The study region encompassed an area between 67.0°– 88°26′N and 152°–178°54′W, and was divided into three geographical provinces based on bathymetry: the shallow continental shelf, the continental slope, and the deep ocean. The shallow continental shelf was at a depth of less than 100 m, including Chukchi Shelf and the Alaska’s northwest coast. The continental slope was located in the northern and northeastern parts of the Chukchi Sea at a depth of 100–500 m, included the Chukchi Abyssal Plain, the Chukchi Cap and the Northwind Ridge. The deep ocean was at a depth of 500–4 000 m, included the Canadian Abyssal Basin, the Mendeleev Abyssal Plain and Alpha Ridge. The 50 phytoplankton sampling stations included 23 stations in the shallow continental shelf, five stations in the continental slope and 22 stations in the deep ocean. Based on the geographical location, as well as the hydrological and investigation time, six stations representing different latitudes and depths were selected to analyze the regional disparity in the vertical distribution of phytoplankton (Fig. 1).

The study was conducted between July 20, 2010 and August 30, 2010, as part of CHINARE 2010, aboard the Chinese icebreaker R/V Xuelong. One liter Phytoplankton samples were collected using 10 L Niskin bottles at 50 stations in three regions. A total of 91 water samples were collected, including 50 surface and 41 vertical stratified samples collected from eight depths (10, 20, 30, 50, 75, 100, 150, and 200 m) depending on the water depth. All samples were immediately fixed with Lugol’s solution (final concentration of 2%). After 24 h, buffered formaldehyde was added (final concentration 2%). Determinations and counts of 25 mL subsamples were analyzed under an inverted Zeiss-Z1 microscope (Carl Zeiss MicroImaging, Göttingen, Germany) at 200× or 400× magnification using the Utermöhl method (Utermöhl, 1958). Identification of algal species was based on Round et al. (1990), Tomas (1997), Sun and Liu (2002), and Lee (2008). Temperature and salinity data were collected using a Sea-Bird 911 CTD probe (Seabird Electronics, Inc., Bellevue, WA, USA). Nutrient samples were collected using Niskin water bottles and processed using a Model 300 automated nutrient analyzer (Alpkem, Clackamas, OR, USA; Grasshoff et al., 1999). The ice coverage was visually estimated from the bridge of the R/V Xuelong.

The species data were log₁₀(x+1) transformed before analysis to obtain consecutive distributions. The species richness (d) and species dominance (Y) were calculated using the following formulae:

where n is the number of organisms of the i species, N is the total number of organisms in the sample, f is the occurrence frequency of i species, and S is the species number in the sample.

Relationships between phytoplankton community and environmental variables were analyzed by CANOCO version 4.5. Only those taxa that were observed in more than 10% of the samples and with more than 1% in the total abundance were included in analyses. Both of the species data and the environmental variables were log₁₀(x+1) transformed before analyses. Detrended correspondence analysis (DCA) for the species data was used to determine the methods that would be applied. As was suitable for unimodal ordination, canonical correspondence analysis (CCA) was chosen since that the maximum gradient length of the four axes was between 3 and 4.

During the departure trip from July 20 to August 6, 2010, ice conditions worsened from the shallow continental shelf to the continental slope and further into the deep ocean, the coverage and thickness of sea ice gradually decreased. Most of the Chukchi Shelf became an ice-free area. Only seven out of 16 stations were covered with ice. In the eastern Chukchi Sea and in the northwest of Point Barrow, ice coverage exceeded 80%. Stations R08 and R09 near the Herald Shoal were located at the ice edge. All 12 stations in the deep ocean were completely covered with ice.

The water temperature decreased significantly as the voyage proceeded north. The temperature in the shallow continental shelf was relatively high. The temperature in the southern Chukchi Sea was approximately 5°C. The temperature in the continental slope declined nearly to 0°C. The temperature in the deep ocean fell further to below 0°C, ranging from –1.53 to –0.19°C.

The distribution pattern of salinity was consistent with temperature. Salinity in the southern Chukchi Sea was the highest (31.0). Salinity declined significantly along the continental slope. Salinity in the deep ocean was relatively low, ranging from 25.97 to 30.64. It increased from the southern to the northern parts of this region, salinity in the southern Canadian Basin was below 27.0, with minimum salinity (25.96) recorded at Sta. MS02, and salinity in the northern area of the western Arctic Ocean was above 30.0, with maximum salinity (30.64) recorded at Sta. Bn11 in the northern Alpha Ridge.

Nutrients in the shallow continental shelf were abundant; levels were lower in the south than in the north. The nutrient concentrations in the southern Chukchi Sea and along the coast of Alaska were relatively low. The silicate concentration at Sta. R03 was 0.01 μmol/L, and the dissolved inorganic nitrogen (DIN) concentration at Sta. CC08 was 0.11 μmol/L, which were the lowest in this region. The nutrient concentration in the northern Chukchi Sea was relatively high. The silicate concentration at Sta. R08 was 19.00 μmol/L, and the phosphate concentration at Sta. R09 was 0.72 μmol/L, the highest in this region. Nutrient concentrations at two stations in the continental slope were similar. The distribution of nutrient concentration across the entire deep ocean was relatively even, except for the particularly high silicate concentration at Stas Bn10 and Bn11.

During the return trip in August 22–30, 2010, most of the sea ice had melted, with ice-free conditions in the shallow continental shelf and in the continental slope. Although the ten stations in the deep ocean were covered with ice, the ice coverage had reduced, with more than 70% ice coverage at Stas SR18, SR20 and SR22 in the northern Mendeleev Abyssal Plain and less than 50% ice coverage at Stas SR16 and SR17 in the southern Mendeleev Abyssal Plain. Ice coverage at Stas M01, M02, M03, M04 and M05 in Chukchi Abyssal Plain was between 10%–30%.

The temperature in the three regions was significantly elevated. The temperature in the shallow continental shelf ranged from 1.76°C to 7.81°C. Stations SR10 in the northern Chukchi Sea and Sta. SR05 in the center of the Chukchi Sea had the lowest and highest temperatures, respectively. The temperature in other stations was approximately 5.0°C. Average temperature in the continental slope was above 0°C. Temperatures increased from northern to southern regions. The lowest and highest temperatures were found at Sta. M07 in the Chukchi Abyssal Plain (–0.72°C) and at Sta. SR11 in the continental slope of the Chukchi Sea (1.21°C), respectively. The average temperature in the deep ocean ranged from –1.55°C to –1.03°C, increasing from northern to southern regions, with the lowest and highest temperatures at Sta. SR22 in the northern Mendeleev Abyssal Plain and at Sta. M04 in the Chukchi Abyssal Plain, respectively.

The salinity of the three regions generally decreased from north to south. Salinity in the shallow continental shelf ranged from 29.43 to 31.99. Salinity at Stas SR03 and SR05 in the central waters was below 30.00. Salinity at Sta. SR01 in the southern Bering Strait, and at Stas SR08 and SR09 in the northern Herald shoal was above 31.0. Salinity in the continental slope was significantly lower than that in the shallow continental shelf, ranging from 26.96 to 27.68. The minimum and maximum salinity was recorded at Stas M07 and SR11, respectively. Salinity in the deep ocean ranged from 26.22 to 30.00. Salinity at Stas SR18, SR20 and SR22 in the northern Mendeleev Abyssal Plain exceeded 29.00. The salinity in remaining stations was below 28.00. The lowest salinity was observed at Sta. M05 in the south of Chukchi Abyssal Plain.

Nutrient concentrations in the three regions were higher in the return trip than in the departing trip. Nutrients in the shallow continental shelf were abundant, and higher in the southern Chukchi Sea compared with that in the northern Chukchi Sea. For example, the phosphate, DIN, and silicate values of Sta. SR01 were 0.94 μmol/L, 4.71 μmol/L, and 17.02 μmol/L, respectively, the highest observed in the shallow continental shelf, while nutrients at Stas SR08 and SR09 in the north of the Herald Shoal were the lowest for these respective nutrients in the shallow continental shelf. Nutrient content were relatively high in the continental slope and in the deep ocean, with small difference among stations. Phosphate concentration in these two regions was higher than that in the shallow continental shelf, while the concentrations of DIN and silicate were lower than that in the shallow continental shelf.

A total of 157 phytoplankton taxa (>5 μm) belonging to 69 genera were identified in the study area, including representatives of the following phyla of algae (Table A1): Cryptophyta (1), Katablepharidophyta (1), Haptophyta (1), Heterokontophyta (2), Bacillariophyta (99), Dinophyta (47), and Euglenophyta (6). Bacillariophyta was the dominant phytoplankton found in the western Arctic Ocean on this voyage, with numerous genera accounting for 62.13% of the total number of species and 95.86% of the total phytoplankton abundance. Dinophyta was the second most abundant class of phytoplankton, accounting for only 29.94% of the total number of species and 3.26% of the total phytoplankton abundance. The abundances of other taxonomic groups were considerably lower, comprising only 0.88% of the total phytoplankton abundance. The dominant species were Navicula pelagica and Thalassiosira nordenskioeldii, accounting for 31.23% and 14.12% of the total phytoplankton abundance, respectively. The other frequently encountered species of diatoms with a station occurrence rate above 20% included Chaetoceros diadema, Chaetoceros furcellatus, Leptocylindrus danicus, Thalassiosira rotula, Nitzschia longissima, Thalassionema nitzschioides, Navicula vanhoeffenii, and Cylindrotheca closterium, accounted for more than 1% of the total phytoplankton abundance. The frequently encountered species of Dinophyta included Gyrodinium fusiforme and Protoperidinium bipes, with a station occurrence rate of 30% and 28%, respectively. However, the abundances only accounted for 0.2% and 0.1% of total phytoplankton abundance, respectively.

The species richness (d) of phytoplankton in the shallow continental shelf was the highest, while the lowest was recorded in the deep ocean, and decreased from low latitude to high latitude. In addition, the species richness (d) of phytoplankton during the departure trip was higher than that during the return trip (Table 1). A total of 137 phytoplankton species were found in the shallow-water zones at a depth of less than 500 m, while only 81 phytoplankton species were found in the deep-water zones at over a 500 m depth. The number of phytoplankton species at Stas R03 and CC01 in the southern Chukchi Sea reached more than 30. The number of species was especially high in the waters near the Herald Shoal in the northern Chukchi Sea. For example, Stas R09 in the northern Herald Shoal had the greatest number of 39. A total of 141 species of phytoplankton were found in the whole region during the departure trip, while only 84 species of phytoplankton were found in the whole region during the return trip.

During the departure trip, the average phytoplankton abundance was as high as 797.07×10² cells/L. Along the shallow continental shelf, the average abundance was (1 467±3 390)×10² cells/L (12.75×10²–13 419.35×10² cells/L). Phytoplankton abundance was concentrated at Stas R08 and R09 at the edge of floating ice in the northern Herald Shoal. Station R09 had the maximum abundance (13 419.35×10² cells/L), which mainly consisted of Navicula spp. (9 053×10² cells/L), Fragilariopsis spp. (2 656×10² cells/L), Thalassiosira spp. (860×10² cells/L), and Nitzschia spp. (740×10² cells/L). These four genera accounted for 99.2% of the total abundance. The predominant species included Navicula pelagica (7 973×10² cells/L), Fragilariopsis cylindrus (940×10² cells/L), Navicula vanhoeffenii (813×10² cells/L), Pesudo-nitzschia seriata (287×10² cells/L), Thalassiosira nordenskioeldii (273×10² cells/L), and Nitzschia longissima (200×10² cells/L). The second highest abundance (1 827×10² cells/L) was located at Sta. CC01, which mainly included Chaetoceros spp. (1 545×10² cells/L) and Leptocylindrus spp. (75×10² cells/L), accounting for 84.6% and 4.1% of the station abundance, respectively. The coast of Alaska had poor nutrient and relatively low abundance. The values at Stas CC08, C02, and C06 were 128.5×10² cells/L, 105.5×10² cells/L, and 115.25×10² cells/L, respectively. In the northwest of Point Barrow, which had high ice coverage and low temperature, small areas of low abundance were observed. The abundance at Stas Co01, Co03, Co05, Co10, and C09 was all below 55×10² cells/L, with the lowest recorded at Sta. Co03 (12.75×10² cells/L). Phytoplankton abundance was dramatically lower in the continental slope, with a mean of (51.88±5.48)×10² cells/L. The abundance at Stas Co05 and S23 were also relatively low, with 48.00×10² cells/L and 55.75×10² cells/L, respectively. The abundance in the deep ocean was even lower, with an average of (27.65±18.67)×10² cells/L. The values at Stas S25, MS01, and MS02 were 6.75×10² cells/L, 13.0×10² cells/L, and 6.75×10² cells/L, respectively. Station Bn11, the northernmost station of the western Arctic Ocean, had more than 90% ice coverage, high salinity, and low temperature. The low temperature inhibits phytoplankton photosynthesis, leading to the abundance at only 7.50×10² cells/L. The seeding of ice algae to the water column at the center of the deep ocean resulted in abundances at Stas Bn05 and Bn06 of 50.0×10² cells/L and 66.5×10² cells/L, respectively, the latter representing the maximum abundance found in this region (Fig. 2).

During the return trip, the phytoplankton abundances declined significantly, with an average of 84.94×10² cells/L. Along the shallow continental shelf, the average abundance was (223.1±275.7)×10² cells/L, ranging from 15.6 cells/L to 746.0×10² cells/L. Due to the early melting of sea ice in the southern Chukchi Sea, the upper layers of seawater had low nutrient content following high consumption of nutrients by phytoplankton. In addition, the enhancement of solar radiation maximized the surface temperature, which resulted in the formation of a strong halocline and blocked the transfer of nutrients to the surface layer from nutrient-rich deep waters, thereby reduced the abundance in the surface layer. Stations SR01, SR03, SR05, and SR07 had an abundance of less than 100×10² cells/L, with Sta. SR03 having the lowest value. While the abundance in the northern Chukchi Sea was more than 200×10² cells/L. Station SR08 had the highest abundance because its temperature and salinity were higher than that in the other stations. The phytoplankton bloom of eurythermal species Cylindrotheca closterium and Protoperidinium bipes caused the high abundance in Sta. SR08. The abundances of these species were 155×10² cells/L and 302.5×10² cells/L, accounting for 20.78% and 40.55% of the total abundance at Sta. SR08, respectively. The average abundance in the continental slope was (12.41±6.29)×10² cells/L (range: 7.50×10²–19.50× 10² cells/L), which was significantly lower than that in the shallow continental shelf. The pattern of phytoplankton distribution was consistent with the patterns of temperature and salinity, with an increasing trend from northern to southern regions. The average abundance in the deep ocean was (9.70±7.99)×10² cells/L (range: 0.80×10²–27.00×10² cells/L) and was lower than that in the shallow continental shelf and continental slope. As the sea ice coverage exceeded 70%, the northern Mendeleev Abyssal Plain had insufficient sunlight and low temperature, which affected the growth of autotrophic phytoplankton such as diatoms. Therefore, the abundance decreased below 10.0×10² cells/L at Stas SR22, SR20 and SR18, with Sta. SR22 having the lowest value. The Chukchi Abyssal Plain, located in the south of the deep ocean, had low sea-ice coverage (10%), resulted in the large amount of meltwater and low salinity of less than 27.0 at the seawater surface. Seawater stratification was more significant under these conditions, and blocked the replenishment of nutrients from the lower layers, thereby reduced the abundance in the upper layers. At the center of the deep ocean, ice coverage was approximately 30%. Ice algae released from melting sea ice had a seeding effect and significantly contributed to the relatively high phytoplankton abundance in this region (Fig. 2). In short, phytoplankton abundance decreased gradually from the shallow continental shelf to the continental slope and further in the deep ocean on both departure and return trips (Table 1).

The vertical distribution of phytoplankton abundance was noticeably different between each region. In the shallow continental shelf, Stas CC04 and SR10 were located in ice-free areas with depths of 51 m and 77 m, respectively. At Sta. CC04, the halocine and thermocline appeared at a depth of 20 m, with the water temperature decreasing from 5.83°C at a depth of 10 m to 3.39°C at 20 m, and salinity increased markedly from 31.86 at a depth of 10 m to 32.01 at 20 m. Nutrient concentration increased significantly at a depth of 30 m, where the phosphate value peaked. Phytoplankton abundance also peaked at a depth of 30 m at this station. At Sta. SR10, thermocline and halocline were found at a depth of 20 m. From depths of 10–20 m, the water temperature dropped dramatically from 2.32°C to –1.29°C, and the salinity increased observably from 30.09 to 31.90. Nutrient concentration gradually increased from the depth of 20 m to 75 m. The maximum phytoplankton abundance was found at a depth of 30 m at this station. Stations S23 and M07 were located in the continental slope with depths of 360 m and 381 m, respectively. Station S23 had more than 40% ice coverage, salinity increased gradually from the surface to the bottom, and there was no obvious thermocline or halocline. Phytoplankton abundance and nutrient concentration both peaked at a depth of 20 m at this station. At Sta. M07, where sampling was conducted during the return trip, the sea ice had completely melted away. Direct solar radiation and the melting of sea-ice caused the surface water to exhibit high temperatures and low salinity. The temperature gradually declined while salinity gradually increased in deeper layers; therefore, there was no thermocline or halocline at this station. Nutrient concentration increased significantly starting at the depth of 50 m and remained at a high level to the depth of 200 m. Phytoplankton abundance peaked at the depth of 50 m at this station. Stations Bn06 and M01 were located in the deep ocean with depths of 3 613 m and 2 310 m, respectively. Station Bn06 in the high-latitude had more than 90% ice coverage, while Sta. M01 in the mid-latitude had less than 30% ice coverage during the return trip. The findings at these two stations were similar, with temperature below 0°C at all depths, and no remarkable thermocline was found. Nutrients were abundant at all depths, increasing significantly from a depth of 50 m and remaining at a high level to a depth of 200 m; meanwhile, salinity increased dramatically at the depth of 50 m. Phytoplankton abundance also peaked at the depth of 50 m. In general, the maximum abundances were confined to the lower layers of the euphotic zone close to the layers of the halocline, thermocline, and nutricline. In the shallow continental shelf, the maximum abundance of phytoplankton occurred at depths below the halocline and thermocline. In the continental slope, the phytoplankton abundance peaked only in layers close to the nutricline. In the deep ocean, the phytoplankton abundance peaked in the deeper layers, which was consistent with the locations of the halocline and nutricline, but not correlated with water temperature (Fig. 3).

The shallow continental shelf was located at lower latitude, and there was more than one month between the departure and return trips. Sea ice had completely melted by the return trip, the Pacific Ocean water of high temperature and salinity inflowed into this region unimpeded. In addition, direct solar radiation and adequate sunlight caused water temperature in the shallow continental shelf to increase significantly from (3.43±2.80)°C to (5.02±1.79)°C, so that there was wide spatial variability in the composition of the dominant phytoplankton species. Phytoplankton abundance decreased dramatically from (1 466.67±3 390)×10² cells/L to (223.12±275.68)×10² cells/L. Furthermore, the species composition changed markedly. During the departure trip, diatoms accounted for 96.74% of the total phytoplankton abundance, and pennate diatoms were the dominant group, pinnate diatoms and centric diatoms accounted for 64.98% and 35.02% of total diatom abundance, respectively. For example, the predominant species N. pelagic (443.90×10² cells/L) and T. nordenskioedii (201.23×10² cells/L), which belong to pennate and centric of diatoms, accounted for 31.28% and 14.18% of total diatom abundance, respectively. Dinoflagellates accounted for 1.88% of the total phytoplankton abundance. During the return trip, the proportion of diatoms increased to 98.04%, and centric diatoms were the dominant group, accounting for 70.6% of total diatom abundance. The predominant species C. diadema (130.80×10² cells/L) and L. danicus (26.50×10² cells/L) accounted for 59.79% and 12.11% of total diatom abundance, respectively. Whereas the proportion of pennate diatoms decreased to 29.4%. For example, the abundances of N. pelagic and Cylindrotheca closterium decreased to 16.21×10² cells/L and 10.40×10² cells/L, accounted for 7.41% and 4.75% of total diatom abundance, respectively. As the same time, the proportion of dinoflagellates in the total phytoplankton abundance decreased to 1.46%. In contrast, in the high latitude deep ocean, the time between the departure and return trips was short, and there was relatively small sea ice coverage change and all stations in the region were still covered with ice. The only significant change was that the thickness and extent of ice coverage declined between the two trips. Temperature at most stations in this region was below 0°C with a small decline between the trips, decreasing from (–1.19±0.55)°C to (–1.26± 0.20)°C. There was small abundance variability in phytoplankton between the depart and return trips, decreasing from (27.65± 18.67)×10² cells/L to (9.70±7.99)×10² cells/L, the range of variation was within the same level of magnitude. In addition, the dominant genera of phytoplankton had small change between the two trips. The proportion of pennate diatoms decreased from 55.0% to 51.82% between the two trips, while that of centric diatoms increased from 33.2% to 35.6%, changes in the proportion of both genera occurred to a lesser extent in the deep ocean than in the shallow continental shelf. The dominant phytoplankton species was the same during the departure and return trips. Thalassionema nitzschioides, accounted for 12.4% and 7.71% of the total phytoplankton abundance, respectively. Navicula spp. accounted for 5.58% and 5.96%, and Nitzschia spp. accounted for 3.78% and 4.03% of the total abundance during the departure and return trips, respectively.

The average surface phytoplankton abundance of the entire sea area was 512.06×10² cells/L (range: 0.8×10² –13 419.35×10² cells/L). The highest abundance was observed in the north of the Herald Shoal, followed by the southern part of the Chukchi Sea. Phytoplankton abundance off the coast of Alaska on the east side of the Chukchi Sea was significantly lower than on the west side of the Chukchi Sea. However, the abundance in the region northwest of Point Barrow was higher than off the coast of Alaska (Table 2). The spatial differences in the phytoplankton community in the western Arctic Ocean represent a characteristic feature of many aquatic environments. The southern Chukchi Sea connects to the Bering Sea via the Bering Strait. The western Chukchi Sea connects to the East Siberia Sea via the De Long Strait, and the eastern part connects to the Beaufort Sea in the northeast of Point Barrow. The northern part comprises the Chukchi Borderland as well as the Canadian Basin. The north central region of the Chukchi Sea is the Herald Shoal with a water depth of 30 m. The differences in the height between the Pacific Ocean and the Arctic Ocean drive the North Pacific Current. As a result, water with higher temperatures, salinity and nutrient content flows into the Chukchi Sea via the Bering Strait. The seawater inflow from the Pacific Ocean comprises three water masses. These include Anadyr Water from the west side of the strait (with low temperatures, high salinity, and high nutrient content), Alaska Coastal Water at the east side of the strait (with high temperatures, low salinity, and low nutrient content), and Bering Shelf Water in the middle of the strait (Coachman and Aagaard, 1988; Shi et al., 2004; Zhao et al., 2010). The seawater in the north of Herald Shoal is derived from Anadyr Water in the winter and early spring seasons, and exhibits low temperatures, high salinity, and high silicate content. In addition, the Herald Shoal has a significant effect on the motion of water mass and melting process of sea ice, and during sea ice melting, the seeding of ice algae and the enhancement of optical radiation stimulate ice-edge phytoplankton blooms (Heimdal, 1989; Zhao et al., 2010). Therefore, Sta. R09 in the north of Herald Shoal had lower temperature, lower salinity, and higher nutrient concentration than other stations of the shallow continental shelf. The temperature and salinity at Sta. R09 were as low as –1.224°C and 30.0, which were lower than the averages of the shallow continental shelf during the same period. Furthermore, the values of phosphate, DIN, and silicate were as high as 0.73 μmol/L, 0.53 μmol/L, and 11.67 μmol/L, respectively. In particular, the silicate concentration was significantly higher than the average of the shallow continental shelf during the same period (Table 1). In addition, ice algae released during ice melting had a seeding effect on phytoplankton blooms. For example, this region has higher populations of ice algae such as Nitzschia frigida than other areas. Therefore, the peak abundance of phytoplankton caused by ice-edge phytoplankton blooms was recorded at Sta. R09 in this study. Directly affected by the nutrient-rich Pacific water, the southern Chukchi Shelf represents an area of intensive development of phytoplankton throughout the spring and summer seasons (Grebmeier et al., 2006a; Sukhanova et al., 2009). The values of DIN and silicate in the shallow continental shelf were high during the departure trip despite high nutrient consumption by phytoplankton, similar to the values recorded during the return trip when ice was melting (Table 1). Stations CC01 and CC04 were located in the zone of a cyclonic circulation formed by the western branch of the Bering Sea Current in the southern Chukchi Sea. Owing to the regional features of the hydrophysical and hydrochemical regimes, Stas CC01 and CC04 in the southern Chukchi Sea had the second highest phytoplankton abundance. The narrow shelf of the Beaufort Sea receives considerable coastal runoff due to the Beaufort Gyre, which lowers the salinity of the nearshore waters and supplies a large amount of terrigenous material. Another possible reason for the formation of the high levels of phytoplankton abundance and biomass in the north-northwest of Point Barrow may lie in the upwelling that periodically occurs in this region and delivers deep water from the adjacent basin upward over the Barrow Canyon (Aagaard and Roach, 1990; Woodgate et al., 2005). In the present study, Sta. Co07 was located on the shelf of the Beaufort Sea in the northwestern Point Barrow. Affected by river freshwater input, it had a salinity of 0.61 lower than the average at the other stations on the east side of the Chukchi Sea (30.49). With the effect of upwelling, the temperature of Sta. Co07 was 3.52°C lower than the average at the other stations on the east side of the Chukchi Sea (2.22°C). In addition, there were abundant nutrients in both the upwelling and offshore currents. The overlaying and joining of these currents enhanced the phytoplankton abundance of Sta. Co07, which was an order of magnitude higher than other stations on the east side of the Chukchi Sea (Table 2). However, the phytoplankton abundance at Stas Co01, Co03, and Co05 was significantly lower than those from the west side of the Chukchi Sea due to its proximity to the coast of Alaska.

The relationship between environmental factors and phytoplankton was analyzed by CCA (Table 3). The first two axes explained 31.4% of the total variance in the phytoplankton abundance. Four significant canonical axes explained 33.4% of the phytoplankton variation. All canonical axes accounted for 58.7% of the variation in the phytoplankton data. We obtained a two-dimensional distribution figure of species, sample distribution, and environmental factors. The result showed that there were two distinct communities of phytoplankton in the Western Arctic Ocean: continental shelf community, which were positively correlated with temperature, encompassed samples collected in the Chukchi Shelf and the Alaska’s northwest coast as well as Stas SR11and SR12 in the continental slope. The community was characteristic of high species diversity and abundance, and primarily dominated by neritic planktonic diatoms such as Chaetoceros debilis, C. diadema, Leptocylindrus danicus, and Nitzschia longissima. Deep ocean community mainly distributed in abysmal sea and deep basin and affected directly by ice coverage. The assemblages of phytoplankton were represented by pennate diatoms such as Navicula pelagic, Cylindrotheca closterium, and Thalassionema nitzschioides. The latter community was subdivided into two groups; the first situated in Chukchi Abyssal Plain and Chukchi Cap as well as Stas Co05, S23 and M07 in continental slope; and the second situated in the Beaufort Sea, Mendeleev Abyssal Plain, and Alpha Ridge. Ice conditions worsened from the low latitude shelf to the high latitude basin as the voyage proceeded north, with the increase of ice coverage, the water temperature gradually decreased, and the two were negatively correlated. Thus, temperature and ice coverage were the most important environmental factors influencing phytoplankton distribution in the surveyed sea. Among the three types of nutrients, silicate had the largest effect on phytoplankton, which had significant influence on Thalassiosira antarctica and T. nordenskioeldii (Fig. 4). The above findings support the conclusion that light and water temperature are more important than nutriment for arctic phytoplankton (Heimdal, 1989; Lin et al., 2009). This clarifies the correlation between ecological characterization of phytoplankton and the environment. The entire western Arctic Ocean was not nutrient limited in this survey, DIN and phosphate was particularly abundant. For example, the concentrations of DIN and phosphate at Stas R09, where phytoplankton bloom occured, were as high as 0.53 μmol/L and 0.73 μmol/L, respectively (Table 2). Therefore, the effects of these two nutrients on phytoplankton were relatively small among the five environmental factors studied.

Previous studies have reported the relationship between seasonal and inter-annual variations of Arctic phytoplankton species with sea ice conditions. With regard to the inter-annual variation, diatoms were dominated by a “sub-ice species”, Melosira arctica, during a period of high ice cover in 1994 in the deep basins; however, during a period of low ice cover in 2008, it was dominated by “pelagic species” such as Nitzschia spp., Fragilariopsis spp., Navicula spp., and Actinocyclus spp. (Coupel et al., 2012). Regarding the seasonal variation, the phytoplankton species composition varied from spring to summer in the southern part of the Chukchi Sea. In early spring (May), the dominant species was the pennate diatom Fragilaria striatula. Two months later during summer, 97.5% of the phytoplankton were represented by the late-spring centric diatom T. nordenskioeldii (Sukhanova et al., 2009). Springer and McRoy (1993) also documented the dominance of the late-springtime and summertime diatom assemblages (Chaetoceros spp., Thalassiosira spp., and Leptocylindrus spp.) in this phytoplankton-rich region during summer. The species succession of Arctic phytoplankton was closely related to the sea ice conditions. Pennate diatoms such as Navicula spp. and Fragilariopsis spp. represent an important part of the algal community in Arctic ice. However, centric diatoms such as Chaetoceros spp., Cylindrotheca spp., and Rhizosolenia spp. are marine planktonic diatoms (Hsiao, 1980; Okolodkov, 1992; Booth and Horner, 1997). In the fjords of Svalbard, the relationship between ice type and the dominant algae species in the water column underneath shows that flagellates prevail under poor light conditions (thick ice, early in the season), while the dominant diatom T. nordenskioeldii appears later in the season at the ice-edge and in the open waters. The phytoplankton succession could be summarized as a change from mostly heterotrophic flagellates to diatoms, followed later in the season by Dinophyceaes (Wiktor1999).

The present study reveals that even during the same season and at the same station, the phytoplankton species composition shows a significant rapid succession due to variations in sea ice conditions. Slightly more than a month elapsed between repeated sampling at the same stations during the return and departure trips. During the departure trip, Stas R08 and R09 were located at the ice-edge with sea ice of 1 m thick. Pennatae dominated the phytoplankton taxa, accounting for 73.05% and 83.35% of the total phytoplankton, respectively. Navicula spp. and Fragilariopsis spp. were the dominant species, accounting for 48.28% and 78.26% of the total phytoplankton abundance, respectively. However, during the return trip, Stas SR08 and SR09 were located in an ice-free region, with a drastic increase in water temperature. Centric diatoms became the dominant group of successional phytoplankton, accounting for 66.80% and 58.27%, respectively. Chaetoceros spp. and Leptocylindrus spp. were the dominant genera, accounting for 60.64% and 53.8% of the total abundance, respectively (Table 4). Due to global warming, the Arctic sea ice is quickly thinning and shrinking. The global consequences of greenhouse warming at polar latitudes are likely to show first in shallow adjacent seas of the Arctic Ocean, especifically the Pacific-influenced regions of the Chukchi Sea and East Siberian Sea (Walsh, 1989). Therefore, the Chukchi Sea is the region that is likely to most significantly manifest global climate change. This region is also ideal for studying the mechanisms of rapid changes in Arctic sea ice and the effects of climate change on marine ecosystems. The Arctic phytoplankton blooms are directly related to timing, duration, and location of ice melt. Sea ice coverage during the departure trip was higher than that during the return trip. Therefore, a comparison was made of the phytoplankton succession at the same locations to compare between the two trips, which reflected the response of phytoplankton to sea ice conditions. This study approach will be useful in determining both past and future changes in relation to predicted sea ice decline.