of phytoplankton community. Moreover we have to know abundances of cyano bacteria and other picophyto­plankton separately, since they have a distinctively different PC: CHL (FUR UY A 1990). One of the simple way to do so is a direct count of the cell using an epifluorescence microscope (MURPHY & HAUGEN 1985, 0DATE 1989, SHAPIRO & HAUGEN 1988' w ATERBURY et al. 1979' w ATERBURY et al. 1986) .

In this paper we estimate more accurate PC from size-fractionated concentration of CHL and cell density of phytoplankton than one from a bulk PC: CHL in the temperate neritic water of Funka Bay, from spring to summer, during which considerable variations of size and species compositions of phytoplankton community have been reported (MAITA & 0DATE 1988, 0DATE 1989). The temporal variations of phytoplankton abundance and size composition based on the measured CHL will be compared with those based on the estimated PC from the size-fractionated CHL and cell density. And then, discrepancy arising from the size-dependent PC: CHL and abundance of cyanobacteria will be evaluated.

Materials and Methods

30'

42°N

'--"'-__ __.__ ____ ....._ __ ____, 30' 141° E 130°E 140°E

Fig. 1 . Location of the sampling station in Funk a Bay.

Sample Collection Seawater was collected on the T /S Ushio Maru using a Van Dorn sampler from six depths; 0, 10, 30, 50, 70, and 90 m, at the central station of Funka Bay (92 m depth, Figure 1) from February 7 to August 22, 1989, during which the station was occupied twelve times. The seawater samples were kept in dark until the following processes. Although water temperature

130

and salinity were profiled using a CTD between the surface (0 m) and the near bottom layer (85-90 m), the data from 0, 5, 10, 20, 30, 40, 50, 60, 80 m, and near bottom layer were chosen to illustrate temporal changes of the water column properties .

Chlorophyll a Measurements Aliquot of the seawater sample (100-1, 000 ml) was sequentially filtered through a Nuclepore filter (diameter, 47 mm) with 10 µm pore size, and through filters of 2 and 0. 2 µm pore size. Another aliquot of the water sample (100-200 ml) was directly filtered onto a Whatman GF/F glass fiber filter (diameter, 25 mm) for a bulk measurement so that all cells are quantitatively recovered through the sequential process of size-fractionation. A vacuum pressure was less than 100 mm Hg to collect all cells without damage (Lr 1986, TAGUCHI & LAWS 1988). After the filtration the filters were immediately placed in opaque glass vials containing 10 mL of 90 % acetone and stored for at least 24 h duration at -20 °C in a freezer. Concentration of chlorophyll a (CHL) was determined by the fluorometry (PARSONS et al. 1984).

Cell Caunting Following the same procedure as CHL, the seawater sample (10-100 ml) was size-fractionated into three size groups using Nuclepore filters (diameter 25 mm) in the laboratory. Phytoplankters collected onto the filters were enumerated within ten minutes after the every filtration using an epifluorescence microscope (blue excitation), by the methods of 0DATE (1989).

Yellow (phycoerythrin) fluorescing cells both in the > 10 µm and the 10-2 µm size fractions were not included in the following analyses, since their cell size were less than 2 µm. Although chlorophyll a concentration in these size fractions also included the yellow fluorescing cells, their contribution was probably small since the cell densities of yellow fluorescing cells were usually more than one order of magnitude less than that of red (chlorophyll) fluorescing cell. The red fluorescing cells both in the > 10 µm and the 10-2 µm size fractions are thought to be eukaryotes, considering the relatively large cell size (WATERBURY et al. 1986) . In the 2-0. 2 µm size fraction, yellow fluorescing cells and red fluorescing cells were commonly observed. The red fluorescing cells may include both small eukaryotes and prokaryotes (prochlorophytes) (CHISHOLM et al. 1988) . Since discrimination between these two groups could not be conducted in the present study, we called them as other picophytoplankton.

Phytoplankton Carbon Estimation Phytoplankton carbon (PC) was estimated from following processes for the > 10 µm and the 10-2 µm size fractions. Firstly, the mean CHL per cell was obtained from the size-fractionated CHL and the cell density in the respective size fractions, and then cell volume was calculated using the conversion equation. The relationship between cellular CHL and cell volume established by FURUYA (1990) is rewritten for the purpose of the present study as,

0DATE et al.: Phytoplankton Carbon and Chlorophyll a 131

Log (cell volume) = 2.04 + 1.37 Log (cellular CHL) (1). Next the cellular PC was calculated using the conversion equations derived from STRATHMANN (1967) [equations (2) and (3)],

Log (cellular PC) = -0.422 + 0.758 Log (cell volume) (2) Log (cellular PC) = -0.460 + 0.866 Log (cell volume) (3).

And the PC in the > 10 µm and the 10-2 µm size fractions were subtotaled by multiplying the cell density in the respective size fraction. We assumed that the dominant taxa in the respective size fractions were diatoms [equation (2)] and algae other than diatoms [equation (3)] (DURBIN et al. 1075, MAITA & 0DATE 1988, MA.LONE 1980, SHAPIRO & HAUGEN 1988).

For cyano bacteria and other picophytoplankton in the 2-0. 2 µm size fraction, we used the factors of 250 fgC · cell- 1 (KANA & GLIBERT 1987) and 59 fgC ·cell- 1 (LI et al. 1992) , respectively.

Feb

1 0 •

g 30 •

1 0 •

g 30 •

.s::. g 50 c

70

9 0 Salinity

o.--__.-.,..._.___._....----~...-~ ......... -.---_,...._,..__.'rr"~,.._, 1 0 •

g 30 •

70 • . . . . . Slgma-t ---~ • •

90.._~_._...;.._~_._~~..__~__,_~~_.._~~..._;.____,

Fig. 2. Vertical profiles of water temperature, salinity, and sigma-t from February 7 to August 22, 1989.

132

Results Ocearwgraphic Structure Surface water temperature increased from April 12, and exceeded 20 °C on August 22 (Figure 2) . The Oyashio Water ( < 33. 0 practical salinity unit) (OHTANI & Kmo 1980) was observed in the upper 30 m on March 23. Thereafter, salinity in the surface water gradually decreased and formed the Summer Funka Bay Water, which is characterized by high water temperature and low salinity resulted from snow melting water (OHTANI & Krno 1980). The Oyashio Water reached at depth of ca. 70 m on July 3. Saline water ( > 33. 25) was observed at depth of 50 m on August 9, suggesting an intrusion of the Tsugaru Warm Water (OHTANI & Krno 1980). From May to August, the water column was stratified. In particular, strong pycnocline was developed in the upper 30 m in August. These changes in the physical properties are consistent with earlier studies (e. g. 0HTANI & Kmo 1980).

Distribution of Chlorophyll a Concentration Vertical distributions of bulk and size-fractionated concentrations of chloro­phyll a ( CHL) are shown in Figure 3. In the upper 50 m of the water column, the bulk CHL was high (>lOµg·l- 1

) on February 23 and March 8, indicating an out burst of a spring phytoplankton bloom. On March 23 and April 12, the bulk CHL decreased to < 5 µg · 1- 1 in the upper 10 m, although the concentration remained high at depth of 30 m. From May 9 to August 22, the bulk CHL was between 0. 3 and 2. 5 µg · 1- 1 in the upper 30 m, and was less than 0. 3 µg · 1- 1 at depths below 50 m.

In the > 10 µm size fraction, the concentrations higher than 10µg·1- 1

occurred in the upper 50 m during the spring bloom and at 30 m depth on April 12, as observed in the bulk CHL (Figure 3). The concentrations

Feb Mar Apr May June July Aug

1 0 •

.c ~ 50 • 0

70

I I fl t1i',"" ... ,

10 ·: :·:~·f\'.·\ \. : : ~o.s:\ \0~1 f I \ I \ \

i 3 0 ·: : • ""-;"' .} ~ : • - : : 0.3'',/,/ .1=. I l / ,' ,'

~ 50 .\ \ • • /li c \ ....... __ .... : :

...... ' ... , : : 7 0 • \ • :\. : :.

0.1 I I I \ I : ~

9 Q \ \ ~I I

. \ . . \~~~·~~~)· ·.:· 0.1

\ .... -----~... : . ',,·,,, . t'• . ' ... !_ .... '.

\ : • ' ... ! .. ./ •

. .• .. 10-2µm 2-0.2 µm

Fig. 3. Vertical profiles of bulk chlorophyll a and size-fractionated chlorophyll a concentrations ( µg · 1- 1

) from February 7 to August 22, 1989.

.s::. g. 5 0 c

0DATE et al.: Phytoplankton Carbon and Chlorophyll a

Eukaryotes (>10µm) Eukaryotes (10-2µm)

Cyanobacteria Other picophytoplankton

Fig. 4. Vertical profiles of cell densities (cells·l- 1) of eukaryotes in

the > 10 µm and the 10-2 µm size fraction, cyanobacteria, and other picophytoplankton in the 2-0. 2 µm size fraction from February 7 to August 22, 1989.

133

\. • H •

'5X10~-

higher than 0.5 µg· l- 1 were also observed in the upper 10 m on June 19 and July 3 and in the upper 30 m on July 28. The temporal and vertical distribution of CHL in this size fraction was very similar to that of the bulk CHL, implying that large variations in CHL of the total phytoplankton community are caused by the > 10 µm size fraction.

The CHL in the 10-2 µm and the 2-0. 2 µm size fraction did not exceed 1. 0 µg · l- 1 throughout the survey period (Figure 3). The relatively high concentrations ( >O. 3 µg · l- 1

) of the 10-2 µm size fraction were observed in the upper 50 m during the spring bloom, and at the surface during July.

On the other hand, the CHL in the 2-0. 2 µm size fraction exceeded 0. 3 µg · l- 1 in the upper 10 m in February and in the upper 30 m on March 8. In this size fraction, the CHL higher than 0. 3 µg · l- 1 also occurred in the upper 30 m from May 29 to June 19 and in the upper 10 m from July 3 to August 22.

Distribution of Phytoplankton Cell densities of the phytoplankton populations in the > 10 µm, the 10-2 µm, and the 2-0. 2 µm size fractions are shown in Figure 4. These cell densities varied largely in the upper 30 m of the water column. The cell density of eukaryotes in the > 10 µm size fractions was high in the upper 50 m from February 23 to April 12 (2-8 x 106 cells· 1- 1

) • The high density was also noted in the upper 30 m on July 28 (2 x 106 cells· l- 1

). The low cell density occurred between the surface and depth of 70 m of the water column on May 9 (3-9Xl0 4 cells·l- 1

) and during August (0.8-7Xl0 4 cells·l- 1).

The cell density of eukaryotes in the 10-2 µm size fraction varied

134

between 7 x 10 4 and 8 x 10 6 cells· 1- 1 throughout the water column. The cell density was high in the upper 50 m from February 7 to March 8 (1-2 x 10 6 cells · 1- 1

). In the surface layer, the cell density decreased to 3 x 10 5 cells· 1- 1 on March 23 when the CHL in this size fraction was low (Figure 3) . After then it exceeded 10 6 cells · 1- 1 in the surface layer on May 9 and reached the maximum on August 22 at depth of 10 m (8 x 10 6 cells· 1- 1

), although the CHL in this size fraction was not high (Figure 3). Cyanobacteria declined from February 23 (1x10 6 cells· 1- 1

) to April 12 (7-9 x 10 4 cells · 1- 1

) (Figure 4) in the upper 10 m. The cell density in the upper 10 m sharply increased by more than two orders of magnitude from June 19 (6-9 x 10 5 cells· 1- 1

) to July 28 (2x10 8 cells· 1- 1), although the CHL

in the 2-0. 2 µm size fraction did not increase so largely (Figure 3) . The maximum density of cyanobacteria was observed at depth of 10 m on August 22 ( 4 x 10 8 cells · 1- 1

) •

As same as cyano bacteria, the cell density of other picophytoplankton decreased from February to March, and increased in the upper 30 m from April to May (Figure 4), during which the CHL in the 2-0. 2 µm size fraction also increased (Figure 3). After then the contour line of 5 x 10 6

cells · 1- 1 of other picophytoplankton almost corresponded to that of 0. 1 µg · 1- 1 of the CHL in this size fraction. The CHL in this size fraction was higher than 0. 5 µg · 1- 1 at depth of 30 m on May 29 and of 10 m on July 3,

"' ~ 10

2

.c c. e 0 :;: 0

' •• 151 0

0 •11 I· A ..... 151

II 1'iafl =·/ 11 .. A .. Eukaryotes (>10µm)

151 11 II

102 cc • •

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e A

A

10 1

• Eukaryotes (10-2µm)

10 1 .............. ~....._._.._...................,,_....__................._.........,__._~ .............

1 O 1 10 2 1 0 3 10 4 1 0 2 1 0 3 1 0

4 e Feb. 7

"7'10 3 .---.-........... .....,-..,.....,...,..........,.-r-,......,.,.....,..,-.-.-......., .......... .......,.,,.10 3 ........... ,.,.,.,.,..---.--.-.-.........,r--"--.--...,....,.....--...--.-..,-,.....,,...,-~ ...J

E Cl ~ c 0

~ 1 o2

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e fl8 90 IB~ .. ,<;J ~ •

: II r:l •

.. ,

• 151 Arl'o•

d c ,. c •

0 .......

I

Other picophytoplankton

103 104

Cell density (cells mL ' 1)

o Feb. 23

• Mar. 8

c Mar. 23

• Apr. 12

o May 9

• May 29

A June 19

• July 3

151 July 28

11 Aug. 9

• Aug. 22

Fig. 5. Relationships between cell density and chlorophyll a concentration for each size fraction.

0DATE et al.: Phytoplankton Carbon and Chlorophyll a 135

where the cell density of other picophytoplankton was high ( > 5 x 107 cells · 1 -1).

Relationship between Cell Density aml Chlorophyll a Concentration Figure 5 shows the relationships between the cell density and the CHL in each size fraction. In the > 10 µm size fraction, the CHL was significantly correlated with the eukaryotes cell density (r = 0. 83, p < 0. 001, n = 72) . Some CHL data collected on February 7 (four out of six), 23 (all out of six), on March 8 (one out of six), and on July 3 (two out of six) were about one order of magnitude higher than the others of which cell density was the same, implying that CHL content per cell was high and/ or that cell size was relatively large in the > 10 µm size fraction.

""" '....J 0)

3

C1I

>. .c a. 0 0 :c (.)

'....J

0

20

1 5

1 0

5

0

250

t3 > 1 O~t m D 10-2pm

[Il 2-0.2~1m

a

b

3 200 ~ >10~1m (eukaryotes)

D 10-2µm (eukaryotes) t:: 0 ..0 cu t)

t:: 0 ~ c: C1I a. 0 >. .c a.

...J :c (.)

0 a.

150

100

50

0 ·250

200

150

100

50

c

D 2-0.2pm (other picophytoplankton)

• 2-0.2~1m (cyanobacteria)

ol.......m--~--L.....m---------------------~ Feb Mar Apr May June July Aug

Fig. 6. Temporal changes in size-fractionated chlorophyll a concentra­tion (a), estimated phytoplankton carbon (b), and phytoplankton carbon to chlorophyll a ratio (c). Data are the averages within the water column of 0-30 m.

136

The relationships between the cell density of eukaryotes and the CHL in the 10-2 µm size fraction (r= 0. 21, n = 72) and between the cell density of cyanobacteria and the CHL in the 2-0.2µm size fraction (r=0.22, n=72) were not significant (p>0.05). The former relationship on each day were, however, significant (r=0.82-0.97, p<0.05 or <0.01, n=6) except for on April 12, July 28, and August 22 (r=0.25-0.55, p>0.05 n=6). This may indicate that the CHL were also associated with the cell density of eukaryotes, althongh there were large temporal fluctuations of CHL content per cell and/ or cell size within the 10-2 pm size fraction. A significant relationship was observed between the cell density of other picophytoplankton and the CHL in the 2-0.2µm size fraction (r=0.66, p<0.001 n=72).

Phytoplankton Carbon Biomass Phytoplankton cell carbon (PC) was estimated from the size-fractionated samples. The size-fractionation by using membrane filters is the one of the most convenient technique in the biological oceanography (HILMER & BATE 1989). There is, however, always uncertainty in the validity of this tech­nique. One of validation is to compare the sum of three size fractions with the total CHL by the bulk measurement with a single filter. They are significantly related (r=0.99, p<0.001 n=72) although the slope of the regression line (1.044) was significantly defferent from 1.0 (p<0.01) while the intercept of the line (-0. 088 µg · 1- 1

) was not significantly different from the origin (p>0.05). The maximum defference between two methods was 13 % between 0. 5 and 17. 0 µg · 1- 1 of the bulk CHL. Therefore all data shallower than 30 m were employed for the estimation of cell carbon.

Temporal changes of the size-fractionated CHL, the estimated PC, and the calculated PC: CHL are shown in Figure 6a, b and c, respectively. The total CHL by the sum of three size fractions was high from February 23 to April 12, i.e. , a spring phytoplankton bloom (Figure 6a) , during which more than 90 % of the sum was accounted for by the > 10 µm size fraction. During the period of phytoplankton bloom, almost the same trend as the CHL was detected by the PC (Figure 6b), and then the temporal variation of PC:CHL was small (13-14) (Figure 6c). Dominant size class in phytoplankton carbon in this period was also the > 10 µm size fraction, as shown by the CHL.

In the thermally stratified water column observed from May 9 to August 22 (Figure 2) the sum of CHL in three size fractions varied between 2 and 10 % of the highest CHL observed during the period of phytoplankton bloom (on March 8) (Figure 6a). The lowest PC:CHL of 8 was calculated on May 9 (Figure 6c) , when the 2-0. 2 µm size fraction predominated the CHL and the cell density of cyanobacteria was not high (Figure 4). Almost the same CHL concentrations were detected between in May and August, although the cell densities of cyanobacteria were quite different each other on those days (Figure 4). In the stratified water column the total PC was 2 to 40 % of the highest PC on March 8. The PC from July 28 to August 22, during which cyano bacteria were abundant, were not so low as detected by the CHL (Figure 6b) . Hence, the higher PC: CHL than 40 were calculated

0DATE et al.: Phytoplankton Carbon and Chlorophyll a 137

(44-245) during this period.

Discussion It is well documented that considerable seasonal variation of phytoplankton community is observed in temperate to high latitude neritic water, and that diatoms predominate the community during spring phytoplankton bloom period (MALONE 1980, RAYMONT 1980). Seasonal variations are also observed in abundance and size composition of chlorophyll a ( CHL) in these waters (CAMPBELL & CARPENTER 1987' DURBIN et al. 1975' LARSSON & HAGSTROM 1982, MAITA & 0DATE 1988, MALONE 1980). The present results further confirm that large variations of the total CHL are caused by the larger size fraction as same as these previous studies (see also CHISHOLM 1992) . The predominance of the larger size phytoplankters has been well documented in relate to nutrient regime (FURUYA & MARUMO 1983, MAITA & 0DATE 1988, MALONE 1980' p ARSONS & TAKAHASHI 1973' TAGUCHI et al. 1992) .

Recent studies show the temperature related distribution of cyanobacteria, which are one of a dominant group in picophytoplankton community (EL HAG & FOGG 1986, JOINT et al. 1986, MURPHY & HAUGEN 1985, 0DATE 1989, WATERBURY et al. 1986). In this study, cell density of cyanobacteria increased sharply by more than two orders of magnitude from June 19 to July 28. From July 28 to August 22, cyanobacteria numerically surpassed other picophytoplankton, being consistent with the observations of 0DATE (1989).

We used the conversion equations (FURUYA 1990, STRATHMANN 1967) and the conversion factors (KANA & GLIBERT 1987, Lr et al. 1992) to estimate phytoplankton carbon (PC) biomass. The estimated PC varied between 3. 3 and 202 µg C · 1- 1

, which is less than usually reported values of particulate organic carbon in this bay ( 100-300 µg C · 1- 1

) (MAITA & YAN ADA 1978, YA­N ADA & MAITA 1978). Even during the spring bloom the high estimates of PC have never exceeded the amount of particulate organic carbon.

Morever, the PC: CHL calculated in this study were between 8-245, which is similar to the previously reported range of PC: CHL. For instance HOBSON et al. (1973) estimated PC from microscopic observations and obtained PC: CHL ranging from 10 to 145. Almost the same range of PC: CHL is obtained by the similar method (Lr et al. 1992, RIEMANN et al. 1989). Analyzing linear regression between particulate organic carbon and CHL, PC: CHL ranging from 20 to 300 is obtained (EPPLEY et al. 1977, LOREN­ZEN 1968, STEELE & BAIRD 1962). A wide range of PC: CHL (7-500) is

TABLE 1. EXAMPLES OF SIZE-FRACTIONATED CHLOROPHYLL a CONCENTRATION ( µg • 1- 1

) AND PHYTOPLANKTON CARBON ( µgC • 1- 1) • THE RELATIVE ABUNDANCE

OF MARCH 8 AND AUGUST 22 TO MAY 9 ARE SHOWN IN PARENTHESES.

Chlorophyll a Carbon Factor Sum of three Percent composition Sum of three Percent composition of

Date size fractions > 10 µm 10-2 µm 2-0. 2 µm size fractions > 10 µm 10-2 µm 2-0.2 µm difference (A) (B) (B/A)

March 8 15.05(36.7) 93.7 3.2 3.1 202 .1(61.2) 95.5 3.9 0.3 1. 7

May 9 0.41( 1.0) 18.6 20.5 60.9 3. 3( 1.0) 30.6 34.5 35.0

August 22 0.33( 0.8) 19.3 20.9 59.8 81.5(24. 7) 1.1 0.7 98.2 30.9

138

calculated from direct measurements of PC and CHL using cultures of phytoplankton (CAPERON & MEYER 1972' EPPLEY & RENGER 197 4' GIESKES 1991, HUNTER & LAWS 1981, RIEMANN et al. 1989). Using 14C labeling technique, PC: CHL is obtained between 41 and 272 (REDALJE & LA ws 1981, WELSCHMEYER & LORENZEN 1984). From the pigment budget model, PC:CHL ranging from 10 to 150 is estimated (WELSCHMEYER & LORENZEN 1985) . These previously obtained results support that the present estimation of PC: CHL may be appropriate.

Our results show that relationships between phytoplankton cell density and CHL in the > 10 µm and the 2-0. 2 µm size fraction are significant, although samples were collected from different seasons. In the 10-2 µm size fraction the CHL significantly correlated with the cell density on each day except for on April 12, July 28, and August 22. These results do not discourage usage of CHL as an indicater of phytoplankton abundance. When we compare phytoplankton a bun dances collected from temporally and regionally different waters, there are some problems.

First, we have to take care the size composition of phytoplankton communities when we compare the phytoplankton abundance with CHL, since PC: CHL of eukaryotes varies with cell size (FURUYA 1990). In this study temporal fluctuation of the total PC was almost the same as that of total CHL during the phytoplankton bloom period (February 7 to April 12) , since the size class in phytoplankton comm unities was dominated by the> 10 µm size fraction.

Considerable change in the size composition was observed between bloom and non-bloom periods (May 9-29). The CHL between February 7 and April 12 was predominated by the > 10 µm size fraction, while that in May was predominated by the 2-0. 2 µm size fraction. Since the cell densities of cyano bacteria were less than 106 cells · 1- 1 both on March 8 and May 9, the effect arising from the difference of size composition can be evaluated by comparisons between CHL and PC on those days (Table 1) . The total CHL on March 8 was 37-fold higher than that on May 9. On the other hand, the total PC on the former day was 61-fold larger than that on the latter day. This elevated factor in carbon base results from the size dependence of PC: CHL, since we used the conversion equations which imply that larger cells have higher PC: CHL (FURUYA 1990, STRATHMANN 1967) . These calculations have revealed that the defference between the phytoplankton abundance based on CHL and based on PC may be factor of less than two within the observed change of size composition in Funka Bay.

Abundance of smaller eukaryotes would be more emphasized in terms of CHL than in terms of PC, since PC: CHL of them is generally smaller than that of larger eukaryotes (FURUYA 1990) . In the data collected on May 9 the percent contribution of the 2-0.2 µm size fraction based on CHL decreases in PC comparison (Table 1) . The similar trend has been found in the western Pacific Ocean (FURUYA 1990, FURUYA & MARUMO 1983) and the North Atlantic Ocean (Lr et al. 1992) , where picophytoplankton prevails in CHL (GLOVER et al. 1985, GLOVER et al. 1986, 0DATE & MAITA 1988/1989, TAKAHASHI et al. 1985) .

0DATE et al.: Phytoplankton Carbon and Chlorophyll a 139

Secondly, the problems resulted from occurrence of cyanobacteria are considered. In the 2-0. 2 µm size fraction relationship between cell density of cyanobacteria and CHL was not significant (Figure 5). A similar discordance has been noted between vertical profiles of cyano bacteria and CHL in the thermally stratified water column of the Sargasso Sea (Lr et al. 1992, Lr & WooD 1988). On the other hand, other picophytoplankton significantly correlated with CHL. These relationships may largely result from that cyanobacteria contain relatively lower CHL than the same cell sized eukaryotes (FURUYA 1990).

On May 9 and August 22, the cell densities of cyanobacteria were quite different but the size compositions of CHL were almost the same (Table 1) . Hence, the effect arising from the defference of cell density of cyanobacteria can be evaluated by comparisons between CHL and PC on those days. The CHL on August 22, when cyanobacteria were the most abundant, was 80 % of that on May 9. If the comparison was conducted based on the PC, the biomass of phytoplankton on August 22 is 25-fold higher than that on May 9. These calculations indicate that abundance of cyanobacteria is unlikely to be exactly evaluated by the CHL measurement as suggested by 0DATE et al. (1990), and that PC of cyanobacteria-abundant community may be considerably higher than of cyanobacteria-less community, although CHL are the same each other. The factor of difference resulted from the large occurrence of cyano bacteria may become more than ten, which is higher than that resulted from the difference of size composition of phytoplankton community as mentioned former.

MAITA & YANADA (1978) reported that high 14C uptake rate occurred in a stratified water column of Funka Bay, in which phytoplankton abundance has been thought to be low based on CHL measurements. The high 14C uptake may be conducted by cyanobacteria which contain less CHL, since they dominate phytoplankton comm unity in thermally stratified water columns (EL HAG & FOGG 1986, JOINT et al. 1986, MURPHY & HAUGEN 1985, 0DATE 1989' 0DATE et al. 1990' w ATERBURY et al. 1986) . MAITA & 0DATE (1988) have also repoted the high 14C uptake rate per unit CHL in the 2-0. 2 µm size fraction during summer period in this bay.

The evaluation of PC altered the size composition of phytoplankton community detected by CHL on August 22. The percent contribution of the 2-0. 2 µm size fraction in the CHL (59. 8 %) increased to 98. 2 % in terms of PC (Table 1). This is due to the high abundance of cyanobacteria.

In this study we discussed the problems derived from variation of PC: CHL in relation to cell size and from occurrence of cyano bacteria. Besides these two points there are many other factors, which affect the interpretation of CHL data. An empirical relationship to calculate PC: CHL from photon flux density and temperature was provided by GEIDER (1987). Evidence of photoadaptive regulation of cellular CHL content, which generally increase with depth, is known in a stratified water column (CULLEN & LEWIS 1988, Lr et al. 1992, PREzELIN et al. 1986) . This may affect the present estimation of the mean cell volume, which is obtained from CHL and cell density in the > 10 µm and the 10-2 µm size fraction. The effects,

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however, would be minimal to the PC biomass estimated in the stratified water column in summer, since the most dominant size class in summer phytoplankton comm unity is neither the > 10 µm nor the 10-2 µm size fraction but the 2-0. 2 µm size fraction, of which PC was estimated by cell density and cellular PC content, being independent of cellular CHL.

Nutrient availability also alter PC:CHL, that is, low in nutrient rich waters and high in nutrient depleted waters (ANTIA et al. 1963, BANNISTER & LAWS 1980' CAPERON & MEYER 1972' EPPLEY 1968' Lr et al. 1992' RIEMANN et al. 1989, STRICKLAND 1960). This may further emphasize the high PC in summer, since nutrient concentrations in the surface of the stratified water column in summer are very low in Funka bay (MAITA & 0DATE 1988, MAITA & YANADA 1978).

Conclusion In the present study we estimated PC using the size-fractionated CHL and cell density collected from Funka Bay, and then the temporal variation of phytoplankton abundance based on the PC was compared with that based on the CHL. As the results, the difference between these two methods became most considerable when a cell density of cyanobacteria had changed. The defference was relatively small when a size composition of phytoplankton community had changed but cyanobacteria were not abundant. It is concluded that, for estimating phytoplankton a bun dance of Funka Bay using CHL data, the effect resulted from large occurrence of cyanobacteria is severer than that resulted from change in size composition of phytoplankton community. Phytoplankton biomass does not seem to be exactly evaluated by CHL data in summer when a phytoplankton community is predominated by cyanobacteria. The PC biomass of summer phytoplankton community is probably not so low as have been detected by CHL in this Bay.

Acknowledgments We would like to express our sincere gratitude to Dr. K. FURUYA, Biological Oceanography, Mie University, for his helpful suggestions and critical reading of this manuscript. We are also grateful to the captain and crew of the T/S Ushio Maru, Hokkaido University, for their helpful support during the field .observations.

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