Atsushi Yamaguchi

Hokkaido University | Hokudai · Faculty of Fisheries

As part of the research program Western Pacific Environment Study on CO2 Ocean Sequestration for Mitigation of Climate Change (WEST-COSMIC), vertical distribution and community structure of copepods were studied at Station Knot (44°N, 155°E) down to 4000 m depth in the western subarctic Pacific. Vertical carbon flux mediated by copepod communities...

As part of the research program “WEST-COSMIC (Western Pacific Environment Study on CO2 Ocean Sequestration for Mitigation of Climate Change)”, vertical distribution patterns of community structure and size spectra of plankton organisms were studied at three sites: Station SA at 44°N, 155°E (down to 4000 m depth); Station I at 39°N, 147°E (2000 m de...

As part of the research program WEST-COSMIC Phase I (1997–2001), vertical profiles down to the greater depths (0–2000 m or 5800 m) of the plankton community structure composed of heterotrophic bacteria, phytoplankton, protozooplankton and metazooplankton were studied at one station in each subarctic (44°N) and in transitional region (39°N), and two...

In the Arctic Ocean, the life cycles of interzonal omnivorous copepods have been studied, whereas little information is available on the life cycles of mesopelagic carnivorous species. Here, the life cycles of two mesopelagic carnivorous copepods (Paraeuchaeta glacialis and Heterorhabdus norvegicus) are described from vertically stratified samples...

In the northern Bering Sea, ice coverage and retreat timing were low and early, respectively, in the 2017–2018 winter. To evaluate the effect of these anomalous ice conditions, we quantified various environmental parameters (temperature, salinity, mixed-layer depth, nutrients) and the standing stocks of various planktonic taxa (phytoplankton counts...

The most concerning recent ocean changes in temperature issues are known as marine heat waves. Under these conditions, it is important to evaluate the effects of temperature on zooplankton. In this study, we investigated the growth rates of three dominant copepod species (Eucalanus bungii, Metridia pacifica, and Neocalanus plumchrus) in the norther...

Calanus hyperboreus is one of the dominant copepod species in the Arctic zooplankton communities. The impact of climate change varies among regions within the Arctic, implying that C. hyperboreus populations may be differently affected at different locations, but knowledge on seasonal population dynamics in relation to biogeography is scarce. To fi...

Drastic environmental changes were noted in the northern Bering Sea in 2018. A reduction in sea ice affected several trophic levels within the ecosystem; this resulted in delayed phytoplankton blooms, the northward shifting of fish stocks, and a decrease in the number of seabirds. Changes in the community composition of zooplankton were reported in...

DNA metabarcoding on a single organism is a promising approach to clarify the biological interactions (e.g., predator-prey relationships and symbiosis, including parasitism) of difficult-to-culture protists. To evaluate the effectiveness of this method, Radiolaria and Phaeodaria, which are ecologically important protistan groups, were chosen as tar...

We present the description of the new suctorian genus Copterus gen. n. with type species Copterus tyloloricatus sp. n. from the family Acinetidae, inhabiting the prosomes and urosomes of calanoid copepods from the northern North Pacific Ocean. The new taxon is characterized by (i) an ellipsoidal to triangular, asymmetric, dorsoventrally flattened c...

Calanus glacialis/marshallae is a dominant zooplankton species in the Pacific Arctic Ocean that is widely distributed in shelf areas, and it plays a vital role in connecting primary production to higher trophic levels. Its phenology is well adapted to hydrography, but there is little available information about regional and diel changes in populati...

Information on pelagic polychaete community structure in the western North Pacific is available for the subarctic region (Station K2) but not for the subtropical region. Hence, we analyzed day–night vertically stratified samples collected in eight layers within the first 1000 m of the water column during four seasons in 1 year, using the same sampl...

Vertically stratified day and night samples were collected from a depth of up to 1000 m during four seasons at a single station in the western subarctic Pacific. Subsequently, the abundance, biovolume, community structure and population structure of the pelagic amphipods were evaluated from the imaging analysis using ZooScan. The stable isotope rat...

Aetideid copepods dominate the mesopelagic layer of the Arctic Ocean and play an important role in the vertical material flux and biodiversity. However, little information about the lengths and weights of their copepodids is available. In this study, we collected five sympatric aetideid copepods, Chiridius obtusifrons Sars G.O., 1902, Gaetanus tenu...

Knowledge about the taxonomic and size composition of mesozooplankton is of critical importance for both fisheries and oceanography. In this study, we collected an annual time series of mesozooplankton samples in Funka Bay and analysed them using ZooScan to reveal the seasonal changes in taxonomic and size structure. Both zooplankton abundance and...

This study investigated the host-epibiont relationship between calanoid copepods and suctorian ciliates collected widely from the northern North Pacific Ocean. Mid- and deep-water adult female calanoids without dormant stages were found to be infested with suctorians, probably because of their larger size, longer intermolt duration and longevity in...

An increasing body of work has shown the potential impacts of subglacial discharge from marine‐terminating glaciers on the marine environment around Greenland. Upwelling of nutrients associated with rising buoyant plumes near the front of marine‐terminating glaciers plays a key role in maintaining the high productivity of connected fjords. The resp...

The phytoplankton community was analyzed from surface water samples collected at 32 stations along the Pacific coast of Hokkaido during 6–12 October 2021. Phytoplankton cell density ranged between 38 and 9033 cells mL–1. The phytoplankton were classified into four separate communities, and the harmful, red tide, dinoflagellate Karenia selliformis d...

Aetideidae is a calanoid copepod family dominant in the mesopelagic layer of the Arctic Ocean for which little ecological information is available because species identification, especially of early copepodite stages, is difficult. In this study, we developed a species identification flow for the whole copepodite stages of five sympatric aetideid c...

The faecal pellets egested by zooplankton are essential as they influence the vertical material flux in oceans. As mesozooplankton are dominant within the plankton community in the southern Okhotsk Sea during early summer, the vertical material flux via mesozooplankton is expected to be substantial. However, quantitative information on their faecal...

A remarkable early sea-ice reduction event was observed in the northern Bering Sea during 2018. In turn, this unusual hydrographic phenomenon affected several marine trophic levels, resulting in delayed phytoplankton blooms, phytoplankton community changes, and a northward shift of fish stocks. However, the response of the zooplankton community rem...

To understand the environmental factors affecting the density of foraging seabirds across the cold-water belt in the southwestern Okhotsk Sea, we conducted a 1-day (180-km transect length) shipboard seabird survey off the northeastern coast of Hokkaido during summer in 2019, along with acoustic observations of potential prey (zooplankton and fish)...

The southern part of the Okhotsk Sea off Hokkaido has rich fishery resources supported by high primary production. The phytoplankton community has been described by several studies based on size-fractionated chlorophyll a concentration and primary production; however, the details of the species composition of nano- and micro-protist communities hav...

Epibiotic relations between copepods and suctorian ciliates have long been known. However, the attachment sites of suctorians on copepod bodies have not been investigated intensively, although this information may be important to understand the life history of these suctorians and the benefits and harmful effects of epibiosis. We investigated epibi...

Macroecological relationships provide insights into rules that govern ecological systems. Bergmann's rule posits that members of the same clade are larger at colder temperatures. Whether temperature drives this relationship is debated because several other potential drivers covary with temperature. We conducted a near‐global comparative analysis on...

Respiratory oxygen consumption rates (at the two temperatures of 0-50 m and 50-200 m depth strata) and day/night biomass in the top 50 m water column were determined on adult female Metridia pacifica at twelve stations in the western/ eastern subarctic Pacific and one station in the oceanic Bering Sea during summer. At each station, the respiration...

Various zooplankton studies have been conducted in the Indian sector of the Southern Ocean, but studies investigating spatial changes in both meso-and macro-zooplankton populations are limited. To reveal the north-south change of the meso-and macro-zooplankton community structure and the population structure of large copepods, we investigated zoopl...

Pelagic polychaetes are distributed from the sea surface to greater depths of the oceans worldwide, but little information is available regarding their ecology. This study investigated the vertical distribution of abundance, biomass and community structure of pelagic polychaetes at a single station in the western subarctic Pacific based on day–nigh...

The phylogenetic affiliation of Oroscenidae, a family of Radiolaria possessing large siliceous skeletons, was thoroughly revised in this study. Plankton sampling was conducted over seven years in seven sea areas of the Northern Hemisphere. Molecular analyses on the obtained specimens revealed that living oroscenids are phylogenetically different fr...

To evaluate seasonal changes in a protist community (diatoms, dinoflagellates and ciliates) and their production in the southern Okhotsk Sea, water sampling was conducted approximately once a week from July 2016 to July 2017 at Mombetsu Port. Diatoms were the predominant category of the three taxa throughout the study period. The abundances of all...

Community structure and seasonal changes in the population structure of pelagic polychaetes were studied based on zooplankton samples collected by sediment traps moored at 200 m depth in the subarctic and subtropical western North Pacific throughout the year. Eight species belonging to seven genera and seven families occurred at the subarctic stati...

The distribution of growth-inhibiting bacteria (GIB) against the toxic dinoflagellate Alexandrium catenella (Group I) was investigated targeting seagrass leaves and surface waters at the seagrass bed of Akkeshi-ko Estuary and surface waters of nearshore and offshore points of Akkeshi Bay, Japan. Weekly samplings were conducted from April to June in...

Environments along the coast of Greenland are rapidly changing under the influence of a warming climate in the Arctic. To better understand the changes in the coastal environments, we performed researches in the Qaanaaq region in northwestern Greenland as a part of the ArCS (Arctic Challenge for Sustainability) Project. Mass loss of ice caps and ma...

The three currently recognized species comprising the pelagic polystiliferous hoplonemertean genus Protopelagonemertes Brinkmann, 1917 are supposedly distinguishable chiefly with respect to the number of proboscis nerves: 29 (varying from 22 to 30) in P. hubrechti (Brinkmann, 1917); 19–21 (varying from 19 to 22) in P. beebei Coe, 1936; and 36 in P....

To understand the effect of an unusually early sea-ice retreat in the northern Bering Sea in the spring of 2018 on the marine ecosystem of the northern Bering Sea, we compared at-sea observations of seabird density and acoustic observations of prey (fish and zooplankton) biomass during shipboard surveys around St. Lawrence Island in the summers of...

Zooplankton community structure in the northern Bering Sea may change significantly over relatively short periods due to the inflow of different water masses and the seasonal release of meroplankton, although details of these changes are still unclear. We studied the zooplankton community in the northern Bering Sea from June to September of 2017 an...

In recent years, the northern Bering Sea has experienced changes in the timing of sea-ice retreat and in hydrographic conditions during the summer. The influence of these environmental changes on the diatom community has not been examined. In this study, we investigated the spatial changes in the diatom community of the northern Bering Sea during t...

We compared copepod abundance, biomass and community structure in the Atlantic sector (Fram Strait) and Pacific sector (Canada Basin) of the Arctic Ocean by using vertical stratified zooplankton samples collected from 0-1,000 m or 0-1,500 m during daytime in summer. Abundance and biomass were high at the surface layer and decreased with increasing...

The intensity and frequency of harmful algal blooms (HABs) have increased, posing a threat to human seafood resources due to massive kills of cultured fish and toxin contamination of bivalves. In recent years, bacteria that inhibit the growth of HAB species were found to be densely populated on the biofilms of some macroalgal species, indicating th...

We examined the abundance and horizontal and vertical distributions of epipelagic ctenophores and scyphomedusae in the northern Bering Sea using an underwater video camera during July of 2017 and 2018. The effects of environmental and biological parameters on the distribution of these species were evaluated by generalized additive modelling (GAM)....

The community structure and life cycle of dominant chaetognath species were studied in a 0–1000–m water column in the Oyashio region, western subarctic Pacific, six times during the year from March 2003 to February 2004. The abundance of chaetognaths ranged from 3235 to 6691 inds. m⁻². Throughout the study period, ten chaetognath species belonging...

Scaphocalanus magnus is a large copepod species with a worldwide distribution. Information on the ecology of this species is scarce. In this study, S. magnus was collected using vertically stratified net sampling from an ice station in the western Arctic Ocean over a year. The vertical distribution differed between periods of polar night and midnig...

To evaluate the effects of meltwater discharge from marine-terminating glaciers on a fjord protist community in northwestern Greenland during summer, we investigated the distribution, abundance and biomass of the protist community and their relationships with hydrographic parameters. In the standing stock of protists, dinoflagellates (46.4%) and ol...

Arctic ecosystems are altered profoundly by climate changes. However, the responses of Arctic marine and terrestrial ecosystems as well as their biodiversity to global warming remain largely unknown. This article provides comprehensive insights into the results and major findings from the Arctic Challenge for Sustainability (ArCS) Project – an Arct...

The long‐distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ¹⁵NBase), which can be estimated by using compound‐specific isotope analysis. δ¹⁵NBase exclusively reflects the δ¹⁵N of nitrate in...

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plumes near the calving fronts, and these areas serve as an important foraging habitat for seabirds. To investigate the effect of subglacial discharge on the foraging assemblages of surface feeders and divers in a glacial fjord, we conducted boat-based seab...

The Arctic Ocean is characterized as the greatly variable oceanic environment both seasonality and regionally. Such environmental variability would affect regional differences in the respective copepod community structures, though it has not been reported so far. In this study, we analyzed time-series zooplankton samples with focus on large copepod...

Two dominant mesopelagic carnivorous Paraeuchaeta copepods, P. glacialis in the western Arctic Ocean and P. elongata in the western subarctic Pacific, exhibited specialized ontogenetic vertical migration patterns with deeper occurrences of early copepodids (C1) and adults (C6F/M) and shallower occurrences of older copepodids C3-C5. Moult increments...

In the Bering Sea shelf, annual changes in the sea ice extent are large. In this study, we compare the viable diatom resting stages in sediments during the summer of 2017 when the sea ice retreat was late and 2018 when the sea ice retreat was early. South of St. Lawrence Island, the germinating cell number was 10–100 times greater in 2018 than it w...

To evaluate the effect of net mesh size on filtering efficiency and zooplankton collection efficiency, we developed Quad-NORPAC net equipped four-different mesh sizes (63, 100, 150, and 335 µm). Based on zooplankton samples collected by Quad-NORPAC net at 13 stations off Fukushima during 2-12 June 2016, zooplankton settling volume and wet weight (3...

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 north-western Greenland during the summer of 2016 and made microscopic, OP...

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 chaet...

In recent years, marine ecosystems have changed due to the drastic sea ice reduction in the Arctic Ocean, but the relationship between copepod phenology and environmental drivers is unclear. To reveal the relationship, seasonal changes in the flux (abundance of swimmers), population structure, lipid accumulation and gonad maturation of four dominan...

The distribution of resting stage cells of diatoms in the surface sediments and water samples collected in the Chukchi Sea was investigated using the most probable number (MPN) analysis to understand the mechanism underlying diatom blooms in the Arctic region. High densities (3.5 3 10⁵ to 6.8 3 10⁶ MPN cells cm³ wet sediment) of viable resting stag...

This paper reports the first detection of Karenia mikimotoi cells and bloom occurrence in 2015 in Hakodate Bay, Hokkaido, Japan. We investigated the cell densities of K. mikimotoi in Hakodate Bay from May 2015 to November 2016. Regular samplings were conducted 1-3 times every month at the point at the wharf of Hakodate Research Center for Fisheries...

A phaeodarian morphotype, characterized by the feet surrounded with forked pedal spines with anchor-like structures, was collected in the subtropical North Pacific. Considering the morphological and phylogenetic uniqueness, this morphotype is described as Gazelletta kashiwaensis sp. nov. The distribution of this new species is possibly affected by...

In the Bering Sea, warming and reduction of summer sea-ice cover are driving species ranges towards the Arctic. Tanner crab, Chionoecetes bairdi, is a commercially important species in the SE Bering Sea with a northerly range margin in 62 N. In this paper, using plankton samples collected in the Pacific sub-Arctic/Arctic sector during summer, we re...

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...

Microphotographs of the thecal plates of an Alexandrium tamarense vegetative cell found in a seawater sample collected during field sampling. The cell was observed with an inverted epifluorescence microscope under UV light excitation (365 nm), after staining with a fluorescent dye (see Methods). All photographs show the same single cell from variou...

Seasonal changes in salinity at a depth of 0–50 m on the 166°W transect from 55 to 59°N, during the summers of 2004, 2005, 2006, 2009, 2012, and 2013. (DOCX)

Seasonal changes in chlorophyll a at a depth of 0–50 m on the 166°W transect from 55 to 59°N, during the summers of 2004, 2005, 2006, 2009, 2012, and 2013. (DOCX)