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Sampling stations. Distributions of the Oyashio region in April and May are outlined by light gray and mid gray shading, respectively. The Western subarctic gyre (WSG) is shaded dark gray. The Oyashio current is represented by the dark gray arrows. (m) diatom bloom stations in the Oyashio; (n) diatom bloom stations in the Transition; (s) stations in the Transition; (d) stations in the Oyashio; (j) stations in the WSG. The subscripts at each station show the sampling periods: (a) first period in April; (b) second period in April; (c) first period in May; (d) second period in May
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Variations in photosynthetic parameters of phytoplankton during the spring bloom were investigated in the western subarctic Pacific. Light-saturated maximum photosynthesis rate (P-max(B)) and light-limited slope (alpha(B)) in the photosynthesis-irradiance (P-E) curve were almost constant in the first half of the bloom and then concurrently increase...
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... Oyashio is a cold western boundary current of the North Pacific (Fig. 1). The Oyashio water is formed by mixing the subarctic water from the East Kam- chatka Current and the Okhotsk water. A part of the Oyashio water flows southward across the subarctic front and mixes with subtropical Kuroshio water. The water mixing region is called the Kuroshio-Oyashio transition region (hereafter Transition region). The ...
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... and hydrographical analysis. Field obser- vations were performed in the Oyashio and Transition regions off Hokkaido, Japan, during 11 to 25 April and 7 to 19 May 2003 (Fig. 1). We visited 3 stations (A4, A7, and A11) 2 times during each cruise. Tem- perature and salinity were measured with a CTD sys- tem (SBE 911 plus, Sea-Bird Electronics). Areas where the temperature at 100 m depth was < 5°C were defined as the Oyashio region, and > 5°C at 100 m as the Transition region (Kawai 1972). The mixed layer depth ...
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... from 19 to 50 mol photons m -2 d -1 , and the maximum incident PAR was 1927 μmol photons m -2 s -1 during the 2 cruises. There were no significant differences in the incident PAR between the Oyashio, WSG, and Transi- tion regions. Large-scale phytoplankton blooms domi- nated by neritic chain-forming diatoms were observed in the Oyashio region (Fig. 1), and the species compo- sition of diatoms was similar among the stations. On the other hand, pelagic chain-forming diatoms domi- nated in the Transition region (Fig. 1), and the magni- tude of the diatom blooms was relatively low compared to the Oyashio region ([chl a] in the growth phase in Table 1). In the WSG, no diatom blooms were ...
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... incident PAR between the Oyashio, WSG, and Transi- tion regions. Large-scale phytoplankton blooms domi- nated by neritic chain-forming diatoms were observed in the Oyashio region (Fig. 1), and the species compo- sition of diatoms was similar among the stations. On the other hand, pelagic chain-forming diatoms domi- nated in the Transition region (Fig. 1), and the magni- tude of the diatom blooms was relatively low compared to the Oyashio region ([chl a] in the growth phase in Table 1). In the WSG, no diatom blooms were observed during the 2 ...
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... tions of nitrate and silicic acid in the Oyashio region were 1.6 times higher than those in the Transition region (Table 1, Fig. 2c,e). The D-Fe concentrations were also 1.8 times higher in the Oyashio region than in the Transition region (Table 1, Fig. 2f). Chl a concen- trations, diatom fractions, PP, and algal growth rates in the Oyashio and Transition regions were similar to each other ( Fig. 2g-j). ...
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... Finally, 10 coincident satellite data and HPLC data were selected at near-surface depths (<10 m) using the nearest method, as show in Figure 1 To further explore the effect of eddies on phytoplankton, this paper uses the VGPM model to calculate primary productivity (PP) [29]. Yoshie et al. calculated PP/Chl to express the carbon sequestration efficiency per unit of chlorophyll in phytoplankton and calculated PP/POC to express the growth efficiency of phytoplankton [30]. ...
... Its general architecture and initial validation are documented by Bleck [31], followed by further developments by Halliwell To further explore the effect of eddies on phytoplankton, this paper uses the VGPM model to calculate primary productivity (PP) [29]. Yoshie et al. calculated PP/Chl to express the carbon sequestration efficiency per unit of chlorophyll in phytoplankton and calculated PP/POC to express the growth efficiency of phytoplankton [30]. ...
Mesoscale eddies have essential effects on the distribution of the different sizes of phytoplankton and the status of phytoplankton physiology. The impact of mesoscale eddies on phytoplankton size and physiological level in the northern South China Sea is analyzed based on satellite data and HYCOM-simulated results from 2003 to 2018. The results show that there are higher nanophytoplankton levels for high and low nonlinearity in the center of cyclonic eddies. At the same time, the growth rate of phytoplankton increased, and the assimilation of phytoplankton decreased. Moreover, in the center of anticyclonic eddies, a lower nanophytoplankton level is observed in both high and low nonlinearity. At the same time, the growth rate of phytoplankton decreased, and the assimilation of phytoplankton increased. In addition, there is a higher nanophytoplankton level in the mode-water eddy, while the growth rate of phytoplankton is increased, and the assimilation of phytoplankton is decreased.
... Thalassiosira pseudonana is considered an average-sized (Kipp et al., 2019), centric diatom known to both form chains of cells (Waite et al., 1997;Davis et al., 2005;Hildebrand et al., 2007) or remain as a single-celled organism. Species of the genus Thalassiosira are found in many regions of the ocean including coastal and open ocean regions, as well as in estuarine systems (Pratt, 1965;Aizawa et al., 2005;Hoppenrath et al., 2007;Yoshie et al., 2010;Leblanc et al., 2012;Rynearson et al., 2020). Since the sequencing of its genome, Thalassiosira pseudonana has served as a model organism for the general group of diatoms (Armbrust et al., 2004;Parker et al., 2008). ...
Diatoms are important microorganisms involved in global primary production, nutrient cycling, and carbon sequestration. A unique feature of diatoms is their silica frustules, which impact sinking speed, defense against predators and viruses, and growth cycling. Thus, frustules are inherently linked to their role in ecosystems and biogeochemical cycles. However, constraints on cellular silica levels remain unclear and few existing models resolve diatom elemental stoichiometry to specifically include silica. Here, we use a coarse-grained model of the diatom, Thalassiosira pseudonana, compared with laboratory results to illustrate the relationship of silica uptake with elemental stoichiometry of other nutrients. The model-data comparison suggests the balance between growth rate and silica uptake constrains the amount of cellular silica. Additionally, it expresses relationships between silica, nitrogen, phosphorus, and carbon to changing growth rates in nitrogen-limited and phosphorus-limited regimes. First, our model-data comparison suggests Si uptake hits a maximum cellular quota at low growth rates and below this maximum there is independent Si uptake. In each nutrient regime, Si:N, Si:P, and Si:C decrease exponentially with growth rate when Si is below the maximum limit. This is explained by independent Si uptake and increased loss of Si to new cells. These results provide predictions of diatom stoichiometry and allocation, which can be used in ecosystem models to differentiate phytoplankton types to better represent diatoms’ contribution to global biogeochemical cycles and ecosystems.
... The former variation (E k -dependent variability) is mainly caused by photo-acclimation of phytoplankton (Geider et al. 1998;Flynn 2001). The latter variation (E k -independent variability) is suggested to be strongly related to carbon fixation and nitrogen assimilation; however, its mechanism is not well documented for field data (Behrenfeld et al. 2004;Yoshie et al. 2010;Yoshida et al. 2020). ...
... There was a two-fold vertical difference in P * max in offshore Suruga Bay in the warm period, which was almost equivalent to that reported in Japan Sea and the East China Sea during the stratified season (Yoshikawa and Furuya 2008). The value of P * max is enzymatically controlled and temperature dependent in many cases (Eppley 1972;Yoshie et al. 2010;Yoshikawa and Furuya 2008). The relatively higher P * max at the surface compared with that in the subsurface in the present study appeared to be ascribed to the higher surface temperature. ...
... also shown in addition to their actual values (from the data of sampling day; closed squares). The open and closed bars represent warm and cold periods, respectively observation of the data; the former variability is related to photo-acclimation of phytoplankton (Flynn 2001;Geider et al.1998), and the latter is related to nutrient conditions and growth phases (Behrenfeld et al. 2004;Yoshie et al. 2010). Few studies have reported that these two mechanisms occur simultaneously in natural phytoplankton assemblages. ...
Seasonal and vertical variations in photosynthetic–irradiance (P–E) parameters and primary production were examined in offshore Suruga Bay, where both the subtropical warm Kuroshio Water and eutrophic coastal water were influential. The maximum light-utilization coefficient, α*, did not show significant variation between the surface and subsurface layers in both the warm and cold periods. Vertical differences in the maximum photosynthetic rate, \(P_{{\max }}^{*}\), and the light-saturation parameter, Ek, were small even during the warm period. Seasonal variations in the P–E parameters were not significantly related to dissolved inorganic nitrogen (DIN) at the surface layer. These results suggest that low-nutrient and high-light stresses were not serious at the surface layer, even in the stratified season, because of the continuous inflow of high-nutrient river water. Seasonal variations in \(P_{{\max }}^{*}\) and α* were suggested to be regulated by both the Ek-dependent (photo-acclimation of phytoplankton) and Ek-independent (response to nutrient and growth conditions) mechanisms from the correlation analyses. The subsurface chlorophyll maximum (SCM) formation was less obvious, and a major part of the primary production occurred in the upper half of the euphotic zone. The water-column primary production, ΣPP (total), tended to be overestimated by ignoring vertical variations in the P–E parameters and chlorophyll a, especially during the warm period. These features of primary production appeared to be similar to those reported in coastal waters rather than in subtropical waters, playing an essential role in the fishing ground formation in the bay.
... In addition, a massive spring phytoplankton bloom was observed every year in the Oyashio waters at the southwestern edge of the WSG, and the massive bloom had significantly higher chlorophyll a concentrations (over 5 mg/m 3 ) than those in the WSG waters Yoshie et al. 2010;Okamoto et al. 2010;Sugie et al. 2010;Hattori-Saito et al. 2010;Shiozaki et al. 2014;Isada et al. 2010Isada et al. , 2019Kuroda et al. 2019). ...
One of the most important breakthroughs in oceanography in the last 30 years was the discovery that iron (Fe) controls biological production as a micronutrient, and our understanding of Fe and nutrient biogeochemical dynamics in the ocean has significantly advanced. In this review, we looked back both previous and updated knowledge of the natural Fe supply processes and nutrient dynamics in the subarctic Pacific and its impact on biological production. Although atmospheric dust has been considered to be the most important source of Fe affecting biological production in the subarctic Pacific, other oceanic sources of Fe have been discovered. We propose a coherent explanation for the biological response in subarctic Pacific high nutrient low chlorophyll (HNLC) waters that incorporates knowledge of both the atmospheric Fe supplies and the oceanic Fe supplies. Finally, we extract future directions for Fe oceanographic research in the subarctic Pacific and summarize the uncertain issues identified thus far.
... The western subarctic Pacific (WSP) is the largest CO 2 sink by biological activity among the world's oceans (Takahashi et al., 2002) and has high vertical transport efficiency of particulate organic carbon in spring (Honda, 2003;Kawakami et al., 2004Kawakami et al., , 2015. These distinctive characteristics of the WSP are partly due to the massive spring diatom blooms in the Oyashio region off the Kuril Islands (Isada et al., 2010;Suzuki et al., 2011;Yoshie et al., 2010). The WSP is also known to be an HNLC region in summer and winter (e.g., Fujiki et al., 2014;Hattori-Saito et al., 2010;Suzuki et al., 2009;Tsuda et al., 2003). ...
... Therefore, this study examined the effects of Fe availability on the community composition and photophysiology of phytoplankton in the WSP near the Kuril Islands and the eastern Kamchatka Peninsula. Phytoplankton community composition was investigated with scanning electron microscopy (SEM) (e.g., Sugie & Suzuki, 2017), which enabled the identification and quantification of armored cells such as diatoms that were predominant in the study area (Mochizuki et al., 2002;Yoshie et al., 2010;Suzuki et al., 2011Suzuki et al., , 2014. Moreover, the photophysiology of phytoplankton was estimated by photosynthesis-irradiance (P-E) curves, which provide powerful insights into the physiological states of phytoplankton such as light-dark acclimation and responses to nutrient availability (Bouman et al., 2018;MacIntyre et al., 2002;Sakshaug et al., 1997). ...
... Diatoms possess capabilities of exchanging ATP between plastids and mitochondria and of changing the NADPH/ATP ratio (Bailleul et al., 2015). The diatom blooms and diatom-dominated community sometimes showed E k -independent variability (e.g., Isada et al., 2013;Morán & Scharek, 2015;Platt et al., 1992;Yoshie et al., 2010). Yoshie et al. (2010) suggests that the level of ammonia compared with nitrate could contribute E k -independent variability because the difference in the redox state of these nitrogen compounds could affect the quota of photosynthetic energy for carbon fixation and nitrogen assimilation. ...
The western subarctic Pacific (WSP) is known as one of the most productive regions among the world's oceans in spring. However, its oceanic waters are also known as a high‐nutrient, low‐chlorophyll region during summer due to low iron (Fe) availability in seawater. Indeed, recent studies have demonstrated that the distribution of Fe in the WSP is complex and heterogeneous. This study thus investigated the effects of Fe availability on the community composition and photophysiology of surface phytoplankton from coastal to offshore waters in the WSP in the summer of 2014. Although relatively high concentrations (>2 mg/m³) of chlorophyll (chl) a were found in the Sea of Okhotsk and some coastal waters, low chl a concentrations (<1 mg/m³) were commonly observed in offshore waters. Based on dissolved Fe and macronutrient concentrations, we deduced that low Fe availability limited phytoplankton growth in offshore waters, whereas low silicate and/or nitrate levels limited growth in the shelf areas. Scanning electron microscopy also revealed that the centric diatom Chaetoceros exclusively dominated the diatom assemblages in the shelf and coexisted with pennate diatoms in offshore waters, respectively. Primary productivity in surface waters was negatively correlated with the bottom of the euphotic layer or the light saturation index of the photosynthesis‐irradiance curve, which indicates that the phytoplankton assemblages were well acclimated to in situ light conditions regardless of the water masses.
... Morphologically identical or ʺcrypticʺ species have been identified in the ecologically important diatom genus Thalassiosira [9,10]. Globally, Thalassiosira is one of the most abundant and diverse diatom genera [11] and includes species that are ecologically important components of phytoplankton communities in estuarine, coastal, and open ocean regions [12][13][14][15][16][17]. The identification of Thalassiosira species is known to be difficult, especially using light microscopy (LM) due to subtle differences in frustule morphology [18] and some Thalassiosira species are unidentifiable with LM methods [19]. ...
Diatoms generate nearly half of marine primary production and are comprised of a diverse array of species that are often morphologically cryptic or difficult to identify using light microscopy. Here, species composition and realized thermal niches of species in the diatom genus Thalassiosira were examined at the site of the Narragansett Bay (NBay) Long-Term Plankton Time Series using a combination of light microscopy (LM), high-throughput sequencing (HTS) of the 18S rDNA V4 region and historical records. Thalassiosira species were identified over 6 years using a combination of LM and DNA sequences. Sixteen Thalassiosira taxa were identified using HTS: nine were newly identified in NBay. Several newly identified species have small cell diameters and are difficult to identify using LM. However, they appeared frequently and thus may play a significant ecological role in NBay, particularly since their realized niches suggest they are eurythermal and able to tolerate the >25 °C temperature range of NBay. Four distinct species assemblages that grouped by season were best explained by surface water temperature. When compared to historical records, we found that the cold-water species Thalassiosira nordenskioeldii has decreased in persistence over time, suggesting that increasing surface water temperature has influenced the ecology of phytoplankton in NBay.
... The western subarctic Pacific is a preferable field to study cell responses of bloom-forming species because large-scale diatom blooms occur annually in spring in this region 5,20,21 . The hydrography of this region in spring is complex, with dominant water columns: Oyashio Water (OW), Coastal Oyashio Water (COW), and modified Kuroshio Water (MKW) 22 . ...
... The proportion of diatoms in the SCM layer (76.7%) was higher than that in the surface layer (51.4%), suggesting that diatoms have more active transcriptional responses in the SCM layer than at the surface. To elucidate development stages of the diatom blooms at the surface and SCM, we used an index, I devel 20 . I devel was 0.95 and 0.77 at the surface and SCM, respectively, and thus diatoms of the surface and SCM are under late and initial decline phase of the diatom bloom. ...
... The filters were immersed in RNAlater (Ambion, Austin, TX, USA) for 15 min at room temperature, and flash frozen in liquid nitrogen, then stored at −80 °C until analysis. Samples for nutrients and chlorophyll a concentration analysis were collected from nine layers (0, 10,20,30,40,50,60,80, and 100 m depth). Subsamples of 1 L were immediately fixed with acid Lugol's solution (4% final concentration) and preserved at 4 °C. ...
Diatoms play important roles in primary production and carbon transportation in various environments. Large-scale diatom bloom occurs worldwide; however, metabolic responses of diatoms to environmental conditions have been little studied. Here, we targeted the Oyashio region of the western subarctic Pacific where diatoms bloom every spring and investigated metabolic response of major diatoms to bloom formation by comparing metatranscriptomes between two depths corresponding to different bloom phases. Thalassiosira nordenskioeldii and Chaetoceros debilis are two commonly occurring species at the study site. The gene expression profile was drastically different between the surface (late decline phase of the bloom; 10 m depth) and the subsurface chlorophyll maximum (SCM, initial decline phase of the bloom; 30 m depth); in particular, both species had high expression of genes for nitrate uptake at the surface, but for ammonia uptake at the SCM. Our culture experiments using T. nordenskioeldii imitating the environmental conditions showed that gene expression for nitrate and ammonia transporters was induced by nitrate addition and active cell division, respectively. These results indicate that the requirement for different nitrogen compounds is a major determinant of diatom species responses during bloom maturing.
... Furthermore, chlorophyll (Chl) a concentration derived from ocean color remote sensing clearly demonstrates the spatiotemporal heterogeneity of spring phytoplankton bloom in the Oyashio region (Okamoto et al. 2010). However, little is known about the factors regulating phytoplankton physiology and spatial heterogeneity of phytoplankton biomass during the spring bloom in the Oyashio region owing to a lack of extensive in situ observation (Yoshie et al. 2010;Shiozaki et al. 2014). Because the spring bloom is an important event for the biological carbon pump and supports the energy transfer to a wide variety of commercially valuable fishes in the Oyashio region, clarifying the factors controlling bloom heterogeneity in relation to the mixing water masses would accelerate further understanding of the Oyashio ecosystem. ...
... Accordingly, our E k -independent variability observed during OECOS (see "PE parameters" section) could be a consequence of iron stress in the phytoplankton community. Similarly, previous studies (Isada et al. 2009;Yoshie et al. 2010) showed E k -independent variability during the spring bloom in the Oyashio region. It is known that the variability is accompanied by the allocation of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) made by PET chain in a coordinated fashion to regulate the supply of photosynthate to the Calvin-Benson cycle (Behrenfeld et al. 2008;Halsey et al. 2010). ...
Spring phytoplankton blooms play a major role in the carbon biogeochemical cycle of the Oyashio region, western subarctic Pacific, where the seasonal biological drawdown effect on seawater pCO2 is one of the greatest among the world's oceans. However, the bloom often terminates before depleting macronutrients, and the initiation and magnitude of the bloom is heterogeneous. We conducted a high resolution taxonomic and physiological assessment of phytoplankton in relation to the different physicochemical water masses of Coastal Oyashio Water (COW), Oyashio water (OYW), and modified Kuroshio water (MKW) in the Oyashio region from April to June 2007. Massive diatom blooms were found in April. Then, chlorophyll a concentration, cell abundance of diatom taxa, and the maximum photosystem II photochemical efficiency (Fv/Fm) were positively correlated with the mixing ratios of COW, suggesting that the spring bloom in April was strongly affected by the intrusion of COW. In the OYW, intensive blooms occurred from the middle of May under low dissolved iron (DFe) concentration (< 0.26 nM). Redundancy analysis showed that while diatom blooms accompanied by COW were related to DFe concentration, this was not the case in the OYW. These results indicated that diatoms in the OYW possess different iron adaptation strategies compared with diatoms in the water masses affected by COW. This led to the spatial heterogeneity of the Oyashio spring bloom. The results presented here demonstrate that water mass characterization with detailed assessments of phytoplankton taxonomy and physiological status can improve our understanding of marine ecosystems.
... The western subarctic Pacific has one of the highest transport efficiencies of particulate organic carbon (POC) in the water column (Honda, 2003;Kawakami et al., 2004Kawakami et al., , 2015 and the largest biological effect on seasonal changes in surface pCO 2 drawdown in the world ocean (Takahashi et al., 2002). These remarkable biogeochemical features are partly caused by large to vast spring diatom blooms observed in Oyashio (OY) and coastal Oyashio (COY) waters (Chiba et al., 2004;Hattori-Saito et al., 2010;Yoshie et al., 2010;Suzuki et al., 2011), which are biologically highly productive during spring (Isoda and Kishi, 2003;Isada et al., 2010). The OY is the westernmost current of the Western Subarctic Gyre (WSG) and is influenced by both the Eastern Kamchatka current and the Okhotsk Mode water (Yasuda, 2003 et al., 2008). ...
... According to Behrenfeld et al. (2004), the physiological mechanisms responsible for E k -dependent variability generally involve acclimation strategies aimed at maximizing growth under variable light conditions. On the other hand, in the OY region, E k -independent variability, which is the result of co-variation in P B max and α B , was observed from March to May by Isada et al. (2009) and in the post-bloom phase by Yoshie et al. (2010). Additionally, Yoshie et al. (2010) pointed out that water temperature and ammonium levels significantly affected the E k -independent variability. ...
... On the other hand, in the OY region, E k -independent variability, which is the result of co-variation in P B max and α B , was observed from March to May by Isada et al. (2009) and in the post-bloom phase by Yoshie et al. (2010). Additionally, Yoshie et al. (2010) pointed out that water temperature and ammonium levels significantly affected the E k -independent variability. In this study, water temperatures of COY remained relatively low (< 4°C; Table 1) and ammonia did not influence any photosynthetic parameters ( Table 2). ...
Globally, the western subarctic Pacific is known as the region with the largest seasonal drawdown in the partial
pressure of CO2 due to biological activity, i.e., high spring primary production and particulate organic carbon
flux. These distinctive features are mainly caused by intense spring diatom blooms in coastal Oyashio (COY) and
Oyashio (OY) waters. Although phytoplankton assemblages in OY waters are rather well studied, little is known
about COY waters. In this study, photophysiological properties and phytoplankton community composition in
COY waters were investigated during the pre-bloom and bloom periods from March to April 2015. Next-generation
sequencing targeting the 18S rRNA gene revealed that the diatom Thalassiosira generally dominated the
phytoplankton community and showed distinct differences in the diatom communities in shelf and offshore
waters of the COY. Additionally, the relative contribution of Thalassiosira to the total diatom assemblages
showed a positive correlation with maximum photosynthetic rates (PB
max) occurring throughout this study.
Chlorophyll a concentration and primary productivity were also positively correlated with sea surface temperature,
suggesting that temperature was a critical factor for bloom development. Short-term on-deck incubation
experiments were carried out to examine the role of temperature in determining planktonic photosynthetic
processes. Our results showed an increase in PB
max with rising temperature in assemblages from the shelf
COY waters. Similarly, transcription levels of the diatom-specific rbcL gene, which encodes the large subunit of
RuBisCO, also increased with rising temperature in the shelf assemblages. In contrast, temperature had little
effect on the maximum photochemical quantum efficiency (Fv/Fm) of photosystem II. The results suggested that
the transcription activity of the diatom-specific rbcL gene was upregulated by the increase in temperature, and
that led to the higher PB
max values and the spring diatom bloom in the shelf COY region.
... This indicated that the small photosynthetic eukaryotic community composition was dominated by a small number of phylogenetic taxa during these seasons, similar to the phytoplankton bloom in the Oyashio region where larger size diatoms (i.e. >10 μm) predominated (Kasai et al. 1997;Saito, Tsuda and Kasai 2002;Chiba et al. 2004;Yoshie et al. 2010). The other offshore site (A21) was characterized by variable composition throughout the study period, especially during the fall when a warm and high temperature water mass known as the WCR was observed ( Table 1). ...
In this study, we investigated the distribution of small photosynthetic eukaryotes in the near-surface layer of the western North Pacific at four stations, including two oceanic stations where the subarctic Oyashio and subtropical Kuroshio currents influence a transition region and the bay mouth and head of the Sendai Bay, from April 2012 to May 2013. Flow cytometry was applied to sort small photosynthetic eukaryotes (< 5 μm), and high-throughput sequencing of 18S rDNA was performed. Our taxonomic analysis showed that 19/195 operational taxonomic units (OTUs) were frequently distributed among all sites. Composition analysis showed that the OTUs had characteristic patterns and were divided into four main groups. Two groups reflected the low-saline water and winter season, with the characteristic OTUs belonging to diatoms; to note, Chaetoceros and Leptocylindrus were characteristic of low saline water, and two diatom genera (Minidiscus and Minutocellus) and Cryptomonadales-related OTUs were prevalent in the winter. Our results indicated that the community composition of small photosynthetic eukaryotes seasonally changes in a dynamic manner according to variations in water properties.
