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We measured thorium-234 (234Th), particulate organic carbon (POC), and chlorophyll a in the western subarctic North Pacific surface layer in 1997–2008. 234Th, POC, and chlorophyll a in the surface layer showed clear seasonal changes. As a result of approximation by using a trigonometric function, 234Th export flux estimated from the deficiency of 234Th relative to 238U and the ratio of POC to particulate 234Th (POC/234ThP) indicated constant and decreasing trends in the study area from 1997 to 2008, respectively. POC export flux estimated from 234Th decreased in the study area, showing that the strength of the biological carbon pump in this region declined in the studied decade. The ratio of POC/chlorophyll a decreased during the term. These trends were possibly caused by a decline in not the biomass but particle size of phytoplankton. Our results suggest that not only the biological carbon pump but also phytoplankton community structure change in the western subarctic Pacific Ocean in a decade. [ABSTRACT FROM AUTHOR]

The mechanism that controls particulate organic carbon (POC) flux in the deep sea differs depending on the season and sea. The POC produced in the western subarctic North Pacific are known to be transported to the deep sea efficiently, but the direct relationship between the POC flux and physical processes is still unclear. In this study, we evaluated the effect of mesoscale eddies on POC flux in the western subarctic North Pacific. The seasonal and interannual variabilities of POC flux were investigated using data from a time-series sediment trap deployed at 4810 m at station K2 (47°N, 160°E) from 2005 to 2018. POC flux was high during May–November, appearing to reflect spring and fall blooms at the ocean surface. POC flux also showed interannual variability, with twelve peaks that were mostly affected by enhanced bloom just before the peak. Nine peaks of the twelve peaks were affected by mesoscale eddies, which enhanced bloom around K2 by extending the area with a high chlorophyll-a concentration along the coastal region into the offshore region, suggesting that mesoscale eddies strongly impact the interannual variability of POC flux at K2. [ABSTRACT FROM AUTHOR]

We conducted a current velocity and hydrographic time series observation in the upper ocean by deploying a mooring buoy at station K2 (47°N, 160°E) from July 2015 to June 2019 to understand the physical conditions that affect the acidification of the winter mixed layer (ML) water in the western subarctic North Pacific. Disturbances with substantial current velocity shear were excited by the wind changes remotely related to the 2015/2016 and 2018/2019 El Niño events. The density stratification beneath the ML was also weakened in winter, particularly the El Niño years. Based on the bulk Richardson number mixing scheme, we found that the El Niño‐related strengthening of the current velocity shear enhanced the vertical diffusivity beneath the winter ML driven by the near‐inertial current velocity variation. The weakening of the density stratification is ineffective in enhancing the vertical diffusivity. It is suggested that the El Niño‐related large vertical mixing accelerates the acidification of the winter ML water through the enhancement of the entrainment of the deep water that is rich in dissolved inorganic carbon. Plain Language Summary: The acidification of the sea's surface water in the western subarctic North Pacific is slower than the global mean ocean acidification. We observed the current velocity, temperature, and salinity in the upper ocean by deploying a mooring buoy at station K2 (47°N, 160°E) in the western subarctic North Pacific from July 2015 to June 2019 to understand which physical conditions affect the acidification. The current velocity varied noticeably, rotating clockwise near the frequency derived from the Earth's rotation, called the inertial frequency. Moreover, under the remote influence of the very warm sea surface condition in the central and eastern equatorial Pacific, namely El Niño, the vertical differences in the current velocity and the water density became large and small, respectively. The influences of El Niño were found to provide a favorable condition for the occurrences of strong vertical mixing beneath the winter mixed layer due to the near‐inertial current velocity variation. Based on the vertical mixing coefficient, we indicated the acceleration of the acidification due to the strengthening of the upward flux of dissolved inorganic carbon by occurrences of El Niño events. Key Points: Time series of current velocity and water density were obtained at K2 in the western subarctic North PacificShear‐driven vertical mixing is strengthened by the El Niño‐related current velocity changeEl Niño enhances upward dissolved inorganic carbon flux beneath the winter mixed layer and accelerates the acidification [ABSTRACT FROM AUTHOR]

Subpolar gyre regions such as the Western Subarctic North Pacific (WSNP) contain sluggish, low-oxygen water, and are threatened by loss of oxygen (deoxygenation). Our simulations under RCP 8.5 emission scenario suggest that installing pipes to induce artificial downwelling and upwelling (AD and AU) provides short-term solutions to combat deoxygenation in the WSNP. With no engineering, the WSNP's subsurface oxygen decreases by 30–100 mmol/m3 by the year 2100. Continuous implementation of AD and AU instead counters this declining trend, and AD is more effective than AU. The oxygenation effect is primarily a consequence of how the two engineering schemes vertically redistribute oxygen via physical processes. AD directly improves oxygen at depth via advecting surface water toward the ocean interior and subsequent enhanced pycnocline mixing, and AU does so via generating compensatory downwelling outside of the pipes. Both schemes take near 40 years to complete the oxygenation. After that, oxygen reaches a new equilibrium state in the WSNP with no further improvement by the engineering. AD and AU both strongly increase primary production surrounding the deployment sites, but lead only to weak enhancement of aerobic respiration in subsurface water and thus a minor impact on the oxygenation. Other unwanted environmental side effects are negligible compared to those caused by rapid climate change within this century, including outgassing of carbon dioxide, pH decrease, and precipitation reduction.

We conducted a ship observation of two anticyclonic mesoscale eddies (Eddies A and B) between June and July 2016 in the offshore region of the western subarctic North Pacific to clarify the impact of anticyclonic eddies on biological production. Analysis of satellite data reveals that Eddy A formed south of the Alaskan Peninsula on May 2011 while Eddy B formed in the observation area on November 2015. Iron concentration was low both inside and outside Eddies A and B, and there were no large differences between chlorophyll a (chl-a) concentration inside and outside eddies. Satellite-derived chl-a concentration inside Eddy A had been high compared with that in the surrounding area when the eddy was located along the Aleutian Islands and just after it detached from the islands, suggesting that iron originating from the coastal region was consumed during the two-year journey from the coastal region to the observation area. Field measurements also reveal that vertical diffusivity was low both inside and outside Eddies A and B, which could be another factor underlying low iron both inside and outside Eddies A and B and small differences between chl-a concentration inside and outside the eddies. [ABSTRACT FROM AUTHOR]

The margin of the western North Pacific Ocean releases redox-active elements like Mn, Co, and Ce into the water column to undergo further transformation through oxide formation, scavenging, and reductive dissolution. Near the margin, the upper ocean waters enriched in these elements are characterized by high dissolved oxygen, low salinity, and low temperature, and are a source of the North Pacific Intermediate Water. High dissolved concentrations are observed across the Western Subarctic Gyre, with a rapid decrease in concentrations away from the margin and across the subarctic-subtropical front. The particulate concentrations of Mn, Co, and Ce are also high in the subarctic surface ocean and enriched relative to Ti and trivalent rare earth elements. Furthermore, the particles enriched in Mn, Co, and Ce coincide at the same depth range, suggesting that these elemental cycles are coupled through microbial oxidation in the subarctic gyre as the waters travel along the margin before being subducted at the subarctic-subtropical front. Away from the margin, the Mn, Co, and Ce cycles decouple, as Mn and Ce settle out as particles while dissolved Co is preserved and transported within the North Pacific Intermediate Water into the central North Pacific Ocean.

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific Western Subarctic Gyre (WSG) revealed seasonal changes in δ N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting sediment traps (DST; 100-200 m) and moored sediment traps (MST; 200 and 500 m). All particles showed higher δ N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ N(SUS) signature was reflected by δ N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ N(DST) vs. PP regression to δ N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. This new approach to estimate productivity can be a powerful tool for further understanding of the biological pump in the WSG, even though its validity needs to be examined carefully. [ABSTRACT FROM AUTHOR]

Measurements of Th, particulate organic carbon (POC), nutrients and net primary production were made at station K2 (47°N, 160°E) in the western subarctic North Pacific Ocean six times from 3 June to 17 July 2006. Net primary productions varied from 263 to 732 mgC m d. Th was used as a tracer to estimate POC fluxes from the surface layer to 100 m depth. POC fluxes varied from 14 to 284 mgC m d. The maximum POC flux and net primary production in this study were much higher than those observed previously at station K2 (179 mgC m d in July 2003 for POC flux, 552 mgC m d in May 2004 for net primary production). Because maximum chlorophyll a in this study (1.3 µg L) was also much higher than previous values at station K2 (1.0 µg L in August 2004), we assumed that high-frequency observations had allowed measurement of a near annual peak of biological production and biological pump activity. The net primary productions and POC fluxes changed greatly during the study period. Biological production and export in this region varied greatly during this season. The export ratio (POC flux/net primary production) in this season (23 ± 1 %) was less than the annual export ratio in this region (29 ± 1 % in 2002-2005), although this ratio was higher than those in the other oceans (5-10 %). POC export (5.0 ± 0.1 gC m) in late spring-early summer was about half that of net community production (10.2 ± 0.6 gC m) estimated from the drawdown of phosphate during the study period. We suggest that the pathways of carbon export, other than the sinking of particles, were transport by zooplankton and so on. [ABSTRACT FROM AUTHOR]

Abstract: A patch of water in the western subarctic gyre (low iron concentration, <0.02nM) was fertilized twice with 322 and 159kg of iron to induce a phytoplankton bloom. In order to understand the changes in iron distribution and bio-availability throughout the evolution and termination phase of the iron-induced bloom, iron concentrations were measured at stations inside and outside of the iron-fertilized patch, and shipboard culture experiments using iron and desferrioxamine B (DFB) inoculation to regulate iron availability were conducted 5 times with water collected from the center of the iron-fertilized patch on D2, D7, D11, D17 and D23. After the iron fertilization, we observed a significant increase in dissolved iron (1.38nM at 5m depth) at the center of the patch (D1). Dissolved iron concentrations subsequently decreased to an ambient level (~0.08nM) on D16–D17, despite the second iron fertilization made on D6. During the 4-day incubations of the shipboard culture experiments, excess DFB-inoculated treatment inhibited the phytoplankton growth compared to the controls for D2, D7 and D11 patch water. This indicated that available iron existed in the iron-fertilized patch at least until D11. Moreover, iron-inoculated treatments induced growth of large-sized phytoplankton with an accompanying silicate decrease for D7, D11 and D17 patch water, but not for D23 patch water. These results indicated that large diatoms, which can respond to additional iron inoculation, existed in the iron-fertilized patch in evolution and early termination phase of the iron-induced bloom (at least until D17); however, there was no significant amount of large diatoms, which could rapidly respond to iron, in late termination phase (D23) of the iron-induced phytoplankton bloom. [Copyright &y& Elsevier]

Decadal- to multi-decadal variations have been reported in many regional ecosystems in the North Pacific, resulting in an increasing demand to elucidate the link between long-term climatic forcing and marine ecosystems. We detected phenological and quantitative changes in the copepod community in response to the decadal climatic variation in the western subarctic North Pacific by analyzing the extensive zooplankton collection taken since the 1950s, the Odate Collection. Copepod species were classified into five seasonal groups depending on the timing of the annual peak in abundance. The abundance of the spring community gradually increased for the period 1960–2002. The spring–summer community also showed an increasing trend in May, but a decadal oscillation pattern of quasi-30-year cycles in July. Phenological changes coincided with the climate regime shift in the mid-1970s, indicated by the Pacific decadal oscillation index (PDO). After the regime shift, the timing of the peak abundance was delayed one month, from March–April to April–May, in the spring community, whereas it peaked earlier, from June–July to May–June, in the spring–summer community, resulting in an overlap of the high productivity period for the two communities in May. Wintertime cooling, followed by rapid summertime warming, was considered to be responsible for delayed initiation and early termination of the productive season after the mid-1970s. Another phenological shift, quite different from the previous decade, was observed in the mid-1990s, when warm winters followed by cool summers lengthened the productive season. The results suggest that climatic forcing with different decadal cycles may operate independently during winter–spring and spring–summer to create seasonal and interannual variations in hydrographic conditions; thus, combinations of these seasonal processes may determine the annual biological productivity. [ABSTRACT FROM AUTHOR]