Bot. Bull. Acad. Sin. (2001) 42: 287-302

Yang et al. — Coccolithophorid and coccolith distribution

Distribution of coccolithophorids and coccoliths in surface ocean off northeastern Taiwan

Tien-Nan Yang^1,3, Kuo-Yen Wei^1,*, and Gwo-Ching Gong²

¹Global Change Research Center and Department of Geosciences, National Taiwan University, Taipei, 10716 Taiwan, ROC

²Department of Oceanography, National Taiwan Ocean University, Keelung, Taiwan, ROC

(Received October 27, 2000; Accepted July 26, 2001)

Abstract. This study depicts quantitatively the distribution of coccolithophorids and coccoliths during one summer season in the area off northeastern Taiwan where the Kuroshio flows northward and interacts with the shelf waters of the East China Sea. To minimize the influence of diurnal variation of coccolithophorids, only samples taken at sunrise were analyzed. Sea-surface water (2 m in depth) samples were obtained at six stations during the summer of 1996. Forty-one species were identified with Emiliania huxleyi (Lohmann) Hay et Mohler, Palusphaera vandelii Lecal emend. R. E. Norris, Umbellosphaera Paasche spp. and Syracosphaera Lohmann spp. being the predominant forms. Three coccolithophorid communities were recognized: (1) the continental shelf community, dominated by Emiliania huxleyi, Gephyrocapsa oceanica Kamptner and Calciosolenia murrayi Gran, which showed intermediate biodiversity and species evenness; (2) the Kuroshio community, which showed the highest diversity and evenness, with a flora dominated by genus Umbellosphaera Paasche; and (3) the Western North Pacific Central Water community, which had the lowest diversity and evenness, with dominant species Calicasphaera Kleijne and Palusphaera vandelii. The absolute abundance of loose coccoliths ranged from 10.2 × 10⁴ individual coccoliths l^-1 to 22.9 × 10⁴ individual coccoliths l^-1, while those of coccospheres were much less, ranging from 11.5 × 10³ cells l^-1 to 19.7 × 10³ cells l^-1. The largest absolute abundance of coccoliths and coccospheres was found in the Kuroshio path.

Keywords: Coccolithophorid; Diversity; East China Sea; Kuroshio; Standing crops.

Introduction

Coccolithophorids are unicellular, marine, golden-brown algae (Haptophyta) commonly found in near-surface waters in patchy distributions. They are one of the world's major primary producers, contributing about 15 per cent of the average oceanic phytoplankton biomass to the oceans (Berger, 1976). They also produce elaborate, minute calcite platelets (coccoliths), covering the cell to form a coccosphere and supplying up to 60 per cent of the bulk pelagic calcite deposited on the sea floors (Honjo, 1996). Coccoliths can reflect the visible light from surface water during coccolithophorid bloom season (Holligan et al., 1983; Groom and Holligan, 1987; Balch et al., 1991) and thus enhance albedo. Coccolithophorids, therefore, have recently gained much attention as important players in global climate change and carbon cycles (Westbroek et al., 1993 and references therein; Hagino et al., 2000; Cortés et al., 2001; Haidar and Thierstein, 2001).

The large-scale distribution of modern coccolithophorids in the open ocean has been studied in various

parts of the world (for review, see Winter et al., 1994 and references therein). About two decades ago, Okada and Honjo (1970; 1973) and Nishida (1979) completed large-scale studies concerning coccolithophorid biogeography in the Pacific Ocean, with emphasis on the North Pacific. Okada and Honjo (1973) established six coccolithophorid zones in the surface water along the 155°W meridian based on the distribution pattern of characteristic species. Recently, Hagino et al. (2000) documented the spatial dynamics of coccolithophorid assemblages in the Western-Central Equatorial Pacific Ocean, while Cortés et al. (2001) studied the temporal variation pattern of coccolithophorid ecology in water column at the HOT station ALOHA, Hawaii. For the western boundary constraint of the Pacific, Okada and Honjo (1975) conducted a large-scale study of coccolithophorid biogeography of the western Pacific marginal seas. Nevertheless, few detailed analyses of living coccolithophorids of individual marginal seas and biogeographic provinces of the western Pacific have appeared. The present study focuses particularly on the community structure of coccolithophorids in the surface waters (~2 m in depth, hereafter defined as the surface water) of summer-season in the area offshore of northeastern Taiwan, where the northward flowing Kuroshio encounters the Ryukyu Ridge, collides with the continental shelf break, and then deflects to the northeast (Nitani, 1972).

³E-mail: f4224103@ms.cc.ntu.edu.tw

*Corresponding author. Tel: +886-2-23691143; Fax: +886-2-23636095; E-mail: weiky@ms.cc.ntu.edu.tw

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

The Kuroshio, which originates from the North Equatorial Current, is a strong western boundary current that flows northward along the eastern coast of Taiwan and turns northeastward as it enters the Okinawa Trough (Figure 1). As the Kuroshio impinges on the shelf break northeast of Taiwan, some of the surface water intrudes onto the shelf forming a Branch Current (Qiu and Imasato, 1990; Hsueh et al., 1992; Tang et al., 2000) and some of the subsurface water forms a countercurrent hugging the slope (Chung et al., 1993). It has been long observed that the main stream of the Kuroshio shifts its path seasonally: during winter the current path is closer to the shelf break and for the rest of the year the current path shifts seaward (Sun, 1987; Chao, 1991; Tang and Yang, 1993; Tang et al., 2000). The Kuroshio is characterized by high sea-surface temperature and salinity year-round, while the southern East China Sea (ECS) shelf water is saltier and colder during the cold season, and vice versa in the warm season. In July 1996, when our field investigation was conducted, the area of the Kuroshio and the Western North Pacific Central (WNPC) (coined by Sverdrup et al., 1942) water was blanketed with extensive warm surface

water, and a cold eddy off northeast Taiwan was also observed from the NOAA/AVHRR imagery of sea surface temperature (Fig. 2c in Tseng et al., 2000).

The physical forcing in turn controls the chemical hydrography, as Gong et al. (1997) documented that retreat of the Kuroshio intrusion from the shelf northeast of Taiwan restored the upwelling center, which resulted in a doubling of the phytoplankton biomass around the cyclonic eddy within two weeks. In contrast with the sophisticated and variable nutrient conditions of the shelf water, the Kuroshio main stream has been more static with lower nutrient contents (Gong et al., 1997; Gong et al., 2000; Liu et al., 2000). However, the hydrographic structures of the southern ECS during the summer are more complicated than in the winter, because the Kuroshio and shelf waters coexist on the shelf break off northeast of Taiwan during the summer (Lie et al., 1998).

Materials and Methods

Hydrographic Data

Temperature and salinity measurements were obtained from sensors loaded on the Seabird CTD-General Oceanic Rosette assembly during the sampling period. Water samples for nutrients measurements were stored in 100 ml polypropylene bottles and frozen immediately with liquid nitrogen. Nitrate was analyzed with a self-designed flow injection analyzer and was reduced to nitrite with a cadmium wire, activated with a copper sulfate solution (Gong, 1992). The precision for the nitrate analysis was 0.3 µM for concentrations of 10 µM or higher.

Coccolithophorid Sample Processing

Sea surface water (~2 m in water depth) samples for coccolithophorid works were taken from more than 30 stations in the area off northeastern Taiwan during Cruise 457 of the R/V Ocean Research I in the summer of 1996 (July 15 through 22). A Seabird CTD-General Oceanic Rosette assembly with 12 Go-Flo bottles (volume of 10 L) was used to obtain seawater samples.

Seawater (about 1-2 L) obtained by the Go-Flo bottles was filtered on board through a Nuclepore^® polycarbonate membrane (47 mm in diameter, pore size 0.4 µm) by applying low-pressure (<100 mm Hg) generated by a vacuum pump. Each membrane with its filtered particles was immediately transferred into a plastic Petri-dish and refrigerated for preservation. Upon returning to the laboratory, Petri-dishes containing membranes were put into a desiccator. A piece of membrane, about 1 cm², was cut and mounted onto an aluminum stub with double-sided tape and was then coated with platinum using a HITACHI E101 ion sputter coater.

To minimize the effects of diurnal variation (Figueroa et al., 1998; Graham et al., 2000), only the six samples taken during sunrise were used for the study. Two Stations, St. 6 and St. 24, were located at the continental shelf and the WNPC water, respectively, while the others, St. 12, St. 17,

Figure 1. Location of sampling stations (solid dots) in the area off northeastern Taiwan. Sea surface waters (~2 m in depth) were collected during summer 1996. Stations 12 and 34, and 17 and 30 are located within the flow path of Kuroshio (depicted by dashed big arrow). Stations 6 and 24 are located at continental shelf of the southern ECS and the WNPC water, respectively. The solid arrow at each station represents the wind direction, and its length represents the relative wind speed during the sampling time. Two parallel dashed arrows delineate the countercurrent existing on the continental shelf off the northern tip of Taiwan (Tang et al., 2000). The closed solid polygon around the countercurrent indicates the upwelling area. Depth contour lines are in meters.

Yang et al. — Coccolithophorid and coccolith distribution

St. 30 and St. 34 were located within the path of the Kuroshio main stream.

Counting Procedure

The coccoliths and coccospheres on the cut membrane portion were examined in horizontal rows and counted in a total of 320 randomly picked viewing fields under 2000 × magnification with a HITACHI S-2400 Scanning Electron Microscope (SEM). In addition, several other fields were examined for rare species.

The formula for estimating the abundance/standing crop of coccoliths/coccospheres per liter of seawater is as follows:

N_T = N_C × 925 / S_V ,

where N_T is the number of total individuals of coccoliths or of coccospheres per liter, N_C is the number of coccoliths or of coccospheres counted in 320 viewing fields, S_V is the filtered volume of seawater, and the value 925 is a ratio constant of the area covered by particles on the membrane (9.62 × 10^-4 m², diameter 35 mm) to that of the 320 fields examined (the length and width of each field under 2000 × magnification is about 65 µm and 50 µm, respectively). The ratio constant was derived as 925 = 9.62 × 10^-4 m² / 320 × 3.25 × 10^-9 m².

Taxonomy

The systematic classification of coccolithophorids is based mainly on the morphology of cells and coccoliths. The identification of the coccolithophorid species was conducted following the taxonomic classification scheme outlined by Jordan and Green (1994 and references therein). Some associations of holococcolith-heterococcolith or holococcolith-holococcolith coccospheres were documented by Cros et al. (2000). For reviews, see Cros et al. (2000) and references therein. In this study we also adopted the nomenclatural taxonomy for those combinations made by Cros et al. (2000). The coccolithophorid taxa tallied are listed in Table 1. The individuals of coccolithophorids counted in 320 fields are presented in Appendix A. In total 41 coccolithophorid species were identified with the addition of one genus of coccolith and one genus of coccosphere.

SEM photographs of selected species, including major dominant forms, are shown in Plates I-IV. Other rare species encountered both in counting and extra examining will be presented elsewhere. Holococcolithophorid species are displayed in Plate I, and II (Figures 1-4), while other heterococcolithophorid species are shown in Plates II (Figures 5-8), III and IV.

Results

Hydrography

In July 1996, the hydrological parameters in the surface water (~2 m in depth) of the six stations were recorded at dawn. The temperature values in surface water showed no difference (ca. 30.1±0.2°C) around the area off

northeastern Taiwan except for St. 6, located at the continental shelf, which was affected by an upwelling from the subsurface water of Kuroshio and had the lowest value of 28.6°C. The phenomenon in the same month, of a warm area covering the Kuroshio path and WNPC water and of a cold eddy off northeastern Taiwan, were also documented from the NOAA/AVHRR sea surface temperature image (Fig. 2c in Tseng et al., 2000). While the values of salinity fell almost in the same range around 34.0±0.2‰, the wind velocities ranged from 3 to 10 m per

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

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Plate I. Pictures exhibited are all holococcolithophorid species. 1, Calicasphaera diconstricta Kleijne. Monomorphic coccosphere with lateral view of calicaliths with two constrictions and distal view of calicaliths showing the central opening. a: A Palusphaera vandelii rhabdolith with styliform process (arrow). (St. 17); 2, Calyptrosphaera dentata Kleijne. Monomorphic coccosphere consisting of oval calyptroform holococcoliths. (St. 30); 3, C. dentata Kleijne. Magnified view of lower right corner of Figure 2 shows the detailed coccoliths with one tooth-like protrusion. (St. 30); 4, Corisphaera gracilis Kamptner. A small dimorphic coccosphere; only the body zygoliths are featured. (St. 12); 5, Homozygosphaera triarcha Halldal et Markali. Monomorphic coccosphere consisting of zygoliths with three arches. (St. 30); 6, Poricalyptra magnaghii (Borsetti et Cati) Kleijne. Dimorphic coccosphere with calyptroform body coccoliths and helladoform circum-flagellar coccoliths. (St. 30); 7, Sphaerocalyptra sp. Collapsed coccosphere showing only body calyptroliths of campanulate forms. (St. 17); 8, Syracolithus sp. type A. Kleijne. Disintegrated monomorphic coccosphere consisting of laminoliths. Cubics are minute salt crystals precipitated from seawater. (St. 17).

Yang et al. — Coccolithophorid and coccolith distribution

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

Yang et al. — Coccolithophorid and coccolith distribution

second, with the highest speed at the shelf station St. 6 and the lowest at St. 24 in the WNPC water. The direction of water current in the investigated area was northward except at St. 34 (Figure 1).

The most important nutrients for coccolithophorids are nitrate and phosphate (Brand, 1994 and references therein). The vertical distributions of nitrate concentrations obtained in the six stations are plotted in Figure 2. The concentrations of nitrate in the top 20 m of seawater in the study area were too low to be detected. The nitracline in Station 6 was the shallowest while that at Station 24 was the deepest. Nitrate displays three distribution patterns in the water column around northeastern Taiwan: turbulent diffusion in shelf water, molecular diffusion in the relatively static WNPC water, and an intermediate mode in the Kuroshio path.

Coccolithophorid Diversity and Standing Crop

The species richness of coccoliths, coccospheres, and coccolithophorids recorded in the six stations are listed in Table 2 and drawn in Figure 3. More species abundance was found in the flow path of the Kuroshio, with the largest at St. 30. Furthermore, various diversity indices of the floral structure were calculated: Simpson's index (l), Shannon's index (H) and the evenness index (E) (Table 2, Figure 4), following Simpson (1949), Shannon and Weaver

Figure 2. Vertical profiles of nitrate concentration at the studied six stations. The continental shelf Station (St. 6) is represented by �, the Kuroshio stations (St. 12, St. 17, St. 30 and St. 34) are represented by l, and the WNPC station (St. 24) is presented by +.

Figure 3. Plots of species richness of (a) coccoliths and coccospheres and (b) coccolithophorids in the studied area. The Kuroshio path has higher values of species richness. The seaward side of the path, Station 30, recorded the highest species richness. Values of species richness are listed in Table 2. The elevation of map in Figures 3-9 is about 30 degrees.

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Plate II. Figsures 1-4: holococcolithophorids; Figsures 5-8: heterococcolithophorids. 1, Syracosphaera nodosa Kamptner. Helladosphaera cornifera, holococcolithophorid form of Syracosphaera nodosa. Dimorphic coccosphere consisting of helladoform circum-flagellar coccoliths (a) and zygoform body coccoliths. This species is recently recorded as part of a Syracosphaera nodosa-Helladosphaera cornifera combination by Cros et al. (2000), who suggested that S. nodosa has the priority over H. cornifera. (St. 17); 2, Unidentified sp. Disintegrated monomorphic coccosphere. (St. 24); 3, Unidentified sp. Magnified view of Figure 2 shows the irregular pore shape in the central area. (St. 24); 4, Unidentified sp. Collapsed monomorphic coccosphere; showing coccoliths with a robust bridge traversing the central area. (St. 17); 5, Acanthoica quattrospina Lohmann. Disintegrated ellipsoidal coccosphere with spines at both poles: a: two long apical spines at one pole and b: one long apical spine and three short apical spines at opposite pole. This species is recently recorded as part of an Acanthoica quattrospina-holococcolithophorid combination by Cros et al. (2000), who suggested that Acanthoica quattrospina has the priority. (St. 17); 6, Calciopappus rigidus Heimdal. Collapsed elongate coccosphere consisting of elliptical body coccoliths and long apical spines (a) around the flagellar area. (St. 17); 7, C. rigidus Heimdal. Magnified view of Figure 6 showing the elliptical body coccoliths. (St. 17); 8, Calciosolenia murrayi Gran. Disintegrated elongated coccosphere shows two terminal spines (a) and some rhombolith-type coccoliths (b). (St. 6).

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

Yang et al. — Coccolithophorid and coccolith distribution

Figure 4. Geographic variations of (a) Shannon's index and (b) evenness index of coccoliths and coccospheres. The coccolithophorid communities along the Kuroshio path and in its slope waters are more diversified than that in the continental shelf water and WNPC water.

Figure 5. Geographic distribution of abundance of coccoliths (a), and standing crops of coccospheres (b). The Kuroshio path shows the highest abundance of coccoliths and coccospheres.

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Plate III. Pictures exhibited are all heterococcolithophorid species. 1, Calciosolenia murrayi Gran. Detailed view of rhombolith-type coccoliths (scapholiths). (St. 6); 2, Ceratolithus cristatus Kamptner. Neosphaera coccolithomorpha, heterococcolithophorid form of Ceratolithus cristatus. Part of disintegrated coccosphere planoliths. This species is regarded as part of a Neosphaera-Ceratolithus combination by Cros et al. (2000), who suggested that the C. cristatus has the priority. (St. 17); 3, Discosphaera tubifera (Murray et Blackman) Ostenfeld. Disintegrated spherical coccosphere consisting of salpingiform rhabdoliths. (St. 12); 4, Emiliania huxleyi (Lohmann) Hay et Mohler. Two coccospheres with distal shield elements and some detached placoliths. (St. 6); 5, Gephyrocapsa oceanica Kamptner. Coccosphere showing distinctive transversal bridges. (St. 6); 6, Ophiaster hydroideus (Lohmann) Lohmann emend. Manton et Oates. Collapsed coccosphere consisting of elliptical body coccoliths and relatively long and narrow link coccoliths forming arm-like appendages (a) at the posterior pole, and circum-flagellar coccoliths with a spine of the anterior pole. (St. 17); 7, O. hydroideus (Lohmann) Lohmann emend. Manton et Oates. Magnified view of Figure 6 showing the detailed body coccoliths and circum-flagellar coccoliths with a spine of the anterior pole. (St. 17); 8, Palusphaera vandelii Lecal emend. R.E. Norris. Disintegrated coccosphere consisting of rhabdoliths with a long, very thin styliform process. (St. 30).

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

Yang et al. — Coccolithophorid and coccolith distribution

(1949) and Hill (1973), respectively. These indices gave consistent results, showing that the Kuroshio community had higher values than either the shelf area or the WNPC water. Stations 12 and 17, downstream from the Kuroshio, had the highest diversity by high values on the evenness and Shannon's index and low values on Simpson's index. On the other hand, the lowest evenness and information content (indicated as H) were seen at WNPC water station 24.

The abundance and standing crops of coccoliths and coccospheres, respectively, were estimated and are shown with a compilation of other works in Table 3, and they are drawn in Figure 5. Again, the Kuroshio community exhibits higher abundance/standing crops of coccoliths and coccospheres than other two areas. The largest abundance or standing crops of coccoliths and coccospheres occurred at St. 34 and St. 12, respectively. The standing crops of coccolithophorids are four orders in magnitude

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Plate IV. Pictures exhibited are all heterococcolithophorid species. 1, Syracosphaera exigua Okada et McIntyre. Dithecate coccosphere with monomorphic endotheca. Caneoliths have a horizontal, ridged distal flange and a central area with an elongate narrow central mound. Some detaching theca present at the upper right corner. (St. 17); 2, S. halldalii Jordan et Green. Disintegrated monothecate, dimorphic coccosphere. Body caneoliths with a flat distal flange and circum-flagellar caneoliths with a protrusion in the central area are seen. (St. 17); 3, S. pirus Halldal et Markali. Coccosphere dithecate with dimorphic endotheca. Shown are some circular cyrtolliths (arrows) with a narrow outer cycle and a central area of radial laths cover the flagellar area and body caneoliths with a small nodular protrusion and circum-flagellar caneoliths with a long protrusion. (St. 30); 4, S. prolongata Gran ex Lohmann. Dithecate coccosphere, cyrtolith not shown, with dimorphic endotheca. Shown here are body caneoliths with a small nodular protrusion in the central area, and a circum-flagellar caneolith with a long protrusion. (St. 17); 5, S. pulchra Lohmann. Coccosphere dithecate with dimorphic endotheca. Shown are caneoliths with distal and mid-wall flanges (arrows), and circum-flagellar caneoliths with a long protrusion in the central area. Exothecate cyrtoliths elliptical, convex with a depression in the central area. (St. 30); 6, Umbellosphaera irregularis Paasche. Disintegrated coccosphere consisting of high funnel-shaped coccoliths (umbelloliths). (St. 34); 7, U. tenuis (Kamptner) Paasche. Disintegrated coccosphere consisting of two sizes of umbelloliths with normal-elliptical central area and distal shield with radial ribs and diagonal ridges with papillae in between. (St. 34); 8, Umbilicosphaera hulburtiana Gaarder. Coccosphere with protruding elliptical placoliths. Central elliptical opening in crater-shaped depression is surrounded by a row of nodules on elements of the central tube. (St. 17).

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

higher than that found by Okada and Honjo (1975) for the marginal seas along the western Pacific, and are more abundant by one to two times those documented by Okada and Honjo (1970; 1973) and Nishida (1979) for the open ocean. Recently, Cortés et al. (2001) also documented higher cell densities than those reported in the Pacific from Jan. 1994 through Aug. 1996 (Table 3), with two episodic increasing standing crops, 40000 and 38000 cells l^-1 in Mar. 1995 and Aug. 1996, respectively.

Coccolithophorid Communities and Their Floral Characteristics

Calciosolenia murrayi Gran, Emiliania huxleyi (Lohmann) Hay et Mohler, Gephyrocapsa oceanica Kamptner, Palusphaera vandelii Lecal emend. R. E. Norris, Umbellosphaera irregularis Paasche, U. tenuis (Kamptner) Paasche, and species of Syracosphaera Lohmann, dominated the coccolith assemblage in the studied area, while species of Calicasphaera Kleijne, P. vandelii, U. irregularis, U. tenuis, and Syracosphaera spp., predominated in the coccosphere population (for quantitative details, see Appendix A). Relative abundances of these main taxa are plotted in Figures 6-9. From continental shelf seaward through the Kuroshio path to the WNPC area, we see an increasing trend in the dominance of P. vandelii. It also appears that Syracosphaera spp. show a similar increasing trend. In contrast, the abundance of E. huxleyi decreases along the track.

The continental shelf water (St. 6) is characterized by the dominance of E. huxleyi, G. oceanica and C. murrayi (Figure 6). The Kuroshio path is characterized by U. irregularis and U. tenuis (Figure 7), but contains also moderately abundant G. oceanica (Figure 6). The WNPC water (St. 24) is characterized by a high content of Calicasphaera coccospheres. Palusphaera vandelii, mostly in coccolith form, is most abundant in the WNPC water and less abundant along the path of the Kuroshio (Figure 8). Syracosphaera spp. was distributed ubiquitously in the studied area (Figure 9).

Figure 6. Geographic pattern of the relative abundances of Calciosolenia murrayi (a), Emiliania huxleyi (b) and Gephyrocapsa oceanica (c) in the studied area. These species distributed mainly on the continental shelf of the East China Sea.

Figure 7. Geographic distribution of Umbellosphaera irregularis (a) and Umbellosphaera tenuis (b) in the studied area. These species dominated in the Kuroshio and its slope waters.

Yang et al. — Coccolithophorid and coccolith distribution

Figure 8. Geographic distribution of dominant species characteristic of the WNPC water: Calicasphaera spp. (a) and Palusphaera vandelii (b). Calicasphaera spp., appeared only as coccospheres, occurred most abundantly in the WNPC water.

1977; Reid, 1980; Cortés et al., 2001; Haidar and Thierstein, 2001) and in the Mediterranean and Red Sea (Kleijne, 1993; Knappertsbusch, 1990). The dominant forms in the WNPC waters, P. vandelii and Calicasphaera , were also reported to be major components of tropical and subtropical open-ocean communities (Kleijne, 1992). The distribution pattern in the studied area suggests that they prefer stratified oligotrophic waters.

Conclusions

The distribution of coccolithophorids near the ocean's surface off northeast Taiwan during summer 1996 was quantitatively documented in terms of diversity, abundance/standing crops and community structure. A total of 41 species were identified and tallied with Emiliania huxleyi, Palusphaera vandelii, Umbellosphaera spp. and Syracosphaera spp. being the dominant species.

The flow path of the Kuroshio main stream contains more diversified coccolithophorids (20~24 species) than the shelf-waters with the lowest species richness (15 species). The standing crops observed at these stations during summer 1996 are of one to four orders of magnitude higher than those of several previous studies in the Pacific Ocean. The absolute abundances of coccoliths range from 10.2 to 22.9 × 10⁴ per liter, while those of coccospheres range from 11.5 to 19.7 × 10³ cells per liter. Generally speaking, the Kuroshio has a higher abundance of coccolithophorids than the continental shelf. Each water-mass has its own dominant characteristic species: E. huxleyi, C. murrayi and G. oceanica for the shelf-water, Umbellosphaera spp. for the Kuroshio, and Palusphaera vandelii and Calicasphaera spp. for the Western North Pacific Central water. The species compositions in each water-mass are affected by the nutrient distributions.

Acknowledgements. The authors would like to thank the officers and crew of the R/V Ocean Research I, Mr. Y.-H. Wen and Prof. Y.J. Yang of the Dept. of Applied Science, Chinese Naval Academy for cruise assistance, nutrients analysis and coccolithophorid data processing, respectively. We are greatly appreciative to Prof. C.Y. Huang of Dept. of Geosciences, National Taiwan University for providing the SEM facility. We

Figure 9. Geographic distribution of the ubiquitous Syracosphaera spp.

Discussion

The coccolithophorid distribution in the investigated area can be subdivided into three communities based upon characteristic species: the East China Sea continental shelf community, the Kuroshio current community and the Western North Pacific Water community. Sea surface temperature and salinity over the investigated area are generally very similar except that the continental shelf station has slightly lower temperature and salinity. The depth of nitracline, in contrast, shows distinctive variations among the three communities, increasing progressively from the continental shelf through the Kuroshio towards the open ocean. It is conceivable that the nitracline pattern is responsible for the distribution of the studied coccolithophorids.

It has been documented that the nitrate demand of E. huxleyi and G. oceanica (<0.1 µmol kg^-1) is much higher than that of U. irregularis (Cortés et al., 2001). The former two species are considered to be opportunistic species thriving in upwelling areas and continental shelf settings, where nutrient concentration in surface water is generally high (Mitchell-Innes and Winter, 1987; Kleijne et al., 1989). On the other hand, Umellosphaerids (e.g., U. irregularis and U. tenuis) are the most characteristic species of the upper photic zone of oligotrophic waters in the Atlantic and Pacific (McIntyre and Bé, 1967; Okada and McIntyre,

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

are grateful to two anonymous reviewers for their constructive comments and suggestions on the manuscript. This study was supported by Grant NSC-84-2611-M-002-005-K2 (to KYW) and NSC-85-2611-M-019-018-K2 (to GCG) from the National Science Council of the Republic of China.

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