Atsushi Yamaguchi | Hokkaido University (original) (raw)

Papers by Atsushi Yamaguchi

Nature Communications, 2015

The first relates to the presentation of sinking particle flux data in Fig. 2c, which was mistake... more The first relates to the presentation of sinking particle flux data in Fig. 2c, which was mistakenly plotted in reference to a previous dataset, rather than Supplementary Data 1. The corrected version of Fig. 2 appears below. The second error concerns the miscalculation of the Particulate Organic Nitrogen (PON) and Carbon (POC) fluxes presented in Supplementary Data 1 and Fig. 3b,c. During the flux calculation process, we mistakenly used the weight of analysed subsamples rather than aliquot samples, which resulted in an underestimation of the mole fluxes of PON and POC in the published Article. Two new columns, detailing the correctly calculated fluxes, have been added to Supplementary Data 1, while the corrected version of Fig. 3 appears below.

Chattonella globosa is known as a red tide species in western Japan from Kyushu, the Seto Inland ... more Chattonella globosa is known as a red tide species in western Japan from Kyushu, the Seto Inland Sea to Tokyo Bay, but the existence has been unknown in the northern Japan.This is the first report of the observation of occurrence of C. globosa from Funka Bay, in Hokkaido, Japan.Cells were globular and 33 -42 µm in diameter with single flagellum, and sometimes per-formed "Amoeba -like movement".We also studied the seasonal variation in density and distribution of C. globosa.This spe-cies was first detected in July 2011 and reached at maximum cell density of 2.610 4 cells L

Polar Science, 2019

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plum... more In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plumes near the calving fronts. To evaluate the effects of this discharge on the zooplankton community in the fjords, we collected sea surface zooplankton samples in Bowdoin Fjord in northwestern Greenland during the summer of 2016 and made microscopic, OPC and ZooScan analyses. Within the three quantitative methods, ZooScan has advantages that can evaluate various parameters (e.g., abundance, biomass, size and taxonomic information) simultaneously and has the ability to eliminate abiotic particles, such as silt and sediment, which are abundant in samples. Based on taxonomic biomass data, the zooplankton community is clustered into three groups, which varied spatially: inner, middle and outer fjord groups. Jellyfish dominated the outer fjord group, and barnacle cypris larvae dominated the middle fjord group. For the inner fjord group, large-sized Calanus spp. and chaetognaths were abundant. Since these species are characterized with oceanic taxa, they would intrude through the deep fjord water and subsequently be upwelled through entrainment of glacially modified plume water. From the NBSS analysis on zooplankton size spectra, the steep slope of NBSS in the middle fjord community suggests that the high productivity was caused by the addition of meroplanktonic cypris larvae.

Polar Science, 2018

In oceans worldwide, chaetognaths are the second most dominant zooplankton taxa and have vital tr... more In oceans worldwide, chaetognaths are the second most dominant zooplankton taxa and have vital transfer roles between herbivores and higher trophic organisms. Despite the chaetognaths' importance, little is known regarding their ecology in the Chukchi Sea, where recent ice reduction is prominent in the Arctic Ocean. We made comparisons of the chaetognath abundance, biomass, community structure (total length and gonadal maturation) and feeding impacts between the years 1991 and 2007. Within the chaetognath community, Parasagitta elegans was the abundant species, and Eukrohnia hamata only occurred in the southern region in 2007. Yearly differences were detected in chaetognath abundance, biomass, total length and gonadal maturation. All of these parameters were greater and more advanced in 1991 than in 2007. The most prominent yearly differences were in the horizontal distributions of chaetognaths, primarily in the northern region in 1991 and the southern region in 2007. The southern chaetognath populations observed in 2007 were considered to be transported by the Pacific water through the Bering Strait. Because of the differences in the original populations between the Chukchi Sea in 1991 and the Bering Sea in 2007, differences in these two years may be due to regional differences. As the Arctic warms, the horizontal distribution pattern, similar with 2007 in this study, will be more pronounced.

PloS one, 2017

The eastern Bering Sea has a vast continental shelf, which contains various endangered marine mam... more The eastern Bering Sea has a vast continental shelf, which contains various endangered marine mammals and large fishery resources. Recently, high numbers of toxic A. tamarense resting cysts were found in the bottom sediment surface of the eastern Bering Sea shelf, suggesting that the blooms have recently occurred. However, little is known about the presence of A. tamarense vegetative cells in the eastern Bering Sea. This study's goals were to detect the occurrence of A. tamarense vegetative cells on the eastern Bering Sea shelf and to find a relationship between environmental factors and their presence. Inter-annual field surveys were conducted to detect A. tamarense cells and environmental factors, such as nutrients, salinity, chlorophyll a, and water temperature, along a transect line on the eastern Bering Sea shelf during the summers of 2004, 2005, 2006, 2009, 2012, and 2013. A. tamarense vegetative cells were detected during every sampling year, and their quantities varied g...

Progress in Oceanography, 2017

A total of 100 mesozooplankton samples collected with NORPAC nets from a 0 to 150-m depth at lati... more A total of 100 mesozooplankton samples collected with NORPAC nets from a 0 to 150-m depth at latitudinal stations (35-44°N) along 155°E each May from 2002 through 2011 were analyzed. The mesozooplankton abundance at each station varied from 39 to 1106 ind. m À3. The mesozooplankton biomass was consistently higher (80-100 mg DM m À3) in the transition domain (40-42°N) than the biomass in the other domains. An empirical metabolic rate-based carbon budget model indicated that production of mesozooplankton suspension feeders was highest (120-175 mg C m À2 d À1) in the transition domain. A comparison between the production of the mesozooplankton suspension feeders and the food requirement of mesozooplankton carnivores indicated that the latter was well fulfilled by the former in the subarctic and transition domains. However, the food requirement of the mesozooplankton carnivores was near equal to or exceeded the production of mesozooplankton suspension feeders in the subtropical domain. As an annual event, the feeding migration of epipelagic fish to the transition and subarctic domains in summer may be interpreted by their utilization of the excess secondary production (production of mesozooplankton suspension feeders).

Harmful algae, 2017

Abundant cyst distributions of the toxic dinoflagellate Alexandrium fundyense (previous A. tamare... more Abundant cyst distributions of the toxic dinoflagellate Alexandrium fundyense (previous A. tamarense north American clade) were recently observed on the north Chukchi Sea shelf and on the eastern Bering Sea shelf, suggesting that A. fundyense is both highly adapted to the local environments in the high latitude areas and might cause toxin contamination of plankton feeders. However, little is known about the physiological characteristics and toxin profiles of A. fundyense in these areas, which are characterized by low water temperatures, weak sunlight, and more or less permanent ice cover during winter. To clarify the physiological characteristics of A. fundyense, the effects of water temperature and light intensity on the vegetative growth and toxin profiles of this species were examined using A. fundyense strains isolated from one sediment sample collected from each area. Using the same sediments samples, seasonal changes of the cyst germination in different water temperatures were...

Journal of Japanese Biochemical Society, 2015

Biogeosciences Discussions, 2015

In the Chukchi Sea, due to the recent drastic reduction of sea-ice during the summer, an increasi... more In the Chukchi Sea, due to the recent drastic reduction of sea-ice during the summer, an increasing formation of atmospheric turbulence has been reported. However, the importance and effects of atmospheric turbulence on the marine ecosystem are not fully understood in this region. To evaluate the effect of atmospheric turbulence on the marine ecosystem, high-frequent sampling (two to four times per day) on the mesozooplankton community and the gut pigment of dominant copepods were made at a fixed station in the Chukchi Sea from 10 to 25 September 2013. During the study period, a strong wind event (SWE) was observed on 18 September. After the SWE, the standing stock of chlorophyll a (chl a) was increased, especially for micro-size (> 10 µm) fractions. Zooplankton abundance ranged 23 610-56 809 ind. m −2 and exhibited no clear changes with SWE. In terms of abundance, calanoid copepods constituted the most dominated taxa (mean: 57 %), followed by barnacle larvae (31 %). Within the calanoid copepods, small-sized Pseudocalanus spp. (65 %) and large-sized Calanus glacialis (30 %) dominated. In the population structure of C. glacialis, copepodid stage 5 (C5) dominated, and the mean copepodid stage did not vary with SWE. The dominance of accumulated lipids in C5 and C6 females with immature gonads indicated that they were preparing for seasonal diapause. The gut pigment of C. glacialis C5 was higher at night and was correlated with ambient chl a, and a significant increase was observed after SWE (2.6 vs. 4.5 ng pigment ind. −1). Assuming C : Chl a ratio, the grazing impact by C. glacialis C5 was estimated to be 4.14 mg C m −2 day −1 , which corresponded to 0.5-4.6 % of the standing stock of micro-size phytoplankton. Compared with the metabolic food requirement, their feeding on phytoplankton accounted for 12.6 % of their total food requirement. These facts suggest that C. glacialis could not maintain their population on solely phytoplankton food, and other food sources (i.e., microzooplankton) are important in autumn. As observed for the increase in gut pigment, temporal phytoplankton bloom, which is enhanced by the atmospheric turbulence (SWE) in autumn, may have a positive effect on copepod nutrition. However, because of the 3880 BGD 12, 3879-3904, 2015 Short-term changes of the mesozooplankton community K. Matsuno et al.

Zoological Studies, 2015

Background An optical plankton counter (OPC) was used to examine spatial and temporal changes in ... more Background An optical plankton counter (OPC) was used to examine spatial and temporal changes in the zooplankton size spectra in the neighboring waters of Japan from May to August 2011. Results Based on the zooplankton biovolume of equivalent spherical diameter (ESD) in 45 bins for every 0.1 mm between 0.5 and 5.0 mm, a Bray-Curtis cluster analysis classified the zooplankton communities into six groups. The geographical distribution of each group varied from each of the others. Groups with a dominance of 4 to 5 mm ESD were observed in northern marginal seas (northern Japan Sea and Okhotsk Sea), while the least biovolume with a dominance of a small-size class (0.5 to 1 mm) was observed for the Kuroshio extension. Temporal changes were observed along the 155° E line, i.e., a high biovolume group dominated by 2 to 3 mm ESD during May shifted to other size spectra groups during July to August. These temporal changes were caused by the seasonal vertical descent of dominant large Neocalan...

Zoological Studies, 2015

Background: Despite its ecological importance, little information is available regarding the spat... more Background: Despite its ecological importance, little information is available regarding the spatial and vertical changes in the calanoid copepod community over large geographical regions. This study investigated the spatial and vertical patterns in calanoid copepod abundance and community structure using zooplankton samples collected between depths of 0 and 2,615 m across the North Pacific from 0°to 56°N. Results: A total of 211 calanoid copepod species belonging to 66 genera and 24 families were identified. Calanoid copepod abundance decreased with increasing depth, and few latitudinal differences were detected. Across the entire region, species diversity peaked near 500 to 2,000 m in depth. The calanoid copepod community was separated into seven groups with distinct spatial and vertical distributions. For all groups, the number of species was low (28 to 37 species) in the subarctic region (north of 40°N) and high (116 to 121 species) in the subtropical-tropical region. The deepest group in the subtropical-tropical region was composed of cosmopolitan species, and this group was also observed in deep water in the subarctic region. Conclusions: In deep water, most of the calanoid copepod community consisted of cosmopolitan species, while an endemic community was observed in the subarctic region. Because the food of deep-sea calanoid copepods originates from the surface layer, sufficient and excess flux in the eutrophic subarctic region may be responsible for maintaining the endemic species in the region.

Polar Biology, 2015

Increasing numbers of Pacific copepods are being transported from the Bering Sea to the Arctic Oc... more Increasing numbers of Pacific copepods are being transported from the Bering Sea to the Arctic Ocean, so there is clear potential to affect the structure and composition of the Arctic food web. We investigated the grazing impacts of Arctic and Pacific copepods in the western Arctic Ocean using shipboard experiments during autumn. Ingestion rates for both Arctic and Pacific species were low and linked to low food availability. The ingestion rates varied with species, but were not related to chlorophyll a. The maximum ingestion rates calculated by the Michaelis-Menten equation were higher in the Arctic species (3.6% body carbon day −1) than in the Pacific species (0.10% body carbon day −1). The community grazing impacts were 0-0.57% remove day −1 , and the Pacific copepods contributed 0.1-17% for this parameter. Even if Pacific copepods are transported into the Arctic Ocean and ingest the natural protist assemblage, their impact is spatially and seasonally limited, and, at present, Pacific copepods are unlikely to cause a shift in the protist biomass of the western Arctic Ocean during autumn.

Based on the vertical stratified zooplankton samples collected during the autumns (September-Dece... more Based on the vertical stratified zooplankton samples collected during the autumns (September-December) of 1996 to 1998, vertical distribution, biomass, and community structure of zooplankton and calanoid copepods in the southern Okhotsk Sea were evaluated and compared with those of the same period in the Oyashio region. In terms of fauna, zooplankton in the Okhotsk Sea are similar to that in the Oyashio region. However, their biomass, community structure and vertical distribution patterns are quite different between these two regions. Zooplankton biomass near the surface layer (0 m to thermocline) in the Okhotsk Sea was less than that of the Oyashio region. To understand the reason for this, we look at the distribution of large copepods in the mesopelagic layer in the Okhotsk Sea (this is epipelagic in the Oyashio region). Standing stocks of most zooplankton taxa were smaller in the Okhotsk Sea, while only the copepod Metridia okhotensis showed an opposite pattern. The abundance of M. okhotensis in the Okhotsk Sea was 30 times greater than that in Oyashio region, and they predominated (60% of the total copepod number) in the Okhotsk Sea. The development of a strong pycnocline in the Okhotsk Sea may be a key feature responsible for these regional differences in the zooplankton community in the Okhotsk Sea and Oyashio region.

Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea... more Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea). Protist 156: 413῍423 Stentiford, G. D. & J. D. Shields 2005. A review of the parasitic dinoflagellates Hematodinium-like infection in marine crustaceans. Dis. Aquat. Org. 66: 47῍70. Takano,Y. & T. Horiguchi 2004. Surface ultrastructure and molecular phylogenetics of fourunarmored heterotrophic dinoflagellates, including the type species of the genus Gymnodinium. Phycol. Res. 52: 107῍116.

Deep Sea Research Part II: Topical Studies in Oceanography, 2012

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice ext... more The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972-2000, there was high interannual variability of areal extent of sea ice during spring (March-April). In 2000, this shifted to a 5-year (2001-2005) period of low ice extent during spring, which transitioned to a 4-year (2007-2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6-7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December-February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001-2005) and cold period (2007-2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

To evaluate the effects of water mass changes and the spring phytoplankton bloom, we analysed sho... more To evaluate the effects of water mass changes and the spring phytoplankton bloom, we analysed shortterm changes in the population structure of the dominant hydromedusa Aglantha digitale in the Oyashio region during March-April 2007. Samples were collected with Bongo nets at night via oblique towing from a depth of 200 m to the surface on eight occasions in March and April of 2007. The abundance, biomass and mature individual compositions of A. digitale rapidly increased from April onward. During April, the reproduction and growth of A. digitale is achieved due to the initiation of the spring phytoplankton bloom and the increase in small copepods. Based on dry mass, the growth rate of A. digitale (1.3% day-1) was lower than the reported maximum value (4.9% day-1) for this species. The rapid exchange of the dominant water masses was also observed during spring in the Oyashio region. Significant effects of water mass exchange were detected in the abundance, biomass and body sizes of A. digitale. Thus, the abundance, biomass and population structure of A. digitale in the Oyashio region may be altered during spring by two factors : internal growth and external water mass exchange.

Biogeosciences Discussions, 2013

At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical ... more At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical gyre, time-series sediment traps were collecting sinking particles when the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident occurred on 11 March 2011. Radiocesium (134 Cs and 137 Cs) derived from FNPP1 accident was detected in sinking particles collected at 500 m by late March 2011 and at 4810 m by early April 2011 at both stations. The sinking velocity of 134 Cs and 137 Cs was estimated to be 8 to 36 m day −1 between the surface and 500 m and > 180 m day −1 between 500 m and 4810 m. 137 Cs specific activity varied from 0.14 to 0.25 Bq g −1 dry weight. These values are higher than those of surface seawater, suspended particles, and zooplankton collected in April 2011. Although the radiocesium may have been adsorbed onto or incorporated into clay minerals, correlations between 134 Cs and lithogenic material were not always significant; therefore, the form of the cesium associated with the sinking particles is still an open question. The total 137 Cs flux by late June at K2 and by late July at S1 was 0.5 to 1.7 Bq m −2 at both depths. Compared with 137 Cs input to both stations by April 2011, estimated from the surface 137 Cs activity and mixed layer depth and by assuming that the observed 137 Cs flux was constant throughout the year, the estimated removal rate of 137 Cs from the upper layer (residence time in the upper layer) was 0.3 to 1.5 % (68 to 312 yr). The estimated removal rates and residence times are comparable to previously reported values.

Nature Communications, 2015

The first relates to the presentation of sinking particle flux data in Fig. 2c, which was mistake... more The first relates to the presentation of sinking particle flux data in Fig. 2c, which was mistakenly plotted in reference to a previous dataset, rather than Supplementary Data 1. The corrected version of Fig. 2 appears below. The second error concerns the miscalculation of the Particulate Organic Nitrogen (PON) and Carbon (POC) fluxes presented in Supplementary Data 1 and Fig. 3b,c. During the flux calculation process, we mistakenly used the weight of analysed subsamples rather than aliquot samples, which resulted in an underestimation of the mole fluxes of PON and POC in the published Article. Two new columns, detailing the correctly calculated fluxes, have been added to Supplementary Data 1, while the corrected version of Fig. 3 appears below.

Chattonella globosa is known as a red tide species in western Japan from Kyushu, the Seto Inland ... more Chattonella globosa is known as a red tide species in western Japan from Kyushu, the Seto Inland Sea to Tokyo Bay, but the existence has been unknown in the northern Japan.This is the first report of the observation of occurrence of C. globosa from Funka Bay, in Hokkaido, Japan.Cells were globular and 33 -42 µm in diameter with single flagellum, and sometimes per-formed "Amoeba -like movement".We also studied the seasonal variation in density and distribution of C. globosa.This spe-cies was first detected in July 2011 and reached at maximum cell density of 2.610 4 cells L

Polar Science, 2019

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plum... more In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plumes near the calving fronts. To evaluate the effects of this discharge on the zooplankton community in the fjords, we collected sea surface zooplankton samples in Bowdoin Fjord in northwestern Greenland during the summer of 2016 and made microscopic, OPC and ZooScan analyses. Within the three quantitative methods, ZooScan has advantages that can evaluate various parameters (e.g., abundance, biomass, size and taxonomic information) simultaneously and has the ability to eliminate abiotic particles, such as silt and sediment, which are abundant in samples. Based on taxonomic biomass data, the zooplankton community is clustered into three groups, which varied spatially: inner, middle and outer fjord groups. Jellyfish dominated the outer fjord group, and barnacle cypris larvae dominated the middle fjord group. For the inner fjord group, large-sized Calanus spp. and chaetognaths were abundant. Since these species are characterized with oceanic taxa, they would intrude through the deep fjord water and subsequently be upwelled through entrainment of glacially modified plume water. From the NBSS analysis on zooplankton size spectra, the steep slope of NBSS in the middle fjord community suggests that the high productivity was caused by the addition of meroplanktonic cypris larvae.

Polar Science, 2018

In oceans worldwide, chaetognaths are the second most dominant zooplankton taxa and have vital tr... more In oceans worldwide, chaetognaths are the second most dominant zooplankton taxa and have vital transfer roles between herbivores and higher trophic organisms. Despite the chaetognaths' importance, little is known regarding their ecology in the Chukchi Sea, where recent ice reduction is prominent in the Arctic Ocean. We made comparisons of the chaetognath abundance, biomass, community structure (total length and gonadal maturation) and feeding impacts between the years 1991 and 2007. Within the chaetognath community, Parasagitta elegans was the abundant species, and Eukrohnia hamata only occurred in the southern region in 2007. Yearly differences were detected in chaetognath abundance, biomass, total length and gonadal maturation. All of these parameters were greater and more advanced in 1991 than in 2007. The most prominent yearly differences were in the horizontal distributions of chaetognaths, primarily in the northern region in 1991 and the southern region in 2007. The southern chaetognath populations observed in 2007 were considered to be transported by the Pacific water through the Bering Strait. Because of the differences in the original populations between the Chukchi Sea in 1991 and the Bering Sea in 2007, differences in these two years may be due to regional differences. As the Arctic warms, the horizontal distribution pattern, similar with 2007 in this study, will be more pronounced.

PloS one, 2017

The eastern Bering Sea has a vast continental shelf, which contains various endangered marine mam... more The eastern Bering Sea has a vast continental shelf, which contains various endangered marine mammals and large fishery resources. Recently, high numbers of toxic A. tamarense resting cysts were found in the bottom sediment surface of the eastern Bering Sea shelf, suggesting that the blooms have recently occurred. However, little is known about the presence of A. tamarense vegetative cells in the eastern Bering Sea. This study's goals were to detect the occurrence of A. tamarense vegetative cells on the eastern Bering Sea shelf and to find a relationship between environmental factors and their presence. Inter-annual field surveys were conducted to detect A. tamarense cells and environmental factors, such as nutrients, salinity, chlorophyll a, and water temperature, along a transect line on the eastern Bering Sea shelf during the summers of 2004, 2005, 2006, 2009, 2012, and 2013. A. tamarense vegetative cells were detected during every sampling year, and their quantities varied g...

Progress in Oceanography, 2017

A total of 100 mesozooplankton samples collected with NORPAC nets from a 0 to 150-m depth at lati... more A total of 100 mesozooplankton samples collected with NORPAC nets from a 0 to 150-m depth at latitudinal stations (35-44°N) along 155°E each May from 2002 through 2011 were analyzed. The mesozooplankton abundance at each station varied from 39 to 1106 ind. m À3. The mesozooplankton biomass was consistently higher (80-100 mg DM m À3) in the transition domain (40-42°N) than the biomass in the other domains. An empirical metabolic rate-based carbon budget model indicated that production of mesozooplankton suspension feeders was highest (120-175 mg C m À2 d À1) in the transition domain. A comparison between the production of the mesozooplankton suspension feeders and the food requirement of mesozooplankton carnivores indicated that the latter was well fulfilled by the former in the subarctic and transition domains. However, the food requirement of the mesozooplankton carnivores was near equal to or exceeded the production of mesozooplankton suspension feeders in the subtropical domain. As an annual event, the feeding migration of epipelagic fish to the transition and subarctic domains in summer may be interpreted by their utilization of the excess secondary production (production of mesozooplankton suspension feeders).

Harmful algae, 2017

Abundant cyst distributions of the toxic dinoflagellate Alexandrium fundyense (previous A. tamare... more Abundant cyst distributions of the toxic dinoflagellate Alexandrium fundyense (previous A. tamarense north American clade) were recently observed on the north Chukchi Sea shelf and on the eastern Bering Sea shelf, suggesting that A. fundyense is both highly adapted to the local environments in the high latitude areas and might cause toxin contamination of plankton feeders. However, little is known about the physiological characteristics and toxin profiles of A. fundyense in these areas, which are characterized by low water temperatures, weak sunlight, and more or less permanent ice cover during winter. To clarify the physiological characteristics of A. fundyense, the effects of water temperature and light intensity on the vegetative growth and toxin profiles of this species were examined using A. fundyense strains isolated from one sediment sample collected from each area. Using the same sediments samples, seasonal changes of the cyst germination in different water temperatures were...

Journal of Japanese Biochemical Society, 2015

Biogeosciences Discussions, 2015

In the Chukchi Sea, due to the recent drastic reduction of sea-ice during the summer, an increasi... more In the Chukchi Sea, due to the recent drastic reduction of sea-ice during the summer, an increasing formation of atmospheric turbulence has been reported. However, the importance and effects of atmospheric turbulence on the marine ecosystem are not fully understood in this region. To evaluate the effect of atmospheric turbulence on the marine ecosystem, high-frequent sampling (two to four times per day) on the mesozooplankton community and the gut pigment of dominant copepods were made at a fixed station in the Chukchi Sea from 10 to 25 September 2013. During the study period, a strong wind event (SWE) was observed on 18 September. After the SWE, the standing stock of chlorophyll a (chl a) was increased, especially for micro-size (> 10 µm) fractions. Zooplankton abundance ranged 23 610-56 809 ind. m −2 and exhibited no clear changes with SWE. In terms of abundance, calanoid copepods constituted the most dominated taxa (mean: 57 %), followed by barnacle larvae (31 %). Within the calanoid copepods, small-sized Pseudocalanus spp. (65 %) and large-sized Calanus glacialis (30 %) dominated. In the population structure of C. glacialis, copepodid stage 5 (C5) dominated, and the mean copepodid stage did not vary with SWE. The dominance of accumulated lipids in C5 and C6 females with immature gonads indicated that they were preparing for seasonal diapause. The gut pigment of C. glacialis C5 was higher at night and was correlated with ambient chl a, and a significant increase was observed after SWE (2.6 vs. 4.5 ng pigment ind. −1). Assuming C : Chl a ratio, the grazing impact by C. glacialis C5 was estimated to be 4.14 mg C m −2 day −1 , which corresponded to 0.5-4.6 % of the standing stock of micro-size phytoplankton. Compared with the metabolic food requirement, their feeding on phytoplankton accounted for 12.6 % of their total food requirement. These facts suggest that C. glacialis could not maintain their population on solely phytoplankton food, and other food sources (i.e., microzooplankton) are important in autumn. As observed for the increase in gut pigment, temporal phytoplankton bloom, which is enhanced by the atmospheric turbulence (SWE) in autumn, may have a positive effect on copepod nutrition. However, because of the 3880 BGD 12, 3879-3904, 2015 Short-term changes of the mesozooplankton community K. Matsuno et al.

Zoological Studies, 2015

Background An optical plankton counter (OPC) was used to examine spatial and temporal changes in ... more Background An optical plankton counter (OPC) was used to examine spatial and temporal changes in the zooplankton size spectra in the neighboring waters of Japan from May to August 2011. Results Based on the zooplankton biovolume of equivalent spherical diameter (ESD) in 45 bins for every 0.1 mm between 0.5 and 5.0 mm, a Bray-Curtis cluster analysis classified the zooplankton communities into six groups. The geographical distribution of each group varied from each of the others. Groups with a dominance of 4 to 5 mm ESD were observed in northern marginal seas (northern Japan Sea and Okhotsk Sea), while the least biovolume with a dominance of a small-size class (0.5 to 1 mm) was observed for the Kuroshio extension. Temporal changes were observed along the 155° E line, i.e., a high biovolume group dominated by 2 to 3 mm ESD during May shifted to other size spectra groups during July to August. These temporal changes were caused by the seasonal vertical descent of dominant large Neocalan...

Zoological Studies, 2015

Background: Despite its ecological importance, little information is available regarding the spat... more Background: Despite its ecological importance, little information is available regarding the spatial and vertical changes in the calanoid copepod community over large geographical regions. This study investigated the spatial and vertical patterns in calanoid copepod abundance and community structure using zooplankton samples collected between depths of 0 and 2,615 m across the North Pacific from 0°to 56°N. Results: A total of 211 calanoid copepod species belonging to 66 genera and 24 families were identified. Calanoid copepod abundance decreased with increasing depth, and few latitudinal differences were detected. Across the entire region, species diversity peaked near 500 to 2,000 m in depth. The calanoid copepod community was separated into seven groups with distinct spatial and vertical distributions. For all groups, the number of species was low (28 to 37 species) in the subarctic region (north of 40°N) and high (116 to 121 species) in the subtropical-tropical region. The deepest group in the subtropical-tropical region was composed of cosmopolitan species, and this group was also observed in deep water in the subarctic region. Conclusions: In deep water, most of the calanoid copepod community consisted of cosmopolitan species, while an endemic community was observed in the subarctic region. Because the food of deep-sea calanoid copepods originates from the surface layer, sufficient and excess flux in the eutrophic subarctic region may be responsible for maintaining the endemic species in the region.

Polar Biology, 2015

Increasing numbers of Pacific copepods are being transported from the Bering Sea to the Arctic Oc... more Increasing numbers of Pacific copepods are being transported from the Bering Sea to the Arctic Ocean, so there is clear potential to affect the structure and composition of the Arctic food web. We investigated the grazing impacts of Arctic and Pacific copepods in the western Arctic Ocean using shipboard experiments during autumn. Ingestion rates for both Arctic and Pacific species were low and linked to low food availability. The ingestion rates varied with species, but were not related to chlorophyll a. The maximum ingestion rates calculated by the Michaelis-Menten equation were higher in the Arctic species (3.6% body carbon day −1) than in the Pacific species (0.10% body carbon day −1). The community grazing impacts were 0-0.57% remove day −1 , and the Pacific copepods contributed 0.1-17% for this parameter. Even if Pacific copepods are transported into the Arctic Ocean and ingest the natural protist assemblage, their impact is spatially and seasonally limited, and, at present, Pacific copepods are unlikely to cause a shift in the protist biomass of the western Arctic Ocean during autumn.

Based on the vertical stratified zooplankton samples collected during the autumns (September-Dece... more Based on the vertical stratified zooplankton samples collected during the autumns (September-December) of 1996 to 1998, vertical distribution, biomass, and community structure of zooplankton and calanoid copepods in the southern Okhotsk Sea were evaluated and compared with those of the same period in the Oyashio region. In terms of fauna, zooplankton in the Okhotsk Sea are similar to that in the Oyashio region. However, their biomass, community structure and vertical distribution patterns are quite different between these two regions. Zooplankton biomass near the surface layer (0 m to thermocline) in the Okhotsk Sea was less than that of the Oyashio region. To understand the reason for this, we look at the distribution of large copepods in the mesopelagic layer in the Okhotsk Sea (this is epipelagic in the Oyashio region). Standing stocks of most zooplankton taxa were smaller in the Okhotsk Sea, while only the copepod Metridia okhotensis showed an opposite pattern. The abundance of M. okhotensis in the Okhotsk Sea was 30 times greater than that in Oyashio region, and they predominated (60% of the total copepod number) in the Okhotsk Sea. The development of a strong pycnocline in the Okhotsk Sea may be a key feature responsible for these regional differences in the zooplankton community in the Okhotsk Sea and Oyashio region.

Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea... more Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea). Protist 156: 413῍423 Stentiford, G. D. & J. D. Shields 2005. A review of the parasitic dinoflagellates Hematodinium-like infection in marine crustaceans. Dis. Aquat. Org. 66: 47῍70. Takano,Y. & T. Horiguchi 2004. Surface ultrastructure and molecular phylogenetics of fourunarmored heterotrophic dinoflagellates, including the type species of the genus Gymnodinium. Phycol. Res. 52: 107῍116.

Deep Sea Research Part II: Topical Studies in Oceanography, 2012

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice ext... more The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972-2000, there was high interannual variability of areal extent of sea ice during spring (March-April). In 2000, this shifted to a 5-year (2001-2005) period of low ice extent during spring, which transitioned to a 4-year (2007-2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6-7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December-February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001-2005) and cold period (2007-2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

To evaluate the effects of water mass changes and the spring phytoplankton bloom, we analysed sho... more To evaluate the effects of water mass changes and the spring phytoplankton bloom, we analysed shortterm changes in the population structure of the dominant hydromedusa Aglantha digitale in the Oyashio region during March-April 2007. Samples were collected with Bongo nets at night via oblique towing from a depth of 200 m to the surface on eight occasions in March and April of 2007. The abundance, biomass and mature individual compositions of A. digitale rapidly increased from April onward. During April, the reproduction and growth of A. digitale is achieved due to the initiation of the spring phytoplankton bloom and the increase in small copepods. Based on dry mass, the growth rate of A. digitale (1.3% day-1) was lower than the reported maximum value (4.9% day-1) for this species. The rapid exchange of the dominant water masses was also observed during spring in the Oyashio region. Significant effects of water mass exchange were detected in the abundance, biomass and body sizes of A. digitale. Thus, the abundance, biomass and population structure of A. digitale in the Oyashio region may be altered during spring by two factors : internal growth and external water mass exchange.

Biogeosciences Discussions, 2013

At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical ... more At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical gyre, time-series sediment traps were collecting sinking particles when the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident occurred on 11 March 2011. Radiocesium (134 Cs and 137 Cs) derived from FNPP1 accident was detected in sinking particles collected at 500 m by late March 2011 and at 4810 m by early April 2011 at both stations. The sinking velocity of 134 Cs and 137 Cs was estimated to be 8 to 36 m day −1 between the surface and 500 m and > 180 m day −1 between 500 m and 4810 m. 137 Cs specific activity varied from 0.14 to 0.25 Bq g −1 dry weight. These values are higher than those of surface seawater, suspended particles, and zooplankton collected in April 2011. Although the radiocesium may have been adsorbed onto or incorporated into clay minerals, correlations between 134 Cs and lithogenic material were not always significant; therefore, the form of the cesium associated with the sinking particles is still an open question. The total 137 Cs flux by late June at K2 and by late July at S1 was 0.5 to 1.7 Bq m −2 at both depths. Compared with 137 Cs input to both stations by April 2011, estimated from the surface 137 Cs activity and mixed layer depth and by assuming that the observed 137 Cs flux was constant throughout the year, the estimated removal rate of 137 Cs from the upper layer (residence time in the upper layer) was 0.3 to 1.5 % (68 to 312 yr). The estimated removal rates and residence times are comparable to previously reported values.