Living diatom assemblages from North Pacific and Bering Sea surface waters during summer 1999

ARTICLE IN PRESS

Deep-Sea Research II 52 (2005) 2186–2205 www.elsevier.com/locate/dsr2

Living diatom assemblages from North Pacific and Bering Sea surface waters during summer 1999 Chieko Aizawa1, Maiko Tanimoto2, Richard W. Jordan Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, Yamagata 990 8560, Japan Received 8 April 2005; accepted 1 August 2005 Available online 20 October 2005

Abstract Diatom assemblages collected during July–August 1999 along an E–W transect comprising sixty-two North Pacific and Bering Sea surface water samples were identified and enumerated. Absolute abundance profiles of centric and raphid pennate diatoms show that the former group dominates in shallow coastal waters and through island passes, whilst the latter group, though less numerous, dominates the oceanic regions. This pattern is interpreted as nutrient-related, with centric diatoms (especially Chaetoceros (section Hyalochaete), Minidiscus and Thalassiosira spp.) preferring eutrophic and upwelling conditions, and raphid pennate diatoms (especially nitzschioid genera like Fragilariopsis, Neodenticula and Pseudo-nitzschia) preferring oligotrophic pelagic conditions. The absolute abundances of the two sections of Chaetoceros (Chaetoceros (ex Phaeoceros) and Hyalochaete) and some species belonging to the Thalassiosirales and Bacillariaceae show distribution patterns that can be interpreted as preferences for shallow or oceanic waters, subtropical or subarctic waters, or ice-related/arctic conditions. r 2005 Elsevier Ltd. All rights reserved. Keywords: Bering Sea; Diatoms; North Pacific

1. Introduction 1.1. Surface water hydrography The subarctic North Pacific is characterized by a counterclockwise gyre (Fig. 1). The Subarctic Corresponding author. Tel.: +81 23 628 4645; fax: +81 23 628 4661. E-mail address: [email protected] (R.W. Jordan). 1 Present address: Center for Deep Earth Exploration, Yokotsuka 237 0061, Japan. 2 Present address: Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060 0810, Japan.

0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.08.008

Current flows W–E approximately along the line of 401N, but on reaching the American continent it separates into the southward-flowing California Current and the northward-flowing Alaska Current. The Alaska Current eventually runs E–W along the Aleutian Islands as the Alaskan Stream; however, waters exiting from the southwestern Bering Sea, close to the Asian continent, mix with the Alaskan Stream to become the East Kamchatka Current. The East Kamchatka Current then mixes with waters exiting the Sea of Okhotsk to form the southward-flowing Oyashio (Ohtani, 1989; Kono and Kawasaki, 1997), which later converges with the northern arm of the Kuroshio to become the Subarctic Current. This circulation pattern is more

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150°

E

180°

65°N

Anadyr R.

150°

120°

W 65°N

Gulf of Anadyr

Yukon R.

BERING SEA

Komandorsky Basin

60°

2187

60°

Kuskokwim R.

Be

rin

SEA OF OKHOTSK

55°

gS

lo

Aleutian Basin

pe

55°

C. Alaska C.

Ka Eas mc t hat ka C.

50° 45°

Unimak Pass

50° Gulf of Alaska

45°

Alaskan Stream

Oyashio C.

40°

40°

Subarctic C.

JAPAN SEA

California C.

35°

35°

PACIFIC OCEAN Kuroshio C.

30°

30° 150°E

180°

150°

120°W km 0 400

Fig. 1. General circulation pattern of surface water currents in the North Pacific and Bering Sea.

complex than drawn in Fig. 1, as the Subarctic and Subtropical Gyres both comprise two separate gyres, called the Alaskan Gyre and Western Subarctic Gyre (Favorite et al., 1976), and the Northwestern Subtropical Gyre and Northeast Subtropical Gyre (Roden et al., 1982), respectively. Alaska Coastal Current waters, which follow the Gulf of Alaska coastline, enter the southeastern Bering Sea through Unimak Pass (Stabeno et al., 1995). These waters initially travel northeastwards close to the Aleutian Islands, continuing northwestwards across the shallow eastern shelf (i.e. along the western Alaskan coast), before exiting through the Bering Strait. Parts of the Alaskan Stream enter the Bering Sea through many of the 14 major passes, especially three deep central and western passes (Kamchatka, Near and Amchitka Straits) and several shallower passes (Buldir and Amukta Passes) (Stabeno et al., 1999). Waters mainly entering through the Amchitka Pass form the eastward-flowing current, the Aleutian North Slope Current (Reed and Stabeno, 1999), which turns northwestward across the shelf break as a slightly modified current, the Bering Slope Current. Some of these off-slope waters eventually flow northwards along the Siberian coast to form a western boundary current, which enters the Gulf of

Anadyr and passes into the Chukchi Sea through the Bering Strait. Some of the Alaskan Stream waters pass into the Aleutian Basin through Near Strait. These northward-flowing waters combine with westward-flowing waters from the Bering Slope Current to form a western boundary current, the Kamchatka Current (Stabeno et al., 1994), which then flows southwards along the Kamchatka Peninsula in the western part of the Komandorsky Basin ( ¼ Kamchatka Basin). This current exits the Bering Sea through the deep Kamchatka Strait and merges with Alaskan Stream waters to form the East Kamchatka Current. 1.2. Regional diatom studies Diatom studies in the Bering Sea began about 150 years ago with a report by Bailey (1856) on the soundings from the Sea of Kamchatka, followed by examinations of planktonic diatoms collected during the S/S Vega Expedition (Cleve, 1883), and of sediments during the ‘‘Albatross’’ Expedition (Mann, 1907). Around this time Japanese coastal diatoms also were being intensively studied (Yendo, 1905; Okamura, 1911; Akatsuka, 1914; Gran and Yendo, 1914). Throughout the rest of the 20th Century, diatomists continued to gather data and to

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improve the regional diatom taxonomy (e.g., Cupp, 1943; Takano, 1990). Some of these studies were restricted in their geographic coverage, e.g., the fjords of Vancouver Island (Shim, 1977; Takahashi et al., 1977; Sancetta, 1989, 1990; McQuoid and Hobson, 1998; Hobson and McQuoid, 2001; Hay et al., 2003) or Tokyo Bay (Yanagisawa, 1940; Yamazi, 1955; Kadota and Hirose, 1967; Han et al., 1989), whilst others covered much larger areas of the open ocean (Allen, 1927, 1929, 1930, 1943; Jouse´ and Semina, 1955; Kawarada, 1957; Iizuka and Tamura, 1958; Ohwada and Kon, 1963; Marumo, 1967; Karohji, 1972; Motoda and Minoda, 1974; Oshite and Sharma, 1974). Despite this, long-term phytoplankton studies, including diatoms, were not established in the North Pacific until Canadian weatherships began visiting Station P (501N, 1451W) in the Alaskan Gyre between 1956 and 1981 (McAllister et al., 1960; Stephens, 1977; Booth et al., 1982; Clemons and Miller, 1984; Horner and Booth, 1990). A sediment trap study initiated during the early 1980’s at the same location as Station P (sometimes called Ocean Station PAPA; OSP) provided much needed seasonal and annual flux data (Takahashi, 1986, 1987; Takahashi et al., 1989). A subsequent long-term sediment trap study of two sites, AB and SA, located in the Aleutian Basin and central Subarctic Gyre, respectively, also has provided a wealth of microplankton seasonality data (Takahashi et al., 2002). However, a similar sampling programme on the western side of the subarctic Pacific was only recently established at Station KNOT (Mochizuki et al., 2002; Onodera et al., 2003; Komuro et al., 2005). Although many diatom data have been collected in the North Pacific and Bering Sea over the last 150 years, modern taxonomic surveys along extensive transects are rare, and often no attempt has been made to correlate individual species distributions to surface water hydrography. As a consequence, marine diatom ecology seriously lags behind that of its freshwater counterpart and that of other marine phytoplankton groups such as coccolithophorids. The aim of the KH99-3 cruise in the summer of 1999 was to take a series of piston cores and site survey data to support an on-going Ocean Drilling Program proposal to drill in the Bering Sea. One of the authors (RWJ) took this opportunity to collect surface water samples during the crossing of the North Pacific via the Bering Sea, with the primary aim of providing basic ecological and biogeographic

data to aid our subarctic piston core analyses. The general microplankton dataset already has been published by Tanimoto et al. (2003); however, in that paper the diatom data were presented only as total centric and total pennate diatoms. So here we report on selected diatom species abundances in more detail and discuss their oceanographic significance.

2. Methods During leg 3 (from Seattle to Tokyo; 29 July–25 August 1999) of the KH99-3 cruise of the R/V Hakuho Maru, surface water samples were collected approximately every 6 hours using the on-board continuous seawater supply obtained from several meters below the sea surface. Of the 62 samples taken by this method (Fig. 2, Table 1), 48 are from the North Pacific (NP) and 14 from the Bering Sea (B). Four liters of seawater were collected in two plastic 2-l water bottles. Temperature, salinity and conductivity were measured using a continuous salinity temperature recorder (Union-Denshi STMK-11-10) connected to the ship’s seawater supply (Table 2), while water depth, GMT and shipboard times and dates were noted from the ship’s display monitor (Table 1). Silicate, nitrate and chlorophyll a concentrations were not determined for the above samples, but mean values for August (or summer) were obtained from the NOAA website (Fig. 3). Each surface water sample was filtered through a 47-mm diameter, 0.45-mm porosity, Millipore HAtype polycarbonate filter using an Eyela Aspirator A-3S (Tokyo Rikakikai Co., Ltd.) filtration apparatus. The filters were not washed on-board the ship, but were immediately air-dried and stored in Millipore plastic Petri slides. Later, each filter was washed individually by being submerged in tap water for a few minutes to remove the salt crystals, and then air-dried again. A small portion (5 or 3 mm2) of each filter was cut out and glued onto an aluminium stub, coated with Pt/Pd in an Eiko IB-3 ion sputter coater, and placed in a Hitachi S-2250N SEM. Each filter portion was completely analysed for microplankton (including the diatoms), and where possible, counts were made at the species level for each microplankton group. Numbers of microplankton (in cell l1) were calculated using the method outlined in Jordan and Winter (2000).

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150°

180°

2189

150°

120°

65°

65°

60°

60°

55°

B5 B1 B14 (AB)

B10

50°

NP15

N

50°

55°

NP19 NP20 (SA)

NP25

45° NP30 40° NP40

NP48

NP10

45°

NP5

NP1 40°

NP35

35°

35° NP45

30°

30° 150°

180° E

150° W

120° km 0

400

Fig. 2. Location of surface water samples collected during KH99-3. AB and SA refer to the two sediment trap sites of Takahashi et al. (2002).

3. Results To make it easier for the reader, the station numbers are presented in such a way as to reflect their geographic position, i.e. with the west and east Pacific on the left- and right-hand side of the figures, respectively. At the top of each figure are a number of oceanographic regimes that were tentatively assigned by Tanimoto et al. (2003) using the data presented in Fig. 4. These zones are similar to those presented by Dodimead et al. (1963). 3.1. Hydrographic data Low-salinity waters associated with shallow water depths at NP1 and NP46–48, i.e. on both sides of the Pacific, are considered here to represent Washington (WCW) and Japanese (JCW) coastal waters, respectively. The warm salty waters off the eastern coast of Japan (NP36–45) are indicative of the western subtropical zone, whilst cooler, lesssaline waters south of the Aleutian Islands belong to the subarctic zone, separated here into their western (NP18–31) and eastern (NP5–14) components. Transitional waters are assigned to the western (NP32–35) and eastern (NP2–4) temperate zones.

The ship sailed through the shallow passes between the Aleutian Islands on two occasions, entering the Bering Sea through the eastern Aleutian Islands (EAI; NP15, NP16) and returning to the Pacific through the central Aleutian Islands (CAI; NP17). It should be noted that there was a small but significant drop in salinity at NP15, NP16, whilst a temperature drop was recorded at NP17. The conditions in the Bering Sea did not appear to be very different from those outside in the subarctic Pacific, but based on the bathymetry the samples were assigned to either the shelf (B1–3) or the Aleutian Basin (B4–14). 3.2. General diatom abundance The total diatom abundance along the KH99-3 transect shows several peaks, most noticeably in the western and eastern subarctic zones and in Tokyo Bay, Japan (Fig. 5A). The latter peak is by far the largest, with up to 8.8  105 cell l1. The diatom component was separated into the contributions made by centric, raphid and araphid diatoms; however, as the latter group rarely exceeded 2000 cell l1, their abundance profile was omitted from Fig. 5. The centric diatoms are clearly

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Table 1 Co-ordinates of each sampling station and the depth of the underlying sea-bed Station

Date

Latitude

Longitude

Depth of seabed (m)

NP1 NP2 NP3 NP4 NP5 NP6 NP7 NP8 NP9 NP10 NP11 NP12 NP13 NP14 NP15 NP16

30-7-99 30-7-99 30-7-99 31-7-99 31-7-99 31-7-99 31-7-99 1-8-99 1-8-99 2-8-99 2-8-99 2-8-99 3-8-99 3-8-99 3-8-99 3-8-99

48134.84N 48149.53N 49104.36N 49114.59N 49124.86N 49135.36N 49142.22N 49152.55N 49159.88N 50132.48N 51115.16N 51153.31N 52140.05N 53115.22N 53158.62N 54119.84N

125141.48W 128158.38W 131147.09W 134112.22W 136138.09W 139106.96W 140144.88W 143113.45W 144159.03W 147111.77W 150107.18W 152146.33W 156104.15W 158141.53W 162132.35W 164151.52W

69 2460 3017 3416 3854 3548 3904 4110 4266 4486 4687 4581 4517 4788 965 62

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14

4-8-99 4-8-99 4-8-99 5-8-99 5-8-99 5-8-99 6-8-99 6-8-99 8-8-99 8-8-99 9-8-99 9-8-99 10-8-99 11-8-99

54123.20N 54120.70N 54125.32N 54151.58N 55157.33N 57120.21N 57138.72N 58148.26N 56145.30N 54147.06N 54102.29N 54102.18N 53123.46N 53130.46N

164151.05W 169131.77W 170113.79W 171120.76W 173123.84W 176103.61W 175152.24W 179104.16W 178132.09E 176154.93E 178140.63E 179101.19E 179133.48W 176159.80W

738 2834 1895 3395 3363 3013 2161 3472 3870 884 2389 2169 1291 3811

NP17 NP18 NP19 NP20 NP21 NP22 NP23 NP24 NP25 NP26 NP27 NP28 NP29 NP30 NP31 NP32 NP33 NP34 NP35 NP36 NP37 NP38 NP39 NP40

11-8-99 12-8-99 12-8-99 12-8-99 13-8-99 13-8-99 13-8-99 13-8-99 14-8-99 14-8-99 14-8-99 15-8-99 15-8-99 15-8-99 16-8-99 16-8-99 19-8-99 19-8-99 19-8-99 20-8-99 20-8-99 21-8-99 21-8-99 21-8-99

51134.06N 50119.76N 48159.62N 49110.66N 49115.90N 49121.53N 49127.33N 49132.76N 49139.45N 49144.73N 49153.53N 48137.31N 47139.72N 46118.63N 45115.51N 44112.83N 43109.69N 42109.63N 41106.95N 40132.50N 39145.54N 39120.66N 38145.20N 38101.03N

177102.77W 175133.13W 173159.84W 177104.21W 179126.13W 177159.51E 175121.61E 172151.44E 169148.40E 168118.95E 166123.93E 162135.87E 160157.84E 158142.83E 157100.06E 155119.83E 156134.68E 158111.05E 159151.01E 158108.00E 155142.58E 154126.24E 152138.13E 150124.87E

shallow 6217 5434 5700 5323 4725 5154 4701 5572 2389 5446 5725 5433 5044 5488 5361 5447 5504 5610 5542 5443 5630 5642 5695

Table 1 (continued ) Station

Date

Latitude

Longitude

Depth of seabed (m)

NP41 NP42 NP43 NP44 NP45 NP46 NP47 NP48

21-8-99 22-8-99 22-8-99 22-8-99 23-8-99 23-8-99 24-8-99 24-8-99

37122.10N 36159.78N 36133.85N 35148.89N 35109.78N 35100.11N 35104.30N 35124.61N

148128.13E 147121.89E 146106.74E 143154.02E 142119.79E 139128.32E 139138.39E 139147.14E

5728 5573 5514 5819 8740 1383 230 31

responsible for the huge peak seen in Tokyo Bay (NP48; Fig. 5B), whilst the contribution of the raphid diatoms is negligible (Fig. 5C). Although the peak off the Washington coast (NP1) is much smaller than that in Tokyo Bay, the pattern is similar, with centric diatoms dominating. On the other hand, the peaks in the western and eastern subarctic zones are largely produced by the raphid diatoms, with the centric diatoms becoming important only in the vicinity of the Aleutian passes and in the southeastern Bering Sea. In this study, over 70 species of centric diatoms and about 15 species of pennate diatoms were identified. 3.3. Species abundances in each zone 3.3.1. Washington coastal waters This region was only represented by a single station (NP1) but was characterized by high abundances (41  104 cell l1) of Thalassiosira oceanica Hasle, Chaetoceros debilis Cleve, and Minidiscus spp. (M. chilensis Rivera and M. trioculatus (F.J.R. Taylor) Hasle). Other species of Thalassiosira (T. eccentrica (Ehrenberg) Cleve, T. nordenskioeldii Cleve, T. pacifica Gran and Angst, and T. proschkinae Makarova) were common, while pennate diatoms were low in abundance. 3.3.2. Eastern Temperate Zone The surface water temperature in this region was slightly higher than in the Subarctic Zone, and so the stations (NP2–NP4) were assigned to the Temperate Zone. Many of the species found in the Washington coastal waters were also present in these transitional waters, but their numbers were drastically reduced. However, some taxa such as Corethron pennatum (Grunow) Ostenfeld, Coscinodiscus marginatus Ehrenberg, Fragilariopsis

ARTICLE IN PRESS C. Aizawa et al. / Deep-Sea Research II 52 (2005) 2186–2205 Table 2 Physico-chemical data and volume filtered for each sample

2191

Table 2 (continued ) Station Volume filtered (l) Temperature (1C) Salinity (PSU)

Station Volume filtered (l) Temperature (1C) Salinity (PSU) NP1 NP2 NP3 NP4 NP5 NP6 NP7 NP8 NP9 NP10 NP11 NP12 NP13 NP14 NP15 NP16

3.5 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 2.7 4.0 3.8 4.0

12.05 14.67 13.41 12.40 11.69 11.99 12.10 12.14 12.31 12.50 12.28 11.83 11.84 11.07 9.17 7.87

29.94 32.00 32.45 32.49 32.46 32.47 32.48 32.49 32.48 32.47 32.62 32.59 32.47 32.50 31.94 31.72

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

8.31 7.85 8.55 8.26 8.38 8.02 8.27 7.78 8.40 8.68 8.46 8.26 8.43 8.56

32.56 32.69 32.45 32.36 32.65 32.54 32.55 32.69 32.69 32.78 32.80 32.76 32.68 32.73

NP17 NP18 NP19 NP20 NP21 NP22 NP23 NP24 NP25 NP26 NP27 NP28 NP29 NP30 NP31 NP32 NP33 NP34 NP35 NP36 NP37 NP38 NP39 NP40 NP41 NP42

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

6.48 9.37 9.72 9.18 9.17 9.00 8.71 9.22 9.43 9.48 9.71 10.03 10.51 10.58 12.09 15.67 20.23 18.13 19.18 25.04 26.09 25.66 27.36 27.77 28.41 28.64

32.86 32.22 32.29 32.40 32.30 32.42 32.42 32.44 32.40 32.43 32.57 32.64 32.53 32.50 32.43 32.32 33.00 32.68 33.11 34.20 34.11 33.88 34.24 33.32 33.93 34.03

NP43 NP44 NP45 NP46 NP47 NP48

4.0 4.0 4.0 4.0 3.0 1.4

27.62 29.31 28.92 26.80 27.45 28.21

33.38 34.04 33.96 33.36 32.78 24.80

pseudonana (Hasle) Hasle, and Pseudo-nitzschia spp. increased in abundance. F. pseudonana was the most dominant diatom with abundances 41  104 cell l1. 3.3.3. Eastern Subarctic Zone The assemblage of this region (NP5–NP14) was characterized by high abundances of F. pseudonana (sometimes 42  105 cell l1), with important yet lesser contributions by Pseudo-nitzschia spp. and a Thalassiosira sp., and in the north by Neodenticula seminae (Simonsen & Kanaya) Akiba & Yanagisawa. Other centric diatoms were low in abundance. 3.3.4. Eastern Aleutian Islands This zone was represented by two stations (NP15, NP16), which were characterized by small centric diatoms such as T. oceanica, Minidiscus spp. and Chaetoceros resting spores. The pennate diatom F. pseudonana was also common. There were also higher abundances of larger centric diatoms, such as Leptocylindrus danicus Cleve, Rhizosolenia spp., and Skeletonema sp., than in the waters to the south. The presence of the resting spores suggests that growing conditions for the vegetative stages became unfavourable. 3.3.5. Bering Sea In this study, the Bering Sea stations were separated into the eastern shelf region (B1–B3) and the Aleutian Basin (B4–B14). The shelf assemblage was characterized by Thalassiosira spp. (e.g., T. trifulta Fryxell, T. conferta Hasle, and T. gravida Cleve), Chaetoceros contortus Schu¨tt, and C. debilis. Although low in abundance, C. diadema (Ehrenberg) Gran was generally found over the shelf or close to the Aleutian Islands. The Aleutian Basin assemblage was similar to the shelf assemblage, but the centric diatoms were lower in abundance while pennate diatoms such as

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Longitude 150˚E

(A)

180˚

150˚W

120˚W

50˚N

Latitude

60˚N

40˚N 30˚N

0

10

20 mean August silicate (µM/l)

30

Longitude (B)

150˚E

180˚

150˚W

120˚W

50˚N

Latitude

60˚N

40˚N 30˚N 0

(C)

5

10 mean August nitrate (µM/l) Longitude 180˚

150˚E

150˚W

15

120˚W

50˚N

Latitude

60˚N

40˚N 30˚N 0

2

4 8 6 mean summer chlorophyll (µM/l)

10

Fig. 3. Distributions of (a) mean August silicate, (b) mean August nitrate, and (c) mean summer chlorophyll in the study area. Maps plotted using data from http://www.nodc.noaa.gov./OC5/WOA01F/

F. pseudonana, N. seminae and Pseudo-nitzschia spp. dominated. 3.3.6. Central Aleutian Islands Only one station (NP17) has been assigned to this zone, but it represents one of the highest total

abundances in this study. The centric diatoms dominated this station (2.5  105 cell l1), presumably due to upwelling conditions caused by the low sill of the Aleutian Islands. Chaetoceros spp. and Thalassiosira pacifica, in particular, show marked increases at this station. For instance, C. debilis

ARTICLE IN PRESS C. Aizawa et al. / Deep-Sea Research II 52 (2005) 2186–2205 Western Subtropical Zone

WTZ

Western Subartic Zone

C A I

Bering Sea Aleutian Basin

EE A A Shelf II

Gulf of Alaska

E T Z

W C W

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

J C W

2193

(A) 30

Temperature (˚C)

25

20

15

10

(B) 34 33 Salinity (PSU)

32 31 30 29 28 27 26 25 (C)

Depth of sea-bed (m)

1000 2000 3000 4000 5000 6000 7000

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

8000

Station number Fig. 4. Profiles of (a) surface-water temperature, (b) surface-water salinity, and (c) bathymetry along the KH99-3 transect. The zonation scheme is explained in the text.

reached 1.8  105 cell l1, with C. contortus, C. decipiens Cleve, and C. radicans Schu¨tt also making noteworthy contributions.

3.3.7. Western Subarctic Zone This zone was dominated by F. pseudonana (104–105 cell l1), and to a lesser degree by other

ARTICLE IN PRESS C. Aizawa et al. / Deep-Sea Research II 52 (2005) 2186–2205 J C W

(A)

Western Subtropical Zone

WTZ

Western Subartic Zone

C A I

Bering Sea Aleutian Basin

EE A A Shelf II

Gulf of Alaska

E T Z

W C W

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

2194

900000 total diatoms

800000 700000 cell l-1

600000 500000 400000 300000 200000 100000 (B) total centric diatoms

800000 700000 cell l-1

600000 500000 400000 300000 200000 100000 (C) 300000

total raphid diatoms

100000 1000 2000 3000 4000 5000 6000 7000 8000 NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

Depth of sea-bed (m)

200000

Station number Fig. 5. Absolute abundance profiles of (a) total diatoms, (b) centric diatoms, and (c) raphid pennate diatoms. Bathymetry and zonation scheme same as in Fig. 4.

pennate diatoms such as Fragilariopsis spp., Neodenticula seminae, and Pseudo-nitzschia spp., and centric diatoms such as pelagic Chaetoceros spp. (e.g., C. atlanticus Cleve, C. concavicornis Mangin, and C. convolutus Castracane) and some Thalassiosira spp. (e.g., T. oceanica).

3.3.8. Western Temperate Zone Apart from a higher abundance of Thalassiosira lineata Jouse´, the assemblage was similar to that of

the Western Subarctic Zone, although significantly reduced in number. 3.3.9. Western Subtropical Zone The average total diatom count in this zone was very low (o104 cell l1), presumably due to the permanently stratified and nutrient-poor waters. These waters were characterized by pennate diatoms such as Nitzschia bicapitata Cleve and Mastogloia spp., with a few species of the centric diatom genera Bacteriastrum and Chaetoceros probably representing

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a tychoplankton assemblage laterally transported from the Japanese coast. F. pseudonana and Pseudonitzschia spp. were also present in significant numbers, but less than found in more northerly waters. 3.3.10. Japanese coastal waters The temperature, salinity and sea-bed depth of Sagami Bay (NP46, NP47) suggest a more pelagic environment than at Tokyo Bay (NP48). Numerically important species included Skeletonema grevillei Sarno & Zingone, Bacteriatrum spp., and several Chaetoceros spp. (including C. affinis Lauder, C. contortus, C. debilis, C. pseudocurvisetus Mangin, and C. laciniosus Schu¨tt). The highest abundances of Detonula pumila (Castracane) Gran were found at NP46, while Pseudo-nitzschia spp. were common at NP47. In Tokyo Bay, several Thalassiosira species were numerous, with contributions from T. allenii Takano (7.4  105 cell l1), T. binata Fryxell (5.3  104 cell l1), T. lundiana Fryxell (1.8  104 cell l1), and T. tenera Proschkina-Lavrenko (1.3  104 cell l1). These species were rarely encountered at other stations. S. grevillei and Chaetoceros pseudocurvisetus were also important, and C. didymus Ehrenberg was only found at this station. Pennate diatoms were especially low in abundance at NP48. 4. Discussion 4.1. Total diatom absolute abundance Marumo (1967) mapped the distribution and abundance of diatoms for the North Pacific, Sea of Okhotsk and Bering Sea from summer samples collected over a number of years by various Japanese research vessels. He showed that the subarctic North Pacific and marginal seas had much higher values (104–106 cell l1) than the subtropicaltemperate waters to the south (generally 102–103 cell l1), except for samples taken from Japanese and American coastal waters (104–105 cell l1). Inshore waters generally have higher nutrient levels than offshore (as seen off Vancouver Island; Whitney and Freeland, 1999) and thus have the potential to support larger phytoplankton populations. The July–August data presented in Fig. 5A strongly support this general picture, with most of our subarctic and coastal samples containing 104–105 diatom cell l1, but with a significant drop (103 cell l1; NP33–45) in absolute abundance in subtropical-temperate waters. The low abundances

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found in the subtropical waters of the Kuroshio are thought to be typical for the whole year (Marumo, 1967), whereas the relatively high abundances further to the north suggest that the subarctic waters are productive even during summer (see also the mean silicate values for August in Fig. 3A). The seemingly year-round diatom production, the high silicate concentrations in the deep waters (which exceed 230-mM; Coachman et al., 1999), and the fact that the underlying sediments are diatomaceous oozes are the main reasons why the Bering Sea is nicknamed the ‘‘sea of silica’’ (Tsunogai et al., 1979). On the other hand, recent studies have suggested that both the Gulf of Alaska and the western Bering Sea are high nutrient (nitrate), low chlorophyll (HNLC) regions (Miller et al., 1991; Banse and English, 1999; Boyd and Harrison, 1999; Shiomoto et al., 2002), but the East Kamchatka Current, Alaskan Stream, waters around the Aleutian Islands, and eastern Bering Sea continental shelf are not, because the phytoplankton populations show seasonality (Banse and English, 1999). The total microplankton and total diatom abundances from the KH99-3 dataset would appear generally to support this idea (Tanimoto et al., 2003), although seasonality does occur at Station KNOT in the Western Subarctic Gyre (Komuro et al., 2005). In the present study, it can be seen that the peaks in diatom abundance are associated with waters close to the Aleutian Islands, either in the Alaskan Stream or in the Bering Sea, and concomitant changes in species composition would appear to represent improved growth conditions (perhaps increased nutrient concentrations associated with upwelling around the shallow sill of the Aleutian Islands). Indeed, Koike et al. (1982) noted that chlorophyll a was inversely correlated with nitrate when they passed through Unimak Pass, suggesting rapid growth of phytoplankton. They also recorded a change in the composition of the diatom assemblage along the transect, with Chaetoceros-Hyalochaete as the dominant group on the Bering Sea side of the pass. Fig. 6B shows that we encountered a similar increase in Hyalochaete after we passed through the Aleutian Islands and entered the southeastern Bering Sea. 4.2. Diatom species composition 4.2.1. NW American coast The diatom assemblage found off the Washington coast (NP1) was partly similar to those assemblages

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(A)

cell I-1

Western Western Subtropical Subtropical Zone Zone

WTZ WTZ

Western Western Subartic Subartic Zone Zone

CC AA II

Bering BeringSea Sea Aleutian AleutianBasin Basin

E E A A Shelf Shelf II

Gulf Gulfof of Alaska Alaska

EE TT ZZ

W W C C W W

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

JJ CC W W

10000 9000 8000 7000 6000 5000 4000 3000 2000 1000

(B) 40000

Chaetoceros (ex Phaeoceros)

Hyalochaete

approx. 250000

cell I-1

30000 20000 10000

(C)

Bacteriastrum

6000

cell I-1

5000 4000 3000 2000

1000 2000 3000 4000 5000 6000 7000 8000 NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

Depth of sea-bed (m)

1000

Station number Fig. 6. Absolute abundance profiles of the two sections of Chaetoceros, (a) Chaetoceros (ex Phaeoceros) and (b) Hyalochaete, and (c) Bacteriastrum spp. Bathymetry and zonation scheme same as in Fig. 4.

found by previous workers, although there were no neritic surf-blooming species (cf. Lewin, 1973) and only low numbers of toxin-producing Pseudonitzschia spp. (cf. Horner and Postel, 1993; Horner, 2001; Wekell et al., 2002). The individual species

abundances were generally low, perhaps suggesting that NP1 is more oceanic and not much influenced by coastal processes. Hay et al. (2003) reported the dominance of Skeletonema costatum (Greville) Cleve, Thalassiosira nordenskioeldii, and T. pacifica

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Gulf of Alaska

E T Z

W C W

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J C W

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(A)

cell I-1

32000

Minidiscus spp.

24000 16000 8000

cell I-1

(B)

T. oestrupii 1600 800 T. gravida

cell I-1

(C) 1600 800

cell I-1

(D)

T. conferta 1600 800

cell I-1

(E)

Detonula spp. 1600 800

cell I-1

(F)

Skeletonema spp.

12000 8000

1000 2000 3000 4000 5000 6000 7000 8000 NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

Depth of sea-bed (m)

4000

Station number Fig. 7. Absolute abundance profiles of some members of the Thalassiosirales: (a) Minidiscus spp., (b) Thalassiosira oestrupii, (c) T. gravida, (d) T. conferta, (e) Detonula spp., and (f) Skeletonema spp. Bathymetry and zonation scheme same as in Fig. 4.

in surface sediments from Effingham Inlet (a western fjord of Vancouver Island, Canada), while Sancetta (1989, 1990), using sediment traps over a 5-year period, found that the early spring bloom was dominated by species of Minidiscus and

Thalassiosira. NP1 lies just south of Effingham Inlet with a surface assemblage in late July dominated by Minidiscus spp. (Fig. 7A), Chaetoceros debilis, and T. oceanica. The Minidiscus spp. were pooled in our diatom counts due to taxonomic

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problems, but a relative abundance count conducted later showed that M. trioculatus and M. chilensis represented 61.2% and 38.5% of the Minidiscus population, respectively (Aizawa & Jordan, unpub. obs.). Both Sancetta (1990) and Hay et al. (2003) reported M. chilensis to be the most dominant species of the genus, but it is almost impossible to distinguish Minidiscus species by LM (Sancetta, 1990), the method often used to enumerate plankton or sediment samples. 4.2.2. Gulf of Alaska Although diatom assemblages from the coastal regions of the Gulf of Alaska are similar to those of the Washington coast, with S. costatum, Thalassiosira, and Chaetoceros spp. dominating (Allen, 1927, 1929, 1930; Waite et al., 1992a, b), pelagic assemblages are somewhat different. Booth (1981) found that only 10 species of phytoplankton represented over 10% of the total assemblage at any station, seven of which were diatoms (F. pseudonana, Neodenticula seminae, Proboscia alata (Brightwell) Sundstro¨m, Cylindrotheca closterium (Ehrenberg) Lewin and Reimann, Corethron criophilum Castracane, Pseudo-nitzschia seriata (Cleve) H. Peragallo, and Chaetoceros convolutus). Five of these (excluding F. pseudonana and P. seriata) as well as Ethmodiscus rex (Rattray) Hendey contributed over 40% of the total biomass. In descending order, the most frequently occurring species were T. lineata, P. seriata, F. pseudonana, and N. seminae. Distribution maps of 54 diatom spp. appeared to show no differences between Subarctic, Alaskan Stream and Alaskan Gyre waters. In our study, many of the species found in the Washington coastal waters were also present in the transitional waters (NP2–NP4) but their numbers were drastically reduced. However, some taxa such as Corethron pennatum, Coscinodiscus marginatus, F. pseudonana (Fig. 8A), and Pseudo-nitzschia spp. (Fig. 8D) increased in abundance, with F. pseudonana being the most dominant diatom. As Horner and Booth (1990) had found earlier, small pennate diatoms were generally more abundant than centric diatoms in the Gulf of Alaska, with the assemblage characterized by high abundances of nitzschioid diatoms. In the present study, F. pseudonana was dominant with important yet lesser contributions by Pseudo-nitzschia spp. and a Thalassiosira sp., and in the north by N. seminae (Fig. 8C). These findings are in stark contrast to those of the sediment trap study at Station PAPA (Takahashi, 1986, 1987; Takahashi et al., 1989), in

which N. seminae represented 87% of the total diatom flux. The nitzschioids (excluding N. seminae), recorded in the trap study as ‘‘Nitzschia spp. group’’, were low in abundance throughout the year compared to the main taxa. As with the recent data from Station KNOT (Komuro et al., 2005), the phytoplankton data from the Gulf of Alaska suggest that the small-sized dominant diatoms, namely F. pseudonana and related taxa, do not arrive at the deep water-moored sediment traps in large quantities. This emphasises the point that sediment trap assemblages are strongly modified assemblages and not truly representative of the surface water production in this region. Wong et al. (1995) demonstrated that the subarctic waters at Station P always contained low chlorophyll levels, despite the year-round presence of nutrient-rich waters during most years (nitrate levels never fell below 7-mM, but in some summers silicate levels were low; Horner and Booth, 1990; Wong and Matear, 1999). Thus, the Gulf of Alaska has been called an HNLC region, with a lack of iron cited as a possible reason for the low primary production (Miller et al., 1991; Boyd and Harrison, 1999). Given these oceanographic conditions, it is not surprising that massive diatom blooms do not occur at Station P, although moderate growth is clearly sustainable (Clemons and Miller, 1984). However, sedimentary records suggest that exceptional blooms did occur in the Gulf of Alaska in the Late Quaternary, presumably due to the influx of iron via meltwater or dust (McDonald et al., 1999). 4.2.3. Aleutian Islands Seemingly few phytoplankton studies have been conducted close to the Aleutian Islands; Aikawa (1932) studied the western Aleutian plankton communities around Near and Rat Islands, while Koike et al. (1982) measured chlorophyll and observed the diatom species on a track through the Unimak Pass (eastern Aleutians). Compared to the Alaskan Stream, the diatom abundance around the islands is much higher and generally dominated by centric diatoms not pennate diatoms. According to these studies, Chaetoceros spp. such as C. atlanticus, C. compressus Lauder, C. criophilus Castracane, C. debilis, and C. laciniosus are the most numerous (Aikawa, 1932; Koike et al., 1982). Other taxa with high abundances recorded in this area were Coscinodiscus spp., Pseudo-nitzschia seriata, Thalassiothrix longissima Cleve and Grunow, and Neodenticula seminae (as Denticula sp.)

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Western Subartic Zone

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Gulf of Alaska

E T Z

W C W

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

J C W

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(A) 280000 240000 200000 160000 120000 80000 40000 (B)

cell I-1

cell I-1

F. pseudonana

F. oceanica

20000 10000

(C)

Neodenticula seminae

cell I-1

40000 30000 20000 10000 cell I-1

(D) Pseudo-nitzschia spp.

20000 10000

(E) 2400 2000 1600 1200 800 400

cell I-1

Nitzschia bicapitata

(F) 2400 2000 1600 1200 800 400 1000 2000 3000 4000 5000 6000 7000 8000

NP48 NP46 NP44 NP42 NP40 NP38 NP36 NP34 NP32 NP30 NP28 NP26 NP24 NP22 NP20 NP18 B14 B12 B10 B8 B6 B4 B2 NP16 NP14 NP12 NP10 NP8 NP6 NP4 NP2

Depth of sea-bed (m)

cell I-1

Mastogloia spp.

Station number Fig. 8. Absolute abundance profiles of some raphid pennate diatoms: (a) Fragilariopsis pseudonana, (b) F. oceanica, (c) Neodenticula seminae, (d) Pseudo-nitzschia spp., (e) Nitzschia bicapitata, and (f) Mastogloia spp. Bathymetry and zonation scheme same as in Fig. 4.

(Aikawa, 1932) and Paralia sulcata (Ehrenberg) Cleve and Thalassiosira decipiens (Koike et al., 1982). In our study, Chaetoceros spp. (especially Hyalochaete species like C. debilis) were most numerous to the north of Unimak Pass and just south of the Adak Strait, with important contributions from small Thalassiosira spp. and Minidiscus

spp. just south of Unimak Pass and N. seminae to the north of Unimak Pass. 4.2.4. Bering Sea The Bering Sea is generally dominated by smallsized phytoplankton, although larger phytoplankton may contribute substantially to the biomass

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(Shiomoto, 1999). The extensive area (450% of the Bering Sea) of shallow eastern shelf allows phytoplankton blooms to develop, especially those of diatoms and coccolithophorids (Iida et al., 2002; Olson and Strom, 2002; Broerse et al., 2003). The shelf may be ice-covered in winter and so a sea-ice diatom flora may be present. The spring bloom is largely composed of centric diatoms (e.g., Thalassiosira and Chaetoceros spp.), although some pennate diatoms are also involved (e.g., Fragilariopsis oceanica (Cleve) Hasle, F. cylindriformis (Hasle) Hasle, and Pseudo-nitzschia seriata). In the summer, elongate centric diatoms such as Proboscia, Rhizosolenia, Guinardia, and Leptocylindrus spp. are numerous, as well as Chaetoceros spp. (Sukhanova et al., 1999). In the surface waters overlying the deeper basins the dominant diatoms in the spring are Chaetoceros (both sections, Hyalochaete and Chaetoceros) and Thalassiosira spp., while smaller-sized diatoms like F. cylindriformis and N. seminae are more numerous in the summer (Sukhanova et al., 1999). Results from a long-term sediment trap study in the Aleutian Basin showed that N. seminae represented 25–90% of the diatom flux, with summer occurrences of Chaetoceros resting spores reaching over 30% (Kurihara and Takahashi, 2002). In this study the Bering Sea stations were separated into the eastern shelf region (B1–B3) and the Aleutian Basin (B4–B14). The shelf assemblage was characterized by Thalassiosira spp., e.g. T. trifulta, T. conferta (Fig. 7D), and T. gravida (Fig. 7C), and Chaetoceros contortus and C. debilis. Although low in abundance, C. diadema was generally found over the shelf or close to the Aleutian Islands. The Aleutian Basin assemblage was similar to the shelf assemblage, but the centric diatoms were lower in abundance while pennate diatoms such as F. pseudonana, N. seminae, and Pseudo-nitzschia spp. dominated. 4.2.5. Northwest Pacific The western subarctic Pacific is dominated by diatoms (Kawarada and Sano, 1972; Odate and Maita, 1988/89; Semina and Mikaelyan, 1993), although recent findings suggest that siliceous nanoplankton such as the Parmales may be more numerous (Komuro et al., 2005). According to Mochizuki et al. (2002), centric diatoms dominate all year round at Station KNOT, with increases in N. seminae and Fragilariopsis spp. only in spring. However, their study could not identify diatoms

o10 mm, and so the smaller, more abundant diatoms were not recorded. Komuro et al. (2005) showed that Minidiscus spp. and F. pseudonana, in particular, were important diatoms seemingly missed by Mochizuki et al. (2002), in addition to coccolithophorids and Parmales. Despite this, N. seminae was never dominant at Station KNOT. In contrast, sediment trap studies in this area (Tsoy and Wong, 1999; Kurihara and Takahashi, 2002; Onodera et al., 2003) have reported that Neodenticula seminae sometimes overwhelmingly (up to 90%) dominates the diatom assemblages all year round, but with abundance peaks in spring. However, Onodera et al. (2003) showed that Chaetoceros spp. (both spores and vegetative cells of Section Hyalochaete) may actually dominate the spring peak in certain years. Despite the traps being moored at approximately the same latitude, Tsoy and Wong (1999) did not record a peak of Chaetoceros at their Station GA, suggesting that the laterally transported Chaetoceros cells/spores do not reach as far as 1651E in anything like the numbers recorded at 1551W by Onodera et al. (2003). Komuro et al. (2005) have suggested that these Hyalochaete spores, commonly found below the pycnocline, are laterally advected from the Kuril Islands, and not part of the locally produced assemblage at Station KNOT. In this study, F. pseudonana was dominant at most stations in the Western Subarctic Gyre, with other Fragilariopsis spp., Neodenticula seminae, Pseudo-nitzschia spp., Chaetoceros, and Thalassiosira spp. making important contributions. Only at a few stations did N. seminae have a higher abundance than F. pseudonana, suggesting that the deep moored sediment traps receive and record modified surface water assemblages. 4.2.6. Subtropical waters It is well known that small bicapitate nitzschioid species dominate diatom assemblages in subtropical–tropical waters (Blain et al., 1997; Fryxell, 2000). However, there have been few detailed taxonomic studies of marine diatoms in the Kuroshio, possibly due to their low abundance. A 2-year sediment trap study by Tanimura (1992) over the Japan Trench, off the Boso Peninsula, showed that three diatom species peaked during the spring, the Nitzschia bicapitata-N. bifurcata Kaczmarska & Licea complex, Thalassionema nitzschioides (Grunow) Grunow ex Hustedt, and Neodelphineis indica (F.J.R. Taylor) Hasle. These assemblages are

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significantly different to those seen in the northern traps and characterize the warmer, more saline Kuroshio waters. In a later paper (Tanimura, 1999), it was shown that T. nitzschioides sensu stricto inhabit more northerly waters, while three of its varieties (parva Heiden, incurvata Heiden and inflata Heiden) are characteristic of the subtropical gyre. In this study, Nitzschia bicapitata and Mastogloia spp. were common, with Bacteriastrum and Chaetoceros spp. probably representing a tychoplankton assemblage laterally transported from the Japanese coast. To the north, F. pseudonana and Pseudo-nitzschia spp. were also present in significant numbers, but less than that found in subarctic waters. Although the varieties of Thalassionema nitzschioides were not distinguished in this study, var. parva was recorded from Station KNOT in August, suggesting it was transported there via incursions of the Kuroshio. 4.2.7. Coastal waters of NE Japan A long-term study of Sagami Bay showed that diatoms were the largest constituent of the phytoplankton assemblage, forming a bloom during the spring months, dominated by species of Skeletonema, Thalassiosira, Chaetoceros, Coscinodiscus, and Rhizosolenia, although red tides of dinoflagellates and blooms of coccolithophorids have been reported during May in different years (Kanda et al., 2003). However, in a previous and more detailed study, Han et al. (1989) showed that near the mouth of Tokyo Bay ( ¼ northern Sagami Bay) in November a number of assemblages, consisting of both neritic and offshore taxa, were present. These included diatoms such as Leptocylindrus danicus, Guinardia striata (Stolterfoth) Hasle (syn. Rhizosolenia stolterfothii), Skeletonema costatum, Chaetoceros and Bacteriastrum spp., Cylindrotheca (Nitzschia) closterium, Nitzschia spp., and Pseudo-

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nitzschia pseudodelicatissima (Hasle) Hasle. Just inside Tokyo Bay there were assemblages containing Leptocylindrus danicus, Cylindrotheca (Nitzschia) closterium, Chaetoceros decipiens, and Skeletonema costatum. Mizushima (1972) reported similar assemblages in summer from Shimizu Harbour, to the south of Sagami Bay, with Skeletonema costatum overwhelmingly dominant, and important contributions from Chaetoceros spp., Pseudo-nitzschia seriata, and Proboscia alata. Winter assemblages were also dominated by S. costatum, with Chaetoceros spp. common (Mizushima, 1970). In the present study, the late August assemblages in Sagami Bay were dominated by Skeletonema grevillei at NP46 and by Chaetoceros debilis, C. pseudocurvisetus, and Pseudo-nitzschia spp. at NP47, with Bacteriastrum spp. common at both stations. In Tokyo Bay several Thalassiosira species were numerous, with Skeletonema grevillei and Chaetoceros pseudocurvisetus also important. Leptocylindrus danicus was more common in Sagami Bay, but was never abundant. 4.3. Ecological considerations In this study some species distributions were either characteristic of, or limited to, certain regions, presumably due to their ecological preferences. Table 3 provides a summary of this information, while more details on specific taxa are given below. The genus Chaetoceros can be subdivided into two sections, Chaetoceros (ex Phaeoceros) and Hyalochaete (Fig. 6A and B). From their profiles it can be seen that Chaetoceros (especially C. atlanticus and C. sp. cf. C. danicus Cleve) has greater abundances in subarctic waters than in coastal, temperate or subtropical waters, whilst

Table 3 Taxa indicative of certain oceanographic areas using data from the present study Subarctic

Subtropical

Coastal and/or island passes

Washington coast

Japanese coast

F. oceanica F. pseudonana N. seminae Proboscia eumorpha Rhizosolenia hebetata T. conferta T. gravida

Bacteriastrum spp. Mastogloia spp. N. bicapitata

Cyclotella spp. Ditylum brightwellii Eucampia spp. Skeletonema spp. Stephanopyxis spp. T. allenii T. binata T. lundiana

T. anguste-lineata T. proschkinae T. rotula

Cerataulina sp. Detonula pumila T. weissflogii

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Hyalochaete has much larger peaks than Chaetoceros but is generally restricted to coastal waters and the vicinity of the Aleutian passes. The most abundant species of Section Hyalochaete, C. compressus, C. debilis, and C. decipiens, are found in both coastal waters and near the island passes, whilst C. laciniosus and C. pseudocurvisetus are more numerous in Tokyo Bay than elsewhere. Bacteriastrum is a closely related genus, however, the profile of the combined species shows that they are only present in warmer, temperate to subtropical waters (Fig. 6C). Genera belonging to the order Thalassiosirales also show distinct distribution patterns. For instance, Thalassiosira oestrupii (Ostenfeld) Hasle has peaks in both subarctic zones as well as in Tokyo Bay (Fig. 7B), whilst T. gravida (Fig. 7C) and T. conferta (Fig. 7D) have their highest abundances in the Bering Sea. Thalassiosira gravida, in particular, is often referred to as a sea-ice-associated diatom. Some species such as T. tealata Takano, T. tenera, and T. allenii were more abundant in Tokyo Bay, whilst T. oceanica and T. sp.1 were numerous throughout most of the transect, but absent or rare in the subtropical waters and Japanese coastal waters. Minidiscus species (Fig. 7A) were largely present in the Washington coastal waters and close to the eastern Aleutian Islands, whilst Detonula spp. (Fig. 7E) and Skeletonema spp. (Fig. 7F) were mainly found in Japanese coastal waters. Of the raphid diatoms, nitzschioid genera belonging to the family Bacillariaceae were always dominant. Both Neodenticula seminae (Fig. 8C) and F. pseudonana (Fig. 8A) were more numerous in the subarctic zones and in the Bering Sea, whilst Pseudo-nitzschia spp. (Fig. 8D) were common to abundant throughout most of the transect, although less numerous in subtropical waters. F. oceanica (Fig. 8B) had its highest abundances in the western subarctic zone, but was also present in the eastern subarctic and temperate zones. Nitzschia bicapitata (Fig. 8E) and species of the non-nitzschioid genus Mastogloia (Fig. 8F) were numerous in the temperate-subtropical zones, and rare or absent in subarctic waters. 5. Conclusions This study provides insights into the distribution of marine diatoms in the North Pacific and clearly highlights the importance and dominance of the small pennate diatom F. pseudonana in the subarctic

Pacific during summer. In stark contrast to sediment trap studies, the subarctic species Neodenticula seminae was rarely the dominant species, suggesting that sinking assemblages are modified during their descent through the water column and/or small diatoms are actively recycled in the photic zone and thus rarely contribute to the diatom flux or underlying sediments. This study also shows the enhanced diatom abundance close to the Aleutian Islands (the so-called ‘‘island effect’’) and the shallow coastal waters off Washington and in Tokyo and Sagami Bays. Acknowledgements We thank the officers and crew of the R/V Hakuho Maru for their assistance during leg 3 of cruise KH99-3. We would also like to thank the two anonymous reviewers for their constructive comments. This project was supported by a Grants-inAid for Scientific Research (No. 13440152), awarded to RWJ and Kozo Takahashi by the Japanese Society for the Promotion of Science (JSPS). References Aikawa, H., 1932. On the summer plankton in the waters of the Western Aleutian Islands in 1928. Bulletin of the Japanese Society of Scientific Fisheries 1 (2), 70–74 (in Japanese with English abstract). Akatsuka, K., 1914. Planktonic diatoms collected off Takashima Island. Suisan Chosa Hobun 8, 1–106. Allen, W.E., 1927. Surface catches of marine diatoms and dinoflagellates made by USS Pioneer in Alaskan waters in 1923. Bulletin of the Scripps Institution of Oceanography 1, 39–48. Allen, W.E., 1929. Surface catches of marine diatoms and dinoflagellates made by USS ‘‘Pioneer’’ in Alaskan waters in 1924. Bulletin of the Scripps Institution of Oceanography 2, 139–153. Allen, W.E., 1930. Quantitative studies of surface catches of marine diatoms and dinoflagellates taken in Alaskan waters by the International Fisheries Commission in the fall and winter of 1927–1928 and 1929. Bulletin of the Scripps Institution of Oceanography 2, 389–399. Allen, W.E., 1943. Summary of results of twenty years of researches on marine phytoplankton. Sixth Pacific Science Congress, pp. 577–583. Bailey, J.W., 1856. Notice of microscopic forms found in the soundings of the Sea of Kamtschatka. The American Journal of Science and Arts 22, 1–6. Banse, K., English, D.C., 1999. Comparing phytoplankton seasonality in the eastern and western subarctic Pacific and the western Bering Sea. Progress in Oceanography 43, 235–288.

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