Abstract The ocean’s biological carbon pump regulates climate by transferring a portion of surface-fixed CO2 to the deep ocean through sinking particulate organic carbon (POC). Although this flux is strongly attenuated in the twilight zone, the seasonal controls on attenuation remain poorly understood. We examined seasonal variations in POC flux, the nitrogen isotope ratio of sinking particulate nitrogen (δ15Nsink), and mineral composition using sediment traps observations at subarctic (K2) and subtropical (S1) stations in the western North Pacific. POC sequestration efficiency at 500 m [Seq(500) = POC flux/NPP] was quantified, with net primary productivity (NPP) reconstructed from δ15Nsink using an empirical framework. Seq(500) remained nearly constant at K2 (7.4–8.1%) but varied substantially at S1 (3.4–6.5%). At K2, CaCO3 and opal contents exhibited complementary seasonal patterns, whereas S1 showed pronounced variability primarily in CaCO3. We propose that mineral composition modulates aggregate settling velocity and adhesive strength, thereby regulating POC attenuation through fragmentation processes. These findings indicate that seasonal shifts in surface ecosystem structure influence the physical properties of sinking aggregates and ultimately control the fate of NPP in the twilight zone.

Hydromedusae are diverse zooplankton that sometimes have great impacts on aquatic ecosystems. We investigated the seasonal occurrences and assemblage structure of hydromedusae in the offshore area of Sagami Bay, central Japan. Throughout the study, 33 holoplanktonic and 9 meroplanktonic hydromedusae species were collected. Unlike the nearshore hydromedusan assemblage, low species richness of meroplanktonic hydromedusae was a characteristic feature offshore. Total abundances drastically changed seasonally, with a spring peak. Muggiaea atlantica, Obelia sp., Solmundella bitentaculata, Rathkea octopunctata, Sugiura chengshanense, Liriope tetraphylla, and Aglaura hemistoma were abundant, with these seven species together comprising 58.7%-97.9% of the hydromedusan assemblage. Seasonal occurrence patterns differed among species. For example, M. atlantica was the most abundant and occurred throughout the year, with a spring peak that was probably affected by water temperature; Obelia sp. showed multiple abundance peaks, which might be driven by advections. A redundancy analysis (RDA) showed that seasonal occurrence of hydromedusae was closely related to both physical and biological factors. Our findings indicate that succession of the offshore hydromedusan assemblage was seasonal, but it was also influenced by short-term hydrographic processes such as the strong intrusion of oceanic water into the bay.

AbstractEnergetic cyclonic mesoscale eddies, which are called cold-core rings and are shed southward from the Kuroshio Extension jet and form closed streamlines, affect the atmosphere through the heat exchange across the sea surface. To investigate the effect of rings on the atmosphere, we performed atmosphere and ocean observations across a cold-core ring centered around 34.5° N, 150.0° E using a research vessel from November 2021 to January 2022 and a shallow-water profiling float from November 23 to 28, 2021. As heat is released from the sea surface, no significant spatial contrast in the sea surface and mixed layer temperatures was detected across the ring. Meanwhile, the sea surface wind was occasionally observed to be weak around the ring, possibly through the air–sea interactions. The wind drop-off maintained a turbulent heat flux small around the ring. The wind field associated with the wind drop-off was examined by the rotary empirical orthogonal function analysis of the satellite sea surface wind data. The minimum of the sea surface wind is found to shift northward relative to the ring center and to be more than approximately 5 m s$$^{-1}$$ - 1 lower than the surrounding region. The shallow-water profiling float deployed around the ring center observed a rapid freshening event in the mixed layer, which can be attributed to the water intrusion from the north of the Kuroshio Extension jet through the interaction with the jet. This suggests that the cold water from the north continually affects the atmosphere without leaving traces in the shipboard sea surface temperature observations.

Although mesopelagic fish dominate the oceanic ecosystem, they remain one of the least investigated components. From the vertically stratified sample collections, we investigated community structure, vertical distribution and diel/ontogenetic vertical migration covering four seasons for larval and juvenile/adult mesopelagic fish at the time-series station K2 in the Western Subarctic Gyre of the North Pacific. We collected 10 and 20 species of larval and juvenile/adult fish, respectively. Among the larval fish community, Leuroglossus schmidti was the most abundant; total abundances except L. schmidti were low throughout the year; species richness of myctophids were low. Among the 20 species of juvenile/adult fish, larvae of only six species were collected; thus, most mesopelagic fish species do not use the Western Subarctic Gyre as their nursery. In the juvenile/adult fish community, Stenobrachius leucopsarus and Stenobrachius nannochir were abundant. Species diversity and total abundance in the warm seasons (summer and autumn) were higher than in the cold seasons (winter and spring). The decreasing of species diversity and total abundance during the cold seasons was probably affected by horizontal migrations of fish for reproduction toward the southern transition or subarctic slope areas. Stenobrachius leucopsarus was distributed at shallow depths with opportunistic diel vertical migration (DVM); in contrast Stenobrachius nannochir occurred at greater depths, without DVM. The distribution depths of S. leucopsarus during day and those of S. nannochir changed seasonally and synchronously; shallowest in autumn, deepest in spring.

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 ratio (delta N-15) was also measured for each species. In total 10 amphipod species were identified in total, and their community structures were separated into four groups. The four communities were characterized by the upper-layer community dominated by Themisto pacifica, and the deep-water community that was characterized by the absence of T. pacifica. The other two groups were observed for the intermediate depths at night only, which was characterized by the low abundance of T. pacifica performing diel vertical migration from those depths. The deep-dwelling species displayed higher delta N-15 and positive relationships were detected with the habitat depth of each species. For the predominant T. pacifica, population structure (cohort) analysis was possible based on equivalent spherical diameter data from ZooScan. This study demonstrates that imaging analysis by ZooScan is applicable for analyses of minor taxonomic groups (amphipods) in zooplankton communities.

Abstract 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 sampling method as St. K2, at the subtropical region (Station S1). At St. S1, 27 species of pelagic polychaetes belonging to 13 genera and six families were identified. The annual mean abundance was 35.0 ind. 1000 m−3 and the biomass was 17.3 mg WW 1000 m−3. At St. S1, the numbers of genera and species were higher and the annual mean abundance and biomasses were much lower than St. K2. The pelagic polychaetes often peaked in the mesopelagic layer at St. K2, with the carnivores and particle feeders peaking in the epipelagic and mesopelagic layers, respectively. At St.S1, the carnivorous species predominated throughout the entire water column, and were most abundant in the epipelagic layer. Thus, In the western Pacific Ocean, the subarctic pelagic polychaete community structure changed vertically with feeding ecology. On the other hand, the subtropical community may be adapted to conditions of high irradiance and light transmission.

We compared stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) of epi-and mesopelagic mesozooplankton communities at subarctic (K-2) and subtropical (S1) sites in the North Pacific Ocean to evaluate the relationship between mesopelagic communities and epipelagic production. Although delta C-13 and delta N-15 varied among the taxonomic groups, ostracods, carnivorous copepods, and chaetognaths tended to exhibit higher delta C-13 and delta N-15 than omnivorous copepods within the same layers. Mesozooplankton delta C-13 and delta N-15 were higher in the upper (200-500 m) and lower (500-1000 m) mesopelagic layers than in the epipelagic layer (0-150 m). delta C-13 and delta N-15 of mesozooplankton were positively correlated at all three depth-ranges at both sites. Although the slopes of linear regressions of delta N-15 vs. delta C-13 exhibited no significant differences among layers or between sites, the intercept was lower for mesozooplankton at S1. We attributed the similarity of slopes to common isotopic fractionation during protein synthesis among the different taxonomic groups, and the lower intercept to primary producers supported by recycled nutrients and atmospheric nitrogen fixation under the high thermal regime at S1. Our results further confirm earlier evidence that metabolic requirements of mesopelagic communities are supported by epipelagic production through the biological pump.

We studied seasonal variations of the mesozooplankton swimmer community collected by a sediment trap moored at 873 m in the Kuroshio–Oyashio Transition region off the east coast of Japan from 5 August 2011 to 23 June 2013, with sampling bottles rotating at 26-day intervals. The total flux of mesozooplankton swimmers varied between 0 and 11.1 individuals m–2 d–1, with a mean of 3.1 individuals m–2 d–1 during the sampling period. In total, 89 taxa were found in the trap material, of which copepods comprised 87.1% of all swimmers on average. Among the Copepoda, Neocalanus cristatus was the most dominant taxon (76.2% of copepods on average during the sampling period), and all of them were stage C5 copepodite to adult. The species composition of the swimmers closely reflected the mesopelagic mesozooplankton of the Oyashio region. Because all N. cristatus observed in the trap were stage C5 to adult, its flux represents a time series of variations in life history and response to environmental changes at the depth of the sediment trap. The fluxes of Neocalanus species (N. cristatus, N. flemingeri, and N. plumchrus) reflected ontogenetic vertical migration, but may have been overestimates of active fluxes if they included dormant individuals that accidentally entered the sediment trap. The apparent active carbon flux of Neocalanus species ranged from 0 to 22.3 mg C m–2 d–1 during the sampling period, with a mean value of 4.9 mg C m–2 d–1.

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-night vertical stratified samples collected from a 0 to 1000 m water column during four seasons covering 1 year. The polychaete abundance and biomass ranged from 0 to 757 ind. 1000 M-3 and 0-6.1 mg WW m(-3), respectively. Ten pelagic polychaete species belonging to nine genera and six families were identified. From cluster analysis based on abundance, the polychaete community was divided into five communities. Each community occurred at different depth layers. Two surface groups seen at 0-200 m were dominated by two carnivorous species: Tomopteris septentrionalis and Typhloscolex muelleri. The deepest group, dominated by the particle feeder Pelagobia longicirrata, was seen at 500-1000 m. Two transition groups occurred in the intermediate depths. These vertical distributions of the pelagic polychaete communities were common throughout the season and day. Water mass, food availability and the oxygen minimum layer are put forth as environmental factors that affect the pelagic polychaete community.

The Chukchi Sea environment changes considerably in physical and biological conditions, driven by the expanding volume of warm Pacific summer water. These environmental changes can affect the migration timing of baleen whales in the southern Chukchi Sea. However, few studies have been conducted in this area to determine the migration timing of bowhead whales (Balaena mysticetus), the only baleen whale species endemic to the Arctic region. In this study, we conducted a fixed passive acoustic monitoring of bowhead whales in the southern Chukchi Sea from July 2012 to October 2015 and compared the occurrence patterns of vocalizations to physical and biological environmental factors. Bowhead whale calls were detected in fall/winter and spring during the ice-freezing and retreating periods, respectively. The fall migration timing of bowhead whales through the southern Chukchi Sea was delayed in the years when the timing of sea ice formation was late, and it formed increasingly later in the years 2013, 2014, and 2012, in that sequence. Moreover, the sea surface temperature decreased before freeze-up, which affected the timing of fall migration of bowhead whales. There was no clear relationship between the occurrence of bowhead whale calls and the abundance of prey, especially in spring, suggesting that most bowhead whales use the southern Chukchi Sea as a corridor during their spring northward migrations. However, the occurrence of bowhead whales and high abundance of zooplankton in October–November present the possibility that bowhead whales expand their feeding area in the southern Chukchi Sea.

We investigated the seasonal occurrence and vertical distribution/migration of larval and juvenile northern smoothtongue, Leuroglossus schmidti (Pisces, Bathylagidae), in oceanic areas of the western subarctic Pacific. This species was the most abundant larval fish and one of the most abundant juvenile/adult fish at the study site. Larval recruitment of this species occurred in early March to mid-October. Larvae were found, however, throughout all seasons, suggesting that some had overwintered. The highest abundance (5.8 individuals m(-2)) of larvae was in summer. During summer and autumn, hatching mainly occurred at 100-150-m depth and larvae migrated toward the surface with growth. Once larvae reached about 20 mm in length, they moved below 100 m, and larger larvae (over 24 mm) exhibited diel vertical migration. The abundance of juveniles integrated through the water column was relatively constant (0.3 individuals m(-2)) throughout the study period. They were collected from below 300 m during the day, and part of the population (13-38%) swam into the epipelagic layer at night. The information on early stages of L. schmidti presented here provides a basis for future investigations of mesopelagic fish ecology in oceanic areas of the western subarctic North Pacific Ocean.

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 station, while twelve species belonging to ten genera and seven families were identified at the subtropical station. Polychaete abundance was 5.37 ± 0.44 ind. m-2 day-1 (annual mean ± standard error) at the subarctic station, and 1.36 ± 0.15 ind. m-2 day-1 at the subtropical station. Polychaete abundance at the subarctic station was high from May to August, but no seasonal patterns were observed at the subtropical station. The dominant species in the subarctic was Tomopteris septentrionalis, which accounted for 62.9% of annual mean abundance; at the subtropical station, the dominant species was Pelagobia sp. (22.8%). In the subarctic, small specimens of T. septentrionalis (<3 mm in body length) occurred only in winter (December-March). No clear seasonal changes in population structure of the subtropical Pelagobia sp. were detected. The latitudinal patterns we observed in the polychaete communities of the western North Pacific were similar to those previously observed in the eastern North Pacific. Changes in the population structure of T. septentrionalis suggest that the life cycle of this species is seasonal in the subarctic region.

The heterotrophic prokaryotic carbon demand (PCD) in the ocean's interior often substantially exceeds (by up to two orders of magnitude) the amount of organic carbon supplied by sinking particulate organic carbon (POC). A hypothesis to explain this carbon imbalance proposes that some non-steady-state processes have not been considered in previous studies based on snapshot data. To test this hypothesis, we collected time-series (2.5 yr) data on sinking POC fluxes using moored sediment traps (trap deployment depths: 200 m, 500 m, and 4810 m) and compared them with the PCD data collected seasonally at two stations in the subarctic and subtropical western North Pacific. The Delta POC supplies (POC) in the 200-500 m and 500-4810 m layers were estimated with a correction, when appropriate, for the non-steady-state effect arising from the change in the POC flux during the transit of POC between the upper and deeper traps. In the 200-500 m layer, POC generally exceeded or equaled PCD. In contrast, in the 500-4810 m layer, Delta PCD generally exceeded Delta POC by up to sevenfold. However, on a yearly basis, this carbon imbalance in the deeper layer decreased, with PCD balancing Delta POC within a factor of 2. Therefore, the enigma of the high PCD relative to the POC flux in deep water is partially resolved by assuming a temporal uncoupling between supply and consumption, which partly equilibrates the carbon budget over a longer (yearly) time scale.

Seasonal changes in zooplankton swimmer (ZS) abundance, biomass and community structure were evaluated based on samples collected by moored sediment traps at a depth of 200 m in the subarctic (SA) and subtropical (ST) western North Pacific. Based on these samples, we made comparisons on two topics: 1) latitudinal (subarctic vs. subtropical) changes in ZS abundance, biomass and community and 2) quantitative differences between the ZS and particle organic carbon (POC) fluxes based on data from moored or drifting sediment traps. The results showed that the ZS flux was greater in the SA (annual mean: 311 ind.m(-2) day(-1) or 258 mg C m(-2) day(-1)) than in the ST (135 ind. day(-1) or 38 mg C m(-2) day(-1)). The peak ZS flux was observed from July-August in the SA and from April-May in the ST. The dominant taxa were Copepoda and Chaetognatha in the SA and Ostracoda and Mollusca in the ST. These latitudinal differences are likely related to the dominance of large-sized Copepoda in the SA, regional differences in the timing of the spring phytoplankton bloom, and the magnitude and size structure of primary producers. The percent composition of ZS to the total C flux (= ZS + POC flux) varied by region: 85-95% in the SA and 47-75% in the ST. These differences between the ZS composition and the total C flux are most likely caused by the dominance of large-sized Copepoda (Neocalanus spp. and Eucalanus bungii) in the SA.

To understand the seasonal dynamics of zooplankton in the southern Chukchi Sea, we use observations from a moored multi-frequency echo-sounder from July 2012 to July 2014. Zooplankton biomass, as indicated by area backscattering strength, was high during autumn and low in early spring; the seasonal peak in zooplankton biomass did not coincide with the spring phytoplankton bloom. This suggests that the seasonal zooplankton dynamics in the southern Chukchi Sea are less influenced by local growth of zooplankton during the spring phytoplankton bloom and more influenced by advection of zooplankton from the Bering Sea. The differences between volume backscattering strengths at 200 and 125kHz suggest that the main acoustic scatterers are large zooplankton (euphausiids and Neocalanus cristatus) in late summer and autumn and small zooplankton (other copepods) in other seasons. The decrease in acoustic backscatter from large zooplankton from winter to early summer also suggests the unsuccessful overwintering of advected Pacific zooplankton. The temporal mismatch between zooplankton and phytoplankton production suggests that there is still tight pelagic–benthic coupling in the southern Chukchi Sea.

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.