Pelagic and coastal diatom ﬂuxes and the …acr/migrated/BeringResources/Articles of... · Three sediment trap moorings were deployed in the northwestern North ... The three stations

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Deep-Sea Research II 52 (2005) 2218–2239

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Pelagic and coastal diatom fluxes and the environmentalchanges in the northwestern North Pacific during

December 1997–May 2000

Jonaotaro Onoderaa,�, Kozo Takahashib, Makio C. Hondac

aDepartment of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, JapanbDepartment of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, JapancJapan Agency for Marine-Earth Science and Tecnology, Mutsu Institute for Oceanography (Yokohama Office), Natsushima 2-15,

Yokosuka 237-0061, Japan

Received 6 June 2003; accepted 31 July 2005

Available online 10 October 2005

Abstract

Three sediment trap moorings were deployed in the northwestern North Pacific (Station KNOT: 441N, 1551E; Station

50N: 501N, 1651E; Station 40N: 401N, 1651E) during December 1997–May 2000 (to January 2000 for Station 40N) in

order to decipher the linkages between the pelagic environmental variation and the inter-annual or intra-annual variation

of the sinking diatom assemblage fluxes. The three stations were located in different water masses, and the diatom sinking

assemblages reflected the water-mass characteristics at each station. Neodenticula seminae, an oceanic species, was

dominant at Station 50N, attributing to the relatively stable water mass there. At Station KNOT, which is located in the

southwestern margin of the Western Subarctic Gyre, the mean total diatom flux was the highest among the three stations

due to the large contribution of neritic and littoral taxa advected from coastal to pelagic areas. The occurrence of large

fluxes of resting spores was one of the most significant characteristics at Station KNOT, because resting spores are

generally rare in the world’s pelagic oceans. Many characteristic species also occurred in the sinking assemblages at Station

KNOT due to the influence of specific water masses compared to those at Stations 50N and 40N. Seasonal variations in the

diatom flux and assemblage composition were notable at Station KNOT. At Station 40N, which is located along the

Subarctic Boundary, both subarctic species and subtropical species dominated in the flux assemblages. Intra-annual

variation of diatom species composition and fluxes at Stations KNOT and 50N occurred due to the increase of neritic and

littoral taxa, attributable to the strong advection of coastal waters following spring and summer 1999. The variation of

diatom species composition and fluxes at each station may have been in response to large- or meso-scale climate changes

between 1998 and 1999 in the North Pacific, although further extensive investigations are required. At Station 40N,

favorable nutrient conditions were responsible for the high total diatom flux and the dominance of Fragilariopsis doliolus in

1999. This may be due to the positive SSTA and significant influences of the subtropical waters in 1999.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Sediment trap; Diatom flux; Resting spores; North Pacific; Costal water

e front matter r 2005 Elsevier Ltd. All rights reserved.

r2.2005.07.005

ng author. Fax: +81 92 642 2686.

ss: [email protected] (J. Onodera).

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ARTICLE IN PRESSJ. Onodera et al. / Deep-Sea Research II 52 (2005) 2218–2239 2219

1. Introduction

Diatoms play an important role as one of themajor primary producers in the middle and highlatitude oceans (Raymont, 1980; Lalli and Parsons,1993). Diatom studies in the high-latitude oceansare important in order to understand the geochem-ical cycles of silicon in addition to biological andbiogeographic aspects, because high diatom produc-tion and the rapid sinking of the large diatomparticles play significant roles in the biologicalpump (Kemp et al., 2000; Smetacek, 1999; Takaha-shi et al., 2002). The subarctic Pacific is an areawhere the deep-sea waters slowly upwell and is oftencalled the ‘‘Silica Ocean’’ (Honjo, 1997), because theweight ratio of opal/CaCO3 is 41 due to theupwelled water containing a large amount ofdissolved silicon.

Diatoms are the dominant phytoplankton groupin the northwestern North Pacific (Taylor andWaters, 1982) and require relatively high concentra-tions of iron compared to coccolithophorids (Mug-gli and Harrison, 1997). This stems from the factthat the concentration of iron in the surface layer ofthe western subarctic Pacific is higher than that inthe eastern subarctic Pacific (Kudo and Yoshimura,2001). In general, the level of vertical mixing iscontrolled by weather conditions. Most of thenutrient supply from the subsurface to the surfacewaters depends on the extent of seasonal verticalmixing. Therefore, the variation of the diatom fluxin the sinking assemblage (Sidocoenosis: Takahashi,1995) in the western subarctic Pacific can becontrasted with the parameters of climate change.The importance of vertical mixing for primaryproduction also was suggested from the study ofbiogenic particle fluxes (e.g., Takahashi et al., 1989).

Since the pioneer study by Honjo (1984) in thenortheastern subarctic Pacific, many studies onsinking particles have been conducted in the NorthPacific (Reynolds and Thunell, 1985; Noriki andTsunogai, 1986; Takahashi, 1986, 1989, 1991b;Honjo and Doherty, 1988; Kawahata et al., 1998;Noriki et al., 1999; Wong et al., 1999; Takahashiet al., 2002; Scharek et al., 1999; Honda et al., 2002;Kuroyanagi et al., 2002). The studies of diatom fluxand the sidocoenosis have been conducted in thecentral and eastern subarctic North Pacific (Taka-hashi, 1986, 1987, 1991a, 1997; Sancetta andCalvert, 1988; Takahashi et al., 1989, 1990, 1994,2000, 2002; Sancetta, 1992; Kurihara and Takaha-shi, 2002). Tsoy and Wong (1999) studied diatom

fluxes in the pelagic northwestern North Pacific forone year. Our diatom flux data represent the diatomflux variations in the pelagic northwestern subarcticPacific for about two years. This study aims todemonstrate linkages between the time-series fluxvariations of diatom sidocoenosis and the pelagicenvironmental variability in the euphotic layer.

2. Oceanographic setting

The northern North Pacific can be subdividedinto the Subarctic Domain and the TransitionalDomain based on differences in hydrographicconditions (Dodimead et al., 1963; Favorite et al.,1976). The Subarctic Domain (Fig. 1) is located tothe north of the Subarctic Front, which is defined bycold water (o4 1C) below 100m depth. The Sub-arctic Front is only clearly recognizable in thewestern side of the North Pacific. In the SubarcticDomain, there are four counterclockwise gyres: theAlaskan Gyre; the Western Subarctic Gyre (WSG);the Bering Sea Gyre; and the Okhotsk Sea Gyre.There are inter-annual and intra-annual variationsof the transport volume of the Alaskan Stream andthe Western Subarctic Current (Ohnishi and Ohta-ni, 1999; Ohnishi, 2001). Station 50N is located inthe center of the WSG, whereas Station KNOT islocated at the southwestern edge of the WSG.Honda et al. (2002) compiled the hydrographiccharacteristics at each station. There are somedifferences in the amplitude of the hydrographicdata, although subarctic waters occur at bothStations 50N and KNOT. This is due to thedifference in the stability of the water masses andthe location of the stations in the WSG (Honda etal., 2002). The Subarctic Boundary is present at40.41N (70.41) along 1651E (Miyao and Ishikawa,2003) and is characterized by a vertical 34 psuisohaline. The Transitional Domain is locatedbetween the Subarctic Front and the SubarcticBoundary. Station 40N is located slightly south ofthe Subarctic Boundary, where the subtropicalwater is characterized by relatively high temperatureand high salinity (Honda et al., 2002).

3. Materials and methods

Time-series sediment traps were deployed atStation 50N (501010N, 1651000E; water depth5546m), Station KNOT (431580N, 1551030E; waterdepth 5375m), and Station 40N (391600N, 1651000E;water depth 5476m) (Fig. 1). The exact deployment

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The Transitional Domain

60°N

50°N

40°N

30°N

120°E 150°E 180° 150°W 120°W

WSG: Western Subarctic Gyre, AG: Alaskan Gyre, BSG: Bering Sea Gyre, OSG: Okhotsk Sea Gyre

Studied sediment trap stations The sediment trap stations, which were previously studied on the diatom fluxes.

Fig. 1. A map showing the locations of Stations KNOT, 50N, 40N (filled triangles) and the other sediment trap stations with diatom flux

data (open triangles) in the northern North Pacific and the Bering Sea with general surface circulation (from Harrison et al., 1999). The

Transitional Domain (Favorite et al., 1976) is illustrated. The Subarctic Domain is located north of the Transitional Domain.

Table 1

Logistic summary of sediment trap deployments conducted at the three stations during December 1997–2000 in the northwestern subarctic

Pacific (Honda, 2001; Honda et al., 2002)

Station/depth

ID

Latitude Longitude Water

depth (m)

Trap depth

(m)

Sampled duration Sample

interval (d)

Type of sediment

trap (m)

50N 3000m 501010N 1651010E 5546 3260 1 Dec 1997–18

May 2000

15.03 or 17.375 McLane

Mark7G21

KNOT 3000m 431580N 1551030E 5375 2957 1 Dec 1997–13

May 2000

15.03 or 17.375 McLane

Mark7G21

40N 3000m 401000N 1651000E 5476 2986 1 Dec 1997–30

January 2000

15.03 or 17.375 McLane

Mark7G21

J. Onodera et al. / Deep-Sea Research II 52 (2005) 2218–22392220

depths of the time-series sediment traps were 3260,2957, and 2986m at Stations 50N, KNOT, and40N, respectively (Table 1). For simplicity, thesedeployment depths will be written hereafter as‘‘3000 m’’. A total of 141 samples were studiedfrom the three sample sets. These samples werecollected using a multi-sampler sediment trapMark7G21, which was able to automatically collect21 successive samples, during December 1997–May2000 (to January 2000 for Station 40N) (Table 1).Sample collection intervals were either 17.375 or15.03 days. After the recovered samples were sievedthrough a 1mm mesh, the o1mm size fractionswere split into 1/1000 or 1/4000 aliquot sizes using a

rotary splitter. The split samples were filteredthrough gridded membrane filters (Gelmans) anddesalted with distilled water, followed by drying inan oven at 50 1C over night. A portion of the filterwas then mounted onto a slide glass with immersionoil type B (Cargiles) in order to render the filtertransparent. A light microscope (Olympus BX50)was used for identification and counting of thediatom valves at magnifications of x200–600. Onlydiatom valve fragments greater than 2/3 of the valvearea were identified and counted, and about0.4–1.5% of the filter area was examined during avalve count. More than 400 valves were counted persample for the quantitative diatom analysis (Boden,


1991), and diatom fluxes were calculated by thefollowing equation:

Flux ðvalves m�2 d�1Þ ¼ ðN=S=DÞ � ðS1=S2Þ � V ,

where N is the number of diatom valves counted, S

the aperture area of the sediment trap (0.5m2), D

the sample interval (17.375 or 15.03 days), S1 thefiltered area of sample filter (535mm2), S2 thecounted area (mm2), and V the aliquot size.

The following literature was used as references fordiatom identification: Cupp (1943), Hendey (1964),Simonsen (1974, 1992), Fryxell and Hasle (1979),Hargraves (1979), Syvertsen (1979), Sancetta (1982),Akiba (1986), Rines and Hargraves (1988), Hasleand Medlin (1990), Pitcher (1990), Takano (1990,1997), Takahashi et al. (1994), Hasle and Syvertsen(1997). The identification and counting were con-ducted at species level as much as possible.

On the taxonomy of Chaetoceros, Hendey (1964)was mainly referred to in this study. The nameChaetoceros has appeared at many taxonomic levels(genus, subgenus, and species). Therefore, thedefinition of Chaetoceros is a bit complicated. Thegenus Chaetoceros contains two subgenera: Chaeto-

ceros ( ¼ Phaeoceros) and Hyalochaete. Wheneverwe refer to the subgenus Chaetoceros it is spelt outto avoid confusion. Here, species of Chaetoceros aretreated as Latin neuter nouns (-um), not masculine(-us). The resting spore flux of Hyalochaete, which isa subgenus of Chaetoceros, is expressed in valvesbecause the frustule structure of a Hyalochaete

resting spore is similar to the vegetative valvestructure except for the absence of a girdle on thespore (Hasle and Syvertsen, 1997). The restingspores are treated as an ordinary sinking diatomassemblage (diatom sidocoenosis: Takahashi, 1995)with vegetative valves because the sinking restingspores are not capable of coming back to theeuphotic layer to contribute to future diatomproduction (Takahashi et al., 1996).

Comparisons of mean fluxes were only possibleafter significant differences were identified by thet-test. The total mass flux and the entire chemicaldata of the samples were published earlier by Honda(2001) and Honda et al. (2002). The time-series dataon the weekly sea-surface temperature (SST) andsea-surface temperature anomaly (SSTA) are avail-able from the IGOSS NMC (Reynolds and Smith,1994). The wind speed data on the sea surface arefrom Special Sensor Microwave/Images (SSM/I) byRemote Sensing Systems (Wentz and Spenser,

1998). In this paper, seasons are defined forconvenience as follows: winter: December–Febru-ary; spring: March–May; summer: June–August;and fall: September–November.

4. Results

4.1. Diatom fluxes at 3000 m at Station 50N

In 1998 total diatom fluxes at 3000m at Station50N increased in March, the first flux peakappearing in April to May (Fig. 2). After the high-flux period, the fluxes showed a decreasing trendwith small flux peaks. However, later a flux peakoccurred in late November. In 1999 total diatomfluxes showed a low-flux period by April, and thenconspicuous peaks in April–May (Fig. 2B). An earlyAugust peak was followed by a strong flux decrease,a late September peak, and a distinct decreasing fluxtrend (Fig. 2B). In April 2000, the total fluxesincreased and reached the overall maximum of theentire sampled period. Total diatom fluxes at 3000mranged from 5.4� 106 to 67.2� 106 valvesm�2 d�1,with a mean of 18.7� 106 valvesm�2 d�1. Annualamplitude of the total diatom flux was larger in 1999than in 1998. Annual means of total diatom fluxwere similar both in 1998 and in 1999 compared tothose at Station KNOT (Fig. 9). The correlationcoefficient between total diatom flux and total massflux was 0.92 (Table 2).

Diatom assemblages at 3000m at Station 50Ncomprised 60 centric taxa and 45 pennate taxa in the51 samples (Table 3). The most dominant species wasNeodenticula seminae, and its mean flux and meanrelative abundance was 13.2� 106 valvesm�2 d�1 and77.4%, respectively (Tables 4 and 5). This taxonconsistently dominated throughout the year (Fig. 3).The remaining species had annual mean contribu-tions below 3% (Table 4). Thalassiosira oestrupii

was the second most significant taxon. The relativeabundances of T. oestrupii were relatively highduring fall 1998 to spring 1999 and in spring 2000(Fig. 3). The third abundant taxon was Chaetoceros

furcellatum resting spores which increased after July1999 (Fig. 3). Hyalochaete spp., Thalassiosira

gravida, Thalassiosira nordenskioeldii, which areknown to occur in coastal waters, clearly increasedin 1999 (Fig. 3). Chaetoceros atlanticum increased inspring and fall, and its highest percentage wasrecorded in spring 2000 (Fig. 3).

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Station 50N

300

250

200

150

100

50

0

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M

15

10

5

0

7

6

5

4

3

2

1

0Opal

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M1998 1999 2000

1998 1999 2000

Tot

al D

iato

ms

[×1

07 val

ves

m-2

d-1

]

Tot

al M

ass

and

Opa

l Flu

xes

[mg

m-2

d-1

]

SS

T [

° C]

(A)

(B)

Fig. 2. (A) Sea-surface temperature (SST) (Reynolds and Smith, 1994); (B) total mass and opal (shaded bars; Honda, 2001), and total

diatom (solid lines) fluxes at Station KNOT 3000m during December 1997–May 2000. The dark shaded areas represent fluxes other than

opal (CaCO3, organic matter, plus lithogenic matter) and hence the tops of the stack bars equal total mass fluxes.


4.2. Diatom species at 3000 m at Station KNOT

In 1998 total diatom fluxes at Station KNOTshowed some flux peaks in January, April–May,August–September, and late October (Fig. 4B). In1999, total diatom fluxes showed a high flux periodfrom June to August, with the maximum peak inearly June (Fig. 4B). This obvious flux peak was notseen in 1998. Relatively small flux peaks alsooccurred in January, March, and October of 1999.Total diatom fluxes ranged from 7.0� 106 to178� 106 valvesm�2 d�1, with a mean of 40.3�106 valvesm�2 d�1. The pattern of the time-seriesflux changes for the total diatoms was similar tothat of the total mass fluxes (Honda et al., 2002)(Fig. 4B). The correlation coefficients r betweentotal diatom flux and total mass flux was 0.92 (Table2). The correlation coefficients between total diatomfluxes versus radiolarian/silicoflagellate fluxes wereinsignificant at Station KNOT (Table 2). Diatomassemblages at 3000m at Station KNOT werecomprised of 91 centric taxa and 47 pennate taxain a total of 53 samples (Table 3).

The dominant species was Neodenticula seminae.The second most significant taxa were vegetativeHyalochaete spp. and their resting spores (Table 4).

This finding was different from that of Station 50Nbecause the dominance of the diatom species in theassemblages changed seasonally (Fig. 5). In 1998,Neodenticula seminae increased in March, peaked inApril, and then decreased towards December. In1999, this taxon increased in March, peaked in May,and then decreased towards December. In 2000, thefluxes increased and peaked in March, and thendecreased. The peak fluxes of Neodenticula seminae

in 1999 and 2000 were less than half of those in 1998(Fig. 5). Vegetative Hyalochaete spp. fluxes in 1998increased in January, and then decreased towardsJuly. The flux peak of Hyalochaete spp. was alsoobserved in October 1998. In 1999, as in 1998, fluxpeaks of Hyalochaete spp. occurred in January.However, the flux peaks of Hyalochaete spp.observed from June to July in 1999 were notrecorded in 1998 (Fig. 5). Relative abundances ofHyalochaete spp. reached maxima in January 1998,October 1998, January 1999, and June 1999.Resting spore fluxes of Chaetoceros spp. increasedin June 1998, reached the annual maximum inAugust 1998, and then decreased. In 1999, theresting spore fluxes significantly increased in June,and recorded their highest values in July, andalthough a minor peak occurred in October there

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Table 2

Correlation coefficients r between the fluxes of total mass, organic carbon, opal, aluminum, total radiolarians, or total silicoflagellates vs.

the major diatom taxa at 3000m at Stations KNOT, 50N, and 40N (a ¼ 95%)

Total massa,b Organic carbona Opala Aluminuma Total radiolariansc Total sili coflagellatesd

50N (N ¼ 51)

Total diatoms 0.92 0.84 0.88 0.64 0.48 0.69

(Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01)Neodenticula seminae 0.93 0.88 0.90 0.62 0.46 0.63

(Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01)Hyalochaete 0.53 0.43 0.65 0.21 0.00 0.00

(vegetative valves) (Po0:01) (Po0:01) (Po0:01) (P ¼ 0:13) (P ¼ 0:99) (P ¼ 0:98)Hyalochaete 0.36 0.84 0.48 0.05 �0.01 �0.01

(resting spores) (P ¼ 0:01) (Po0:01) (Po0:01) (P ¼ 0:72) (P ¼ 0:95) (P ¼ 0:94)

KNOT (N ¼ 53)

Total diatoms 0.92 0.70 0.95 0.84 0.20 0.22

(Po0:01) (Po0:01) (Po0:01) (Po0:01) (P ¼ 0:17) (P ¼ 0:05)Neodenticula seminae 0.35 0.08 0.35 0.42 �0.18 0.25

(Po0:01) (P ¼ 0:57) (Po0:01) (Po0:01) (P ¼ 0:22) (P ¼ 0:07)Hyalochaete 0.86 0.63 0.89 0.77 0.28 0.35

(vegetative valves) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (P ¼ 0:05) (P ¼ 0:01)Hyalochaete 0.77 0.83 0.80 0.64 0.41 �0.01

(resting spores) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (P ¼ 0:90)

40N (N ¼ 37)

Total diatoms 0.73 0.57 0.81 0.38 0.48 0.75

(Po0:01) (Po0:01) (Po0:01) (P ¼ 0:02) (Po0:01) (Po0:01)Neodenticula seminae 0.72 0.49 0.71 0.53 0.60 0.62

(Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01) (Po0:01)Fragilariopsis doliolus 0.51 0.31 0.61 0.07 0.21 0.52

(Po0:01) (P ¼ 0:06) (Po0:01) (P ¼ 0:69) (P ¼ 0:21) (Po0:01)Hyalochaete 0.45 0.42 0.49 0.16 0.26 0.40

(vegetative valves) (Po0:01) (Po0:01) (Po0:01) (P ¼ 0:35) (P ¼ 0:12) (P ¼ 0:01)

aData from Honda (2001).bHonda et al. (2002).cOkazaki et al. (2005).dOnodera and Takahashi (2005).

J. Onodera et al. / Deep-Sea Research II 52 (2005) 2218–2239 2223

was a general decrease (Fig. 5). Relative abundancesof the resting spores were high between June andFebruary in each year. Thalassiosira nordenskioeldii,Bacterosira bathyomphala, and Rhizosolenia hebeta-

ta f. semispina dominated during summer to fall/winter (Table 6).

The annual mean fluxes of pelagic taxa in 1999were lower than those in 1998. The mean fluxes ofcoastal taxa, total mass, and total diatoms in 1999were higher than those in 1998 (Fig. 9).

4.3. At 3000 m at Station 40N

Total diatom fluxes during 1998 increased inFebruary and plateaued off from March throughMay. Flux peaks also were recorded in late August,October, and December. The total diatom flux

during 1999 peaked in May, decreased in June, andthen increased and reached the flux peak in July andAugust (Fig. 6B). Total diatom flux at 3000mranged from 3.8� 106 to 83.3� 106 valvesm�2 d�1,with a mean of 22.0� 106 valvesm�2 d�1. Annualamplitude of total diatom flux was larger in 1999than in 1998 (Fig. 6B). An annual mean of totaldiatom flux during 1999 was significantly higherthan that in 1998 (Fig. 9). While the samples for theperiod between January to May 1999 at 3000mwere unavailable, the mean total mass fluxes at5000m were significantly higher than those in 1998(Honda et al., 2002). The Correlation coefficientbetween total diatom flux and total mass flux was0.77 (Table 2).

Diatom assemblages at 3000m at Station 40Ncomprised 61 centric taxa and 26 pennate taxa in 37

ARTICLE IN PRESSJ. Onodera et al. / Deep-Sea Research II 52 (2005) 2218–22392224

samples (Table 3). The most dominant taxon in1998 was Neodenticula seminae (Table 4), and themost dominant taxon in 1999 was Fragilariopsis

doliolus. (Table 4). The flux of Neodenticula seminae

in 1998 gradually increased in March, reached amaximum in May, and decreased towards July, andexhibit a second peak of in August. In 1999Neodenticula seminae fluxes peaked in May, de-creased in June, and then increased and reached aflux peak from July to August (Fig. 7). The fluxes ofFragilariopsis doliolus increased after November1998, and peaked in May 1999. Fluxes freeeabruptly decreased in June, formed a secondaryflux peak in July, and then decreased after August1999 (Fig. 7). The flux of Fragilariopsis doliolus

ranged from 0.6� 106 to 55.5� 106 valvesm�2 d�1,with a mean of 7.4� 106 valvesm�2 d�1 (Table 5).Thalassionema nitzschioides fluxes in 1998 increasedin January, reached a peak in March, and thengradually decreased to August; the maximum fluxoccurred in October 1998. In 1999, the flux peakwas recorded in July to early August. The flux ofChaetoceros peruvianum was high during Februaryto May 1998, and was highest in May 1999 (Fig. 7).Among the major species fluxes, Chaetoceros

peruvianum and Thalassiosira oestrupii showed aclear seasonality. Their relative abundances in 1999were suppressed by a large Fragilariopsis doliolus

flux.

5. Discussion

5.1. Contribution of diatoms to total mass, and opal

fluxes

Temporal fluctuation patterns of both totaldiatom and total mass fluxes were similar to eachother at each station (Figs. 2B, 4B and 6B).Correlation coefficients between total diatom fluxand total mass flux showed high values (Table 2).The correlation coefficients between total diatomflux and opal flux were also significantly high ateach station (Table 2). In the chemical bulkcomponent, opal usually constituted more than50% at each station (Honda et al., 2002). Thediatom fluxes at each station were higher thanradiolarian and silicoflagellate fluxes as follows: Thecorrelation coefficients between radiolarian fluxesand total mass fluxes were lower (Station 50N:r ¼ 0:425, Station KNOT: 0.198, and Station 40N:0.677) than that between diatom and total massfluxes at the three stations (Okazaki et al., 2005).

The silicoflagellate fluxes also showed the relativelylow correlation coefficients for total mass (Station50N: r ¼ 0:19; Station KNOT: 0.27) and opal fluxes(Station 50N: r ¼ 0:07; Station KNOT: 0.22) atStations 50N and KNOT (Onodera and Takahashi,2005).

Of the diatom species, the contribution ofNeodenticula seminae to total mass and opal fluxeswas significant at Station 50N, just as theirsignificant contributions to total mass and opalfluxes were suggested at Stations PAPA, C, AB, andSA in the subarctic North Pacific (Takahashi, 1997;Takahashi et al., 2000, 2002). The high diatomcontributions to the biogeochemical cycles at Sta-tion 40N were due to the high fluxes of Fragilar-

iopsis doliolus in addition to Neodenticula seminae

flux (Table 2). At Station KNOT, the fluxes ofHyalochaete spp. and the resting spores were themost significant for the biogeochemical cycles. TheNeodenticula seminae contribution at StationKNOT was rather insignificant (Table 2).

5.2. Characteristics in the diatom sidocoenosis and

their temporal variations

5.2.1. Station 50N

The diatom sidocoenosis flora is mainly char-acterized by the high abundance of Neodenticula

seminae, which is a pelagic species (Kanaya andKoizumi, 1966). The high abundance of Neodenti-

cula seminae also was observed at Station GD (Tsoyand Wong, 1999), which was located near Station50N (Fig. 1). This taxon is rare or absent outside ofthe subarctic Pacific (Hasle, 1976). In the surface-water diatom biocoenosis, the Fragilariopsis pseu-

donana group significantly dominates (greater than70%) the subarctic Pacific, while Neodenticula

seminae is the second most abundant species(Hisamichi and Takahashi, 1994). The change ofthe primary dominant species from the Fragilar-

iopsis pseudonana group in the biocoenosis toNeodenticula seminae in the sidocoenosis is due tothe dissolution of the relatively small and thin valvesof the Fragilariopsis pseudonana group, which areeasily dissolved and recycled in the upper layers.

The relatively abundant occurrences of the coast-al species (Hyalochaete spp., and the resting spores,Thalassiosira gravida, Thalassiosira nordenskioeldii)after July 1999 might be due to the nutrient supplyfrom the subsurface layer at Station 50N ratherthan the advection of coastal waters. Karohji (1972)showed that the diatom taxa in the surface waters

ARTICLE IN PRESSTable

3

Encountereddiatom

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theoccurrence

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KNOT

40N

50N

KNOT

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usEhrenberg

’’

’M

elosi

rasp.

’

Act

inocy

clus

sagit

tulu

sVillarealin

VillarealandFryxell

’O

donte

lla

auri

ta(Lyngbye)

Agardh

’’

’

Act

inocy

clussp.

’P

ala

ria

sulc

ata

(Ehrenberg)Cleve

’’

Act

inopty

chusspp.

’’

’P

lank

tonie

lla

sol(W

allich)Schutt

’

Ast

erola

mpra

mary

landic

aEhrenberg

’P

oro

sira

sp.

’’

Ast

erom

phalu

sara

chne(Brebisson)Ralfsin

Pritchard

’’

Pro

bosc

iaala

ta(Brightw

ell)Sundstrom

’’

’

Ast

erom

phalu

sbro

ok

ei(Brebisson)Ralfsin

Pritchard

’’

’P

robosc

iaeu

morp

haTakahashi,JordanandPriddle

’’

’

Ast

erom

phalu

sel

egansGreville

’’

Pro

bosc

iasu

barc

tica

Takahashi,JordanandPriddle

’’

’

Ast

erom

phalu

shep

tact

is(Brebisson)Ralfsin

Pritchard

’’

’P

seudoso

lenia

calc

ar-

avi

s(Schultze)Sundstrom

’’

Ast

erom

phalu

shyali

nusKarsten

’’

’R

hiz

oso

lenia

ber

goniiPeragallo

’’

Ast

erom

phalu

sro

bust

usCastracane

’’

’R

hiz

oso

lenia

heb

etata

Bailey

f.heb

etata

’’

’

Ast

erom

phalu

ssp.

’R

hiz

oso

lenia

heb

etata

f.se

mis

pin

a(H

ensen)Gran

’’

’

Azp

eiti

aafr

icana(Janisch

exA.Schmidt)

FryxellandWatkins

’R

hiz

oso

lenia

seti

ger

aBrightw

ell

’’

’

Azp

eiti

anoduli

fera

(Janisch

exA.Schmidt)

FryxellandSim

s’

Rhiz

oso

lenia

styli

form

isBrightw

ell

’’

’

Azp

eiti

ata

bula

ris(G

runow)FryxellandSim

s’

’’

Rhiz

oso

lenia

sp.

’

Azp

eiti

asp.

’R

opel

iate

ssel

ata

(Roper)Grunow

exPelletan

’’

Bact

eria

stru

mdel

icatu

lum

Cleve

’’

Sk

elet

onem

aco

statu

m(G

reville)Cleve

’

Bact

eria

stru

mfu

rcatu

mShadbolt

’’

Ste

llari

ma

stel

lari

s(R

oper)HasleandSim

s’

’’

Bact

erosi

rabath

yom

phala

(Cleve)

Syvertsen

andHasle

’’

Ste

phanopyx

issp.

’’

’

Chaet

oce

ros

atl

anti

cum

Cleve

’’

’T

hala

ssio

sira

all

enii

’’

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oce

ros

conca

vico

rneMangin

’’

’T

hala

ssio

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angula

ta(G

regory)Hasle

’

Chaet

oce

ros

convo

ltum

Castracane

’’

’T

hala

ssio

sira

angust

e-li

nea

ta(A

.Schmidt)

FryxellandHasle

’

Chaet

oce

ros

per

uvi

anum

Brightw

ell

’’

’T

hala

ssio

sira

anta

rcti

caComber

’

Hyalo

chaet

espp.

’’

’T

hala

ssio

sira

bin

ata

Fryxell

’’

Core

thro

nsp.

’’

’T

hala

ssio

sira

dec

ipie

ns(G

runow)Jørgensen

’’

Cosc

inodis

cus

ast

erom

phalu

sEhrenberg

’’

’T

hala

ssio

sira

ecce

ntr

ica(Ehrenberg)Cleve

’’

’

Cosc

inodis

cus

centr

ali

sEhrenberg

’’

Thala

ssio

sira

fere

linea

taHasleandFryxell

’’

Cosc

inodis

cus

gra

niiGough

’T

hala

ssio

sira

fren

guel

liiKozlova

’

Cosc

inodis

cus

marg

inatu

sEhrenberg

’’

’T

hala

ssio

sira

gra

cili

s(K

arsten)Hustedtvar.

gra

cili

s’

’

Cosc

inodis

cus

ocu

lus-

irid

isEhrenberg

’’

’T

hala

ssio

sira

gra

vidaCleve

’’

Cosc

inodis

cus

radia

tusEhrenberg

’’

’T

hala

ssio

sira

hyali

na(G

runow)Gran

’’

Cosc

inodis

cussp.

’’

Thala

ssio

sira

cf.

kush

iren

sisTakano

’

Dact

yli

oso

lensp.

’T

hala

ssio

sira

linea

taJouse

’’

’

Dit

ylu

msp.

’’

’T

hala

ssio

sira

min

imaGaarder

’

Thala

ssio

sira

nord

ensk

ioel

diiCleve

’’

Lio

lom

apaci

ficu

m(C

upp)Hasle

’

Thala

ssio

sira

oes

trupii(O

stenfeld)Hasle

’’

’N

avi

cula

dir

ecta

(W.Smith)Ralfsin

Pritchard

’’

Thala

ssio

sira

paci

fica

GranandAngst

’’

Navi

cula

sp.1

’

Thala

ssio

sira

pse

udonanaHasleandHeimdal

’N

avi

cula

sp.2

’

Thala

ssio

sira

punct

iger

a(C

astracane)

Hasle

’’

’N

eoden

ticu

lase

min

ae(Sim

onsenandKanaya)AkibaandY

’’

’

Thala

ssio

sira

trif

ult

agroup

’’

’N

itzs

chia

bic

apit

ata

Cleve

’’

’

Thala

ssio

sira

sp.1

’’

’N

itzs

chia

bra

aru

diiHale

’


ARTICLE IN PRESSTable

3(c

onti

nued

)

50N

KNOT

40N

50N

KNOT

40N

Thala

ssio

sira

sp.2

’’

Nit

zsch

iaca

pulu

spala

eSim

onsen

’’

Centric

sp.1

’’

Nit

zsch

iak

ola

czek

iiGrunow

’’

’

Centric

sp.2

’N

itzs

chia

linea

ta(C

astracane)

Hasle

’

Centric

sp.3

’N

itzs

chia

longis

sim

a(Brebisson)Ralfsin

Pritchard

’

Nit

zsch

iam

ari

naGrunow

inCleveandGrunow

’’

Pennates

Nit

zsch

iacf.

mari

naGrunow

inCleveandGrunow

’

Ach

nanth

essp.

’N

itzs

chia

obli

quec

ost

ata

(VanHeurck)Hasle

’

Baci

llari

asp.

’N

itzs

chia

sicu

la(C

astracane)

Hustedt

’’

’

Cocc

onei

sco

stata

Gregory

’’

Nit

zsch

iasu

bli

nea

taHasle

’’

Cocc

onei

ssp.1

’N

itzs

chia

sp.1

’’

’

Cocc

onei

ssp.2

’N

itzs

chia

sp.2

’’

Cocc

onei

ssp.3

’N

itzs

chia

sp.3

’’

’

Del

phin

eissp.

’’

Nit

zsch

iasp.4

’’

Dim

ereg

ram

masp.

’N

itzs

chia

sp.5

’

Dip

lonei

sbom

bus(Ehrenberg)Cleve

’N

itzs

chia

spp.

’

Dip

lonei

ssp.1

’P

innula

ria

quadra

tare

a(A

.Schmidt)

Cleve

’

Dip

lonei

ssp.2

’P

leuro

sigm

adir

ectu

mGrunow

inCleveandGrunow

’’

Dip

lonei

ssp.3

’P

leuro

sigm

anorm

aniiRalfsin

Pritchard

’

Ento

monei

ssp.

’P

leuro

sigm

asp.1

’

Epit

hem

iasp.

’P

leuro

sigm

asp.2

’

Fra

gil

ari

asp.

’P

seudogom

phonem

asp.

’

Fra

gil

ari

opsi

satl

anti

caPaasche

’’

Pse

udonit

zsch

iase

riata

(Cleve)

H.Peragallo

’

Fra

gil

ari

opsi

scy

lindru

s(G

runow)Krieger

’’

Pse

udo-n

itzs

chia

sp.1

’’

Fra

gil

ari

opsi

sdoli

olu

s(W

allich)MedlinandSim

s’

’’

Pse

udo-n

itzs

chia

sp.2

’’

Fra

gil

ari

opsi

soce

anic

a(C

leve)

Hasle

’’

Pse

udo-n

itzs

chia

sp.3

’’

’

Fra

gil

ari

opsi

sri

cher

iiHustedt

’P

seudo-n

itzs

chia

sp.4

’

Fra

gil

ari

opsi

ssp.1

’’

Raphonei

ssp.

’

Fra

gil

ari

opsi

ssp.2

’’

Ropalo

dia

sp.

’

Fra

gil

ari

opsi

ssp.3

’’

Thala

ssio

nem

abaci

llare

(Heiden

inHeiden

andKolbe)

Kolbe

’

Fra

gil

ari

opsi

sspp.

’’

Thala

ssio

nem

anit

zsch

ioid

es(G

runow)Mereschkowsky

’’

’

Gra

mm

ato

phora

sp.

’’

Thala

ssio

nem

asp.

’

Hasl

easp.

’’

Thala

ssio

thri

xanta

rcti

caSchim

per

exKarsten

’

Lic

mophora

sp.

’T

hala

ssio

thri

xlo

ngis

sim

aCleveandGrunow

’’

’

Lio

lom

ael

ongatu

m(G

runow)Hasle

’T

ubula

riasp.

’

Pennate

sp.1

’’

Chaet

oce

ros

deb

ileCleve

’’

Pennate

sp.2

’’

Chaet

oce

ros

dia

dem

a(Ehrenberg)Gran

’’

Pennate

sp.3

’C

haet

oce

ros

did

ym

um

Ehrengerg

’’

Pennate

sp.4

’C

haet

oce

ros

furc

ella

tum

Bailey

’’

Pennate

sp.5

’C

haet

oce

ros

laci

nio

sum

Schutt

’

Pennate

sp.6

’C

haet

oce

roscf.

lauder

iRalfs

’

Pennate

sp.7

’C

haet

oce

roscf.

mit

ra(Bailey)Cleve

’

ChaetocerosradicansSchutt

Chaet

oce

ros

seir

aca

nth

usGran

’

Res

ting

spore

sF

ragil

ari

opsi

soce

anic

a(C

leve)

Hasle

’

Chaet

oce

ros

affi

neLauder

’’

Hyalo

chaet

espp.

’’

’

Chaet

oce

ros

cinct

um

Gran

’’

Thala

ssio

sira

nord

ensk

ioel

diiCleve

’’

Chaet

oce

ros

com

pre

ssum

Lauder

’

Chaet

oce

ros

cost

atu

mPavillard

’


ARTICLE IN PRESS

Table 4

Major diatom taxa, which contributed greater than 1% of total diatoms, and the relative abandances in 1998 and 1999 and all sampled

durations at 3000m at three stations

Station 50N Mean relative abundance

1998 (1 Dec. 1997–31 Dec. 1998) 1999 (1 Jan. 1999–29 Dec. 1999) All sampled duration

(%) (%) (%)

Neodenticula seminae 83.3 Neodenticula seminae 76.0 Neodenticula seminae 75.8

Thalassiosira oestrupii 1.9 Hyalochaete spp.a 3.6 Thalassiosira oestrupii 2.5

Azpeitia tabularis 1.8 Thalassiosira trifulta group 2.7 Chaetoceros spp. resting spores 2.5

Actinocyclus curvatulus 1.7 Actinocyclus curvatulus 2.5 Actinocyclus curvatulus 2.4

Thalassiosira trifulta group 1.6 Thalassiosira oestrupii 2.0 Thalassiosira trifulta group 2.2

Thalassiosira lineata 1.5 Asteromphalus hyalinus 1.5 Chaetoceros atlanticum 1.5

Coscinodiscus marginatus 1.2 Thalassiothrix longissima 1.3 Azpeitia tabularis 1.3

Rhizosolenia hebetata f. hebeta 1.1 Thalassiosira lineata 1.2

Thalassiosira lineata 1.0 Coscinodiscus marginatus 1.1

Rhizosolenia hebetata f. hebeta 1.1

Corethron sp. 1.0

Asteromphalus hyalinus 1.0

Station KNOT

1998 (1 Dec. 1997–31 Dec. 1998) (1 Jan. 1999–11 Jan. 2000) All sampled duration

Neodenticula seminae 34.2 Hyalochaete spp.a 39.1 Hyalochaete spp.a 31.0

Hyalochaete spp. 19.6 Hyalochaete spp. 24.5 Neodenticula seminae 23.0

Hyalochaete spp.a 23.3 Neodenticula seminae 12.4 Hyalochaete spp. 21.8

Thalassiosira nordenskioeldii 3.5 Thalassiosira nordenskioeldii 2.9 Thalassiosira nordenskioeldii 2.8

Thalassiosira oestrupii 2.5 Thalassionema nitzschioides 2.5 Thalassionema nitzschioides 2.3

Thalassiosira lineata 1.9 Thalassiosira trifulta group 1.8 Thalassiosira oestrupii 2.1

Thalassiosira trifulta group 1.3 Thalassiosira oestrupii 1.5 Thalassiosira trifulta group 1.9

Thalassiosira eccentrica 1.3 Rhizosolenia hebetata f. semisp 1.3 Thalassiosira lineata 1.3

Thalassionema nitzschioides 1.2 Fragilariopsis doliolus 1.2

Actinocyclus curvatulus 1.0 Thalassiosira lineata 1.0

Station 40N

1998 (1 Dec. 1997–16 Dec. 1998) (22 May 1999–1 Jan. 2000) All sampled duration

Neodenticula seminae 17.5 Fragilariopsis doliolus 47.8 Fragilariopsis doliolus 32.9

Fragilariopsis doliolus 15.8 Neodenticula seminae 9.4 Neodenticula seminae 13.4

Thalassionema nitzschioides 9.3 Thalassionema nitzschioides 8.3 Thalassionema nitzschioides 8.9

Chaetoceros peruvianum 9.2 Coscinodiscus marginatus 5.1 Chaetoceros peruvianum 6.3

Thalassiosira oestrupii 6.7 Chaetoceros peruvianum 3.9 Coscinodiscus marginatus 5.0

Coscinodiscus marginatus 4.8 Thalassiosira oestrupii 3.7 Thalassiosira oestrupii 5.0

Actinocyclus kutzingii 4.5 Hyalochaete spp. 3.6 Actinocyclus kutzingii 3.7

Thalassiosira sp.1 4.5 Actinocyclus kutzingii 3.2 Thalassiosira sp.1 2.4

Proboscia subarctica 3.8 Ropelia tesselata 1.3 Proboscia subarctica 2.3

Corethron sp. 2.6 Coscinodiscus radiatus 1.2 Hyalochaete spp. 2.0

Thalassiosira eccentrica 2.1 Proboscia subarctica 1.2 Ropelia tesselata 1.7

Ropelia tesselata 2.0 Thalassiosira sp.2 1.1 Corethron sp. 1.6

Thalassiosira trifulta 1.4 Nitzschia bicapitata 1.0 Thalassiosira eccentrica 1.5

Azpeitia tabularis 1.3 Coscinodiscus radiatus 1.3

Coscinodiscus radiatus 1.3 Thalassiosira trifulta 1.0

Thalassiosira lineata 1.3 Thalassiosira lineata 1.0

Stellarima stellaris 1.2

aThe species occurred as resting spores.


around Station 50N was comprised of pelagic taxa.He also suggested that some neritic taxa occasion-ally occurred in the surface waters during summer,which originated from the coast of the Aleutian

Islands or east coast of the Kamchatka Peninsula.However, the correlation coefficients between alu-minum flux and Hyalochaete spp. or the resting sporefluxes at Station 50N were insignificant (Table 2).

ARTICLE IN PRESS

Table 5

Mean fluxes of major diatom taxa for 1998, 1999 and all sampled duration at 3000m depths at three stations

Date Mean diatom flux (� 106 valvesm�2 d�1)

1998 (1 Dec. 1997–31 Dec. 1998) 1999 (1 Jan. 1999–11 Jan. 2000) All sampled duration

Station 50N

Actinocyclus curvatulus 0.29 0.43 0.43

Proboscia subarctica 0.13 0.04 0.08

Thalassiosira nordenskioeldii 0.01 0.11 0.06

Thalassiosira oestrupii 0.33 0.34 0.46

Thalassiosira trifulta group 0.27 0.46 0.40

Neodenticula seminae 14.46 12.75 13.92

Total vegetative valves 17.31 16.27 17.92

Resting spores of Hyalochaete spp. 0.03 0.60 0.45

(1 Jan. 1999 –29 Dec. 1999)

Station KNOT

Hyalochaete 7.46 12.56 8.98

Thalassiosira lineata 0.72 0.53 0.54

Thalassiosira nordenkioeldii 1.35 1.48 1.16

Thalassiosira trifulta group 0.51 0.90 0.78

Fragilariopsis oceanica 0.22 0.63 0.36

Neodenticula seminae 13.19 6.37 9.47

Total vegetative valves 29.19 31.04 28.35

Resting spores of Hyalochaete spp. 8.87 20.03 12.77

(1 Dec. 1997–16 Dec. 1998) (22 May 1999–1 Jan. 2000)

Station 40N

Chaetoceros peruvianum 1.60 1.31* 1.30

Coscinodiscus marginatus 0.84 1.70* 1.02

Thalassioisra oestrupii 1.16 1.14* 1.03

Thalassionema nitzschioides 1.61 2.76* 1.84

Fragilariopsis doliolus 2.73 15.93* 6.78

Neodenticula seminae 3.03 3.12* 2.77

Total vegetative valves 17.35 33.30* 20.63

The numbers with asterisks represent the mean values with only partial seasons due to the malfunction of the sediment trap.


Aluminum flux is considered as an indicator oflithogenic materials (Honda et al., 2002). In general,the lithogenic materials are supplied either byaeolian dust transported in the air or coastalcurrents. If the aluminum flux is truly the indicatorof the terrigenous input, the increase of the coastalspecies in July 1999 may not be caused by theadvection of the coastal waters. The low SSTA atStation 50N in July 1999, �1.4 1C, the lowest duringthe sampled period (Reynolds and Smith, 1994),resulted in weak water column stability. In general,coastal diatoms (some Thalassiosira, Chaetoceros)adapt themselves to relatively unstable environ-ments (Margalef, 1978). The unusual occurrences ofcoastal diatoms at 3000m at Station 50N after July1999 therefore might be due to the exceptionalnutrient supply from the subsurface rather than theadvective influence of the coastal waters.

The annual mean flux of pelagic taxa in 1999 atStation 50N was slightly lower than that in 1998(Fig. 9). The annual mean flux of the coastal taxa in1999 was 11 times greater than in 1998. This mightbe due to the nutrient supply from the subsurfacelayer as mentioned above. However, the mean totaldiatom flux in 1999 was similar to that in 1998because of the increased coastal taxon fluxes,despite the diminished pelagic taxon fluxes in 1999(Fig. 9). Total diatom fluxes during summer 1998were higher than those during summer 1999 (Fig.2B). A similar trend was also observed at StationSA (Takahashi et al., 2002). Sasaoka et al. (2002)reported that the chlorophyll-a concentrationsaround the center of the Western Subarctic Gyreduring summer and fall 1998 were higher thanthose in 1999. This is because the positive SSTAduring summer and fall enhanced the water-column

ARTICLE IN PRESS

Ditylum sp.

0

0.02

0.04

%

0

0.2

0.41998 1999 2000

Thalassiosira nordenskioeldii

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

0

0.2

0.6

4

2

0

%

0.4

1998 1999 2000

3

1

Thalassiosira gravida

3

2

1

0

%

0

0.2

0.4

0.61998 1999 2000

Vegetative Hyalochaete spp.

0

0.2

0.4

0.6 2

1

0

%

1998 1999 2000

Chaetoceros atlanticum

0

2

4

6

%

0

2

4

6

8

101998 1999 2000

Hyalochaete spp. resting spores

0

1

2

3

10

5

0

%

15

20

Thalassiosira trifulta group

0

1

24

2

0

%

1998 1999 2000

Actinocyclus curvatulus6

4

2

0

%

0

1

21998 1999 2000

Thalassiosira oestrupii

0

1

2

3

6

4

2

0%

1998 1999 2000

Neodenticula seminae

0

10

20

30

40

50 100

80

60

40

20

0

%

1998 1999 2000

Station 50N

Flux Relative abundance

D J F JMAM J J A S O N D F MAM J J A S O N D J F MA


4



D J F JMAM J J A S O N D F MAM J J A S O N D J F MAD J F JMAM J J A S O N D F MAM J J A S O N D J F MA





1998 1999 2000

Fig. 3. The fluxes (solid lines) and relative abundances (shaded areas) of major diatom taxa at Station KNOT 3000m during December

1997–May 2000.


stability, in addition to the fact that the negativeSSTA in winter provided a larger amount ofnutrients to the sea surface in spite of the negativewind anomaly (Sasaoka et al., 2002). This sugges-

tion is in harmony with the high diatom fluxesduring summer 1998.

The flux and relative abundance of Chaetoceros

atlanticum significantly increased during spring 2000

ARTICLE IN PRESS

Station KNOT

Tot

al D

iato

ms

[×1

07 val

ves

m-2

d-1

]

600

500

400

300

200

100

0

Tot

al M

ass

and

Opa

l Flu

xes

[mg

m-2

d-1

]

Opal

D J F M A M J J A S O N D J F M A M J J A S O N D J F M A

20

18

16

14

12

10

8

6

4

2

0


25

20

15

10

5

0

SS

T [

° C]

1998 1999 2000

1998 1999 2000

(A)

(B)

Fig. 4. (A) Sea-surface temperature (Reynolds and Smith, 1994); (B) total mass, opal (Honda, 2001), and total diatom fluxes at Station

50N 3000m during December 1997–May 2000. The representation of total mass flux is as in Fig. 2.


(Fig. 3). This taxon occurs in relatively cold waterswith high salinity (Hendey, 1964). The depth of themixed layer in winter 1999/2000 was deepercompared to the previous winters (Honda, 2001;Honda et al., 2002). It is likely that high salinity andfavorable nutrient conditions by active verticalmixing made the relative abundance of Chaetoceros

atlanticum high in spring 2000 (Fig. 3). The fluxes ofthe other abundant species also increased in spring2000 (e.g., Neodenticula seminae, Thalassiosira

oestrupii, Actinocyclus curvatulus). There is a possi-bility that the diatom production in 2000, althougha full data set is not available, could have beenhigher than those obtained in our study period.

At a nearby location, the mean total mass flux at4500m at Station GD (51.51N, 1651E) during1991–1992 was 276.7mgm�2 d�1 (Wong et al.,1995). This value was significantly higher than thatat 5000m at the proximal location of Station 50Nduring December 1997–1999 (91mg�2 d�1) (Hondaet al., 2002). Mean total diatom flux at Station GDduring 1991–1992 (Tsoy and Wong, 1999) wassimilar to that at Station 50N during our sampledperiod. Based on the 8-year record of the amplitudeof total mass and total diatom fluxes at Station SA

during 1990–1998 (Takahashi et al., 2002), thedifference of the mean total mass fluxes betweenStations 50N and GD can be attributed to theamplitude of inter-annual variation. A comparisonalong the 49–501N belt among four stations (50N,SA, PAPA and C) shows that the total diatomfluxes at Station 50N were almost twice those of thetotal diatom fluxes at Stations PAPA and C. Theorganic carbon flux was also high in the westernStation (Honda et al., 2002). However, the primaryproductivity in the upper layer was high in theeastern subarctic Pacific (Honda et al., 2002).Honda et al. (2002) suggested that the high grazingpressure at Station PAPA (Harrison et al., 1999)might explain the low organic carbon flux and thelow ratios of organic carbon flux to primaryproductivity at Station PAPA compared to thoseat Station 50N.

5.2.2. Station KNOT

The diatom floras at Station KNOT wererelatively variable in contrast to those at Station50N, as the water mass structures at Station KNOTare relatively complex compared to the central areaof the WSG (Harrison et al., 1999, 2004).

ARTICLE IN PRESS

Thalassionema nitzschioides

0

1

3

5

%

02468101214

4

2

1998 1999 2000

Chaetoceros furcellatum resting spores

0

10

40

40

20

0

%

30

20

80

601998 1999 2000


Thalassiosira nordenskioeldii

0

2

4

610

5

0

15

20

%

8

101998 1999 2000

Other resting spores of Hyalochaete spp.

0

20

60

4030

20

0

50

40

10

%

1998 1999 2000



0

10

20

30

40

5060

40

20

0

%

Vegetative Hyalochaete spp.

0

20

40

60

30

20

0

50

40

10

%

80


1998 1999 2000

1998 1999 2000

0

1

2

3Thalassiosira oestrupii

0

2

4

6

8

%

1998 1999 2000

0

1

2

3

0

2

4

6

8

10

%

Thalassiosira trifulta group

1998 1999 2000

3

2

1

0 0

2

4

6

%

1998 1999 2000Thalassiosira lineata

Thalassiosira eccentrica

Station KNOT









2

1

0 0

1

2

3

%

1998 1999 2000

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

Fig. 5. The fluxes (solid lines) and relative abundances (shaded areas) of major diatom taxa at Station 50N 3000m during December

1997–May 2000.


The significant occurrences of coastal and sub-tropical taxa at Station KNOT suggest the lateralinput of coastal and subtropical waters, despite thefact that Station KNOT is located in the pelagic

ocean, about 400 km from the nearest coast of theKuril Islands. Hyalochaete spp., which belongs to asubgenus of Chaetoceros, and Thalassiosira nordens-

kioeldii increased during summer to fall, when the

ARTICLE IN PRESS

Table 6

The mean relative abundances (%) of the dominant taxa in each season in the diatom sidocoenosis at Station KNOT 3000m

Seasona 1998 1999 2000

(Dec. 1997–Nov. 1998) (Dec. 1998–Nov. 1999) (Dec. 1999–May 2000)

Winter Spring Summer Fall Winter Spring Summer Fall Winter Spring

Winter

Hyalochaete spp. 27.40 17.72 9.81 21.57 36.21 18.01 28.02 12.66 18.35 12.87

Thalassiosira lineata 2.90 3.23 0.94 0.31 2.86 2.71 0.43 0.20 0.17 0.32

Thalassiosira oestrupii 5.18 2.11 1.67 2.20 3.75 2.11 1.30 1.27 4.43 3.00

Thalassiosira trifulta group 2.17 1.92 1.11 0.85 2.57 2.09 1.64 2.20 4.02 5.01

Spring

Neodenticula seminae 27.65 53.62 34.31 17.00 12.19 33.89 8.05 9.58 13.80 33.86

Rhizosolenia hebetata f. hebetata 0.21 0.86 0.49 0.43 0.38 0.98 0.41 0.47 0.80 3.30

Thalassionema nitzschioides 0.58 1.50 1.37 1.13 2.64 8.16 1.37 1.27 2.63 6.10

Fall

Resting spores of C. furcellatus 4.93 1.19 5.13 3.54 2.05 1.08 26.23 42.20 33.14 8.35

Thalassiosira nordenskioeldii 0.52 0.07 6.78 7.03 0.87 0.12 5.15 5.77 0.87 0.14

Bacterosira bathyomphala 0.03 0.00 0.37 1.03 0.26 0.06 0.58 2.98 1.58 0.43

Rhizosolenia hebetata f. semispina 0.23 0.07 0.29 0.64 0.43 0.37 1.36 1.62 1.30 0.52

Indefinable

Fragilariopsis doliolus 0.16 0.33 0.63 1.36 2.87 3.62 0.82 0.41 1.05 0.17

Fragilariopsis spp. 0.00 0.00 0.00 0.00 0.00 0.00 1.98 0.08 0.00 0.00

Stellarima stellaris 0.06 0.29 0.44 0.30 0.36 2.69 0.68 0.33 0.40 0.47

Thalassiosira eccentrica 0.81 1.66 1.47 1.29 1.79 0.98 0.50 0.43 0.53 1.04

The numbers in bold represent the highest percentages in each year.aWinter: December–February; spring: March–May; summer: June–August; fall: September–November.


sea-surface temperatures were relatively high (Figs.4A and 5). These taxa are known to be essentiallycold-water species in the neritic-littoral waters in thehigh-latitude oceans (Hendey, 1964; Hasle andSyvertsen, 1997). The coastal waters from the regionoff the Kamchatka Peninsula, which had chloro-phyll-a concentrations of approximately 20 mg l�1

(Imai et al., 2002), reached the point of thenortheast 120 km from Station KNOT during May1999 (Sasaoka et al., 2002). Dominant phytoplank-ton taxa in that water were Thalassiosira nordens-

kioeldii and Fragilariopsis oceanica (Mochizukiet al., 2002). On the other hand, the dominantphytoplankton taxa in the surface water at StationKNOT in May 1999 were the pelagic diatomChaetoceros concavicorne and Corethron criophilum

(Mochizuki et al., 2002). The abundance of Chae-

toceros resting spores was low in the upper200m water column in May 1999 (Mochizuki etal., 2002). The high fluxes of Thalassiosira nordens-

kioeldii during July 1999 at 3000m may reflect theinfluence of turbid coastal waters, taking intoaccount the lag time needed for the turbid water

to reach the location of the sediment traps atStation KNOT (Fig. 5). Hyalochaete resting sporesalso significantly increased after June 1999 (Fig. 5).The water mass, which included the spores, mayhave not been the turbid water because this taxonwas minor in the turbid water and the surface waterat Station KNOT in May 1999 (Mochizuki et al.,2002).

Nitrate concentrations at Station KNOT duringsummer, which represented the dominant period forthe coastal taxa, were low in 1998 and 1999(Tsurushima et al., 2002). In addition, the alumi-num flux was high during June–July 1999 (Fig. 8).Total diatom flux at Station KNOT showed highvalues with not only high opal but also with highaluminum fluxes (Table 2). If the aluminum flux isbelieved to be the indicator of coastal waters duringsummer, then it is reasonable to conclude that thehigh diatom fluxes at Station KNOT were affectedby the supply of coastal waters during summerbelow the upper layer. Therefore, as Thalassiosira

nordenskioeldii, and Hyalochaete resting sporesincreased during summer to fall at Station KNOT,

ARTICLE IN PRESS

Station 40N

250

200

150

100

50

0

15

10

5

0

30

25

20

7

6

5

4

3

2

1

0

8

9

1998 1999

Opal

1998 1999

D J F M A M J J A S O N D J F M A M J J A S O N D J


Tot

al D

iato

ms

[×1

07 val

ves

m-2

d-1

]

Tot

al M

ass

and

Opa

l Flu

xes

[mg

m-2

d-1

]

SS

T [

° C]

(A)

(B)

Fig. 6. (A) Sea-surface temperature (Reynolds and Smith, 1994); (B) total mass, opal (Honda, 2001), and total diatom fluxes at Station

40N 3000m during December 1997–May 2000. The total mass flux representation is as in Fig. 2.


it was suggested that a part of these coastal taxawere brought to Station KNOT by the coastalwaters beneath the upper layer. The reason thatthe large diatom fluxes in July 1999 did not reflectSST changes also may be due to the lateral inputof the allochthonous diatom valves. In the sub-arctic Pacific, the mean total diatom fluxes atStations KNOT and SA were significantly higherthan those at other stations (Table 7). Theserelatively high fluxes suggest significant influencesof the coastal waters at Station KNOT and thewaters from the Bering Arc and/or Bering Sea atStation SA (Takahashi et al., 1996), respectively(Fig. 9).

Thalassiosira lineata increased during winter tospring, when the sea-surface temperatures were attheir lowest during the year (Figs. 4A and 5).Fragilariopsis doliolus also increased in winter tospring in 1999. These taxa are known to beessentially warm-water species (Hasle and Syvert-sen, 1997). Favorite et al. (1976) showed that themean latitude of the Subarctic Boundary in winter

was located farther north compared to that in otherseasons. The occurrences of subtropical taxa inwinter might be influenced by subtropical watersthat spun off from the Kuroshio Extension.

5.2.3. Station 40N

The co-occurrences of subarctic and subtropicalwater species at this station are due to its location inthe transitional area. Neodenticula seminae mainlyoccurs in the subarctic North Pacific pelagic ocean(Hasle and Syvertsen, 1997). The abundance ofNeodenticula seminae is low at Station JT, which islocated in the subtropical North Pacific (Table 7).Fragilariopsis doliolus mainly occurs in the subtro-pical, temperate, and coastal waters (Semina, 2003).The temporal change of the dominant species atStation 40N suggests the replacement of the watermasses. The Shannon-Wiener diversity at Station40N is higher than that at Stations 50N and KNOT(Station 40N: H 0 ¼ 1:45322:977 nat; Station KNOT:1.342–2.779nat; Station 50N: 0.477–1.790nat).Radiolarian faunas at Station 40N also showed

ARTICLE IN PRESS

1998 1999

Fragilariopsis doliolus

0

10

20

30

40

50

80

60

40

20

0

%


0

2

4

6

50

40

30

0

%Thalassionema nitzschioides

15

10

5

0

%

0

2

4

Chaetoceros peruvianum

0

1

2

20

10

0

%

Thalassiosira oestrupii

0

1

2

6

4

0

%

8

10

Coscinodiscus marginatus

%

0

5

10

15

20

2560

8

20

10

6

8

20

25

5

4

3

3

2

1998 1999

1998 1999

1998 1999

1998 1999

1998 1999

12

2

1

0

Proboscia subarctica10

8

6

4

2

0

%

1998 1999

1.2

1.0

0.8

0.6

0.4

0.2

0

1998 1999 4

3

2

1

0

%

Nitzschia bicapitata

0

2

4


Station 40N

Rhizosolenia hebetata f. semispina 0.8

0.6

0.4

0.2

0

1998 1999 3

2

1

0

%1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

7

6

5

4

3

2

1

0

1998 1999

Ropelia tesselata%











×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

×106 v

alve

s m

-2 d

-1

Fig. 7. The fluxes (solid lines) and relative abundances (shaded area) of major diatom taxa at Station 40N 3000m during December

1997–January 2000.


significantly high diversity compared to that atStations 50N and KNOT (Okazaki et al., 2005).Miyao and Ishikawa (2003) showed that thelocation of the Subarctic Boundary (employing

contours of sea-surface salinity) along 1651Ewas estimated at 40.41N (70.41) by the CTDobservations from 1996 through 2001. They alsosuggested that the Kuroshio waters flow across the

ARTICLE IN PRESS

Table 7

Comparison of mean diatom fluxes (� 106 valvesm�2 d�1) among sediment trap stations in the middle to high latitude North Pacific and

the Bering Sea

Station Position Depth

(m)

Total

diatoms

N. seminae Hyalochaete

resting spores

Sampled duration References

KNOT 431580N, 1551030E 2957 40.3 9.6 11.42 Dec. 1997–May

2000

This study

(24%) (28%)

50N 501010N, 1651010E 3260 18.7 13.3 0.48 Dec. 1997–May

2000

This study

(72) (3)

40N 401000N, 1651000E 2986 22.0 2.9 + Dec. 1997–Jan.

2000

This study

(13) (0.01)

GDa 511240N, 1651130E 4500 17.2 14.6 — Jul. 1991–Jul. 1992 Tsoy and Wong (1999)

(85) —

GAa 441560N, 1651050E 5330 12.3 9.0 — Jul. 1991–Jul. 1992 Tsoy and Wong (1999)

(74) —

GBa 441440N, 1761520W 5600 1.9 1.4 — Jul. 1991–Jul. 1992 Tsoy and Wong (1999)

(74) —

SA 491000N, 1741000W 4800 29.1 24.1 0.12 Aug. 1990–Aug.

1998

Takahashi (1995),

Takahashi et al. (2002)

(82) (0.4)

AB 531300N, 1771000W 3200 54.7 43.5 1.8 Aug. 1990–Aug.

1998

Takahashi (1997),


(79) (3)

PAPA 501000N, 1451000W 3800 9.0 6.6 0 Sep. 1982–Oct. 1986 Takahashi et al. (2002)

(73) (0)

C 491300N, 1381000W 3500 7.2 4.0 0 May. 1985–Apr.

1986

Takahashi (1997),


(55) (0)

JTb 341110N, 1411580E 8800 — 0 Aug. 1986–May

1987

JTb 341110N, 1411580E 9000 61.4 (0.8) (0) Jun. 1987–Feb.

1988

Tanimura (1992)

JTb 341110N, 1411580E 4000 Mar. 1988–Nov.

1988

Nc 421050N, 1251460W 1000 17.6 Sep. 1987–Apr.

1989

421050N, 1251460W 1500 71.1 ++ (24.8) Apr.–Sep. 1989 Sancetta (1992)

421050N, 1251460W 1000 Sep. 1989–Sep. 1990

Mc 421120N, 1271350W 1000 28.7 ++ 3.9 Sep. 1987–Feb.

1990

Sancetta (1992)

(13.6)

Gc 411330N, 1321000W 1000 9.7 ++ 0 Sep. 1987–Sep. 1990 Sancetta (1992)

Numbers in parentheses represent mean relative abundance of the taxon.

+, the rare occurrence; ++, the occurrence with no flux data in the reference; —, no data.aThe mean fluxes and the relative abundances at Stations GA, GB, and GD were converted from another data unit presented in Tsoy

and Wong (1999).bThe mean fluxes and the relative abundances at Station JT were calculated from the data table in Tanimura (1992).cThe mean fluxes and the relative abundances at Stations N, M, and G were estimated from the figures in Sancetta (1992). Therefore,

these data are rough.


Subarctic Boundary and extend into the SubarcticDomain. The high diversity at Station 40N meansthe influence from both the subarctic and thesubtropical water masses.

The highly dominant period of Neodenticula

seminae at Station 40N suggests that the influenceof the Oyashio waters was relatively significant.Conversely, the dominant period of Fragilariopsis


doliolus suggests that the influence of the Subtropi-cal water was similarly significant. The distributionof Fragilariopsis doliolus in the pelagic oceancorresponds to the frontal zone of the subtropicaland subarctic gyres, where water mixing and highproduction occur (Venrick, 1971; Pokras andMolfino, 1986). The mean of total mass and opalfluxes at 5000m in 1999, when the samples werecollected throughout the year, were significantlyhigher than that of 1998 (Honda et al., 2002). TheSSTA in 1999 was approximately 0 1C or in apositive phase (Reynolds and Smith, 1994). The

mg

m-2

d-1


Aluminum Flux at 3000 m at Station KNOT

1998 1999 2000

4

3

2

1

0

Fig. 8. Aluminum fluxes at 3000m at Station KNOT (Honda,

2001).

200

150

100

50

0

157.4171.5

mg

m-2

d-1

200

150

100

50

0

50

40

30

20

10

0

40

30

20

10

0

50

40

30

20

10

0

13.9

7.1

19.4

29.1

×106

valv

es m

-2 d

-1

Tot

al D

iato

ms

Pela

gic

or C

oast

al/

Tra

nsiti

onal

Dia

tom

s

Total Mass Total Diatoms

Station KNOT Statio

Tot

al M

ass

(1Dec.1997 - 31Dec.1998)(1Jan.1999 - 11Jan.2000)

104.1

(1Dec.1997 - 31Dec.1998

39.344.8

16.9

40

30

20

10

0

14.5

0.1

1998

1998

1998

1998 1999

1998 1999

1998 1999

×106

valv

es m

-2 d

-1

×106

valv

es m

-2 d

-1×1

06 va

lves

m-2

d-1

mg

m-2

d-1

(A)

(B)

(C)

Fig. 9. Inter-annual variations of mean fluxes: (A) total mass; (B) total

and littoral), and Transitional Domain diatom taxa.

surface wind speeds during April 1999 by SSM/Idata were higher than those of April 1998. If the fastwind might have made the mixing strong, the highnutrient conditions in the upper layer at Station40N in 1999 could support significantly high fluxesand relative abundances of Fragilariopsis doliolus.

Acknowledgments

We thank the captain, crew, and scientists onboard R.V. Mirai of Japan Agency for Marine-Earth Science and Technology with respect to theoceanographic observations and the recoveries ofthe sediment trap samples used in this study. Inaddition, we are grateful to the assistance providedby the members of the Paleoenvironmental SciencesLab. at Graduate School of Sciences of KyushuUniversity.

A part of this research has been supported byProf. Tatsuro Matsumoto Scholarship Funds to JO.This study was also partially supported by thefollowing research programs of the Japan Societyfor the Promotion of Science: Projects, B1 No.13440152, B2 No. 15310001, and B No. 17310009.

1998 1999

Pelagic Taxa Coastal & Transitional Taxa

n 50N Station 40N

40

30

20

10

0

50

200

150

100

50

0

116.596.0 102.8

)(1Jan.1999 - 29.Dec.1999) (1Dec.1997 - 31Dec.1998)(22May1999 - 1Jan.2000)

17.7 16.4

33.1

40

30

20

10

0

13.7

1.15.7

2.95.8

17.1

1998 1999

1998 1999

1999

1999

1999

×106

valv

es m

-2 d

-1×1

06 va

lves

m-2

d-1

mg

m-2

d-1

diatoms; and (C) flux sums of the pelagic diatom, coastal (neritic


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