Northwestern North Pacific Research Articles

Country-wide assessment of plastic removal rates on riverbanks and water surfaces.

Plastic pollution in river basins poses a significant environmental challenge, particularly in Japan, located in the northwestern North Pacific, often referred to as a hotspot for plastics. This study quantified the annual recovery of anthropogenic plastic litter from riverbanks and water surfaces, utilizing a nationwide cleanup dataset provided by the Japanese Ministry of Land, Infrastructure, Transport and Tourism. The dataset includes the amount of litter recovered through volunteer and administrative cleanup campaigns from 2016 to 2020 in 109 Japanese river basins. Total plastic recovery ranged from 763 to 1177 tons/year, averaging 938 tons/year. Basin-specific recovery was associated with extreme weather events, such as typhoon approaches and heavy rain, which caused significant flood damage in local regions. The estimated annual plastic recovery was an order of magnitude smaller than the previously estimated annual emission of land-based plastic.

Linking northeastern North Pacific oxygen changes to upstream surface outcrop variations

Abstract. Understanding the response of the ocean to global warming, including the renewal of ocean waters from the surface (ventilation), is important for future climate predictions. Oxygen distributions in the ocean thermocline have proven an effective way to infer changes in ventilation because physical processes (ventilation and circulation) that supply oxygen are thought to be primarily responsible for changes in interior oxygen concentrations. Here, the focus is on the North Pacific thermocline, where some of the world's oceans' largest oxygen variations have been observed. These variations, described as bi-decadal cycles on top of a small declining trend, are strongest on subsurface isopycnals that outcrop into the mixed layer of the northwestern North Pacific in late winter. In this study, surface density time series are reconstructed in this area using observational data only and focusing on the time period from 1982, the first full year of the satellite sea surface temperature record, to 2020. It is found that changes in the annual maximum outcrop area of the densest isopycnals outcropping in the northwestern North Pacific are correlated with interannual oxygen variability observed at Ocean Station P (OSP) downstream at about a 10-year lag. The hypothesis is that ocean ventilation and uptake of oxygen is greatly reduced when the outcrop areas are small and that this signal travels within the North Pacific Current to OSP, with 10 years being at the higher end of transit times reported in other studies. It is also found that sea surface salinity (SSS) dominates over sea surface temperature (SST) in driving interannual fluctuations in annual maximum surface density in the northwestern North Pacific, highlighting the role that salinity may play in altering ocean ventilation. In contrast, SSS and SST contribute about equally to the long-term declining surface density trends that are superimposed on the interannual cycles.

Subseasonal Variation Characteristics of Low-Cloud Fraction in Southeastern and Northwestern North Pacific

The subseasonal variability of the low-cloud fraction (LCF) over the southeastern North Pacific (SENP) and northwestern North Pacific (NWNP) was studied using satellite observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is found that subseasonal variability of the LCF was closely related to variations in the estimated inversion strength (EIS), sea surface wind speed (SSW), sensible heat flux (SHF), sea surface temperature (SST), surface temperature advection (Tadv), relative humidity (RH), surface level pressure (SLP) and surface air temperature (SAT). An increase in the LCF over the SENP is associated with the development of an anomalous anticyclonic circulation, which is located on the west coast of America. The cold advection, together with the subsidence warming associated with the anticyclonic circulation, strengthens the temperature inversion, favoring the development of the LCF. In the NWNP, the maximum LCF anomaly was also correlated with the stable boundary layer. The southerly wind blows airflow over the Kuroshio Extension from the subtropics, which brings warm and moist air. When air flows to the colder sea surface, it is cooled and condensed by the intensified heat exchange. A lead-lag composite analysis indicates that the mechanisms are different between the SENP and the NWNP, possibly due to the different types of low-level clouds over these two regions. In the SENP, the trade cumulus dominates under a strong capping inversion over the subtropics, whereas fog and stratus often occur under a shallow capping inversion in the NWNP. The effects of atmospheric circulation are also discussed.

Correction to: Phylogeography of a salmonid fish, white-spotted charr (Salvelinus leucomaenis), in a historically non-glaciated region in the northwestern North Pacific

Correction to: Phylogeography of a salmonid fish, white-spotted charr (<i>Salvelinus leucomaenis</i>), in a historically non-glaciated region in the northwestern North Pacific

Typhoon-related changes in moisture pathways and sources for precipitation in Eastern China during the three major rainy seasons

Typhoons (TYs) and their interactions with large-scale ambient flow might exert an important control on moisture transport and precipitation. In this study, we used a Lagrangian model, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT), to quantify changes in moisture pathways and sources associated with TY activity in relation to precipitation in eastern China during the three major rainy seasons. We compared TY-active and TY-inactive periods, i.e., the periods with and without TYs (maximum sustained surface wind speeds of up to 32.7 m/s), over the western North Pacific (WNP) to investigate TY-associated changes for the period 1979–2020. During the pre-summer rainy season in South China, anomalous moisture exhibited a north–south dipole pattern, with moisture contributions from the region extending from the South China Sea to the adjacent tropical WNP showing a 17.0% decrease during TY-active compared with TY-inactive periods (34.7% versus 41.8%, respectively), and contributions from the region extending from southwestern China to the northwestern North Pacific showing a 18.9% increase (17.0% versus 14.3%, respectively) and those from the Bay of Bengal showing a 16.6% increase (17.6% versus 15.1%, respectively). The moisture anomaly during the Meiyu rainy season in the Yangtze–Huaihe River Basin had an east–west dipole structure, with the moisture contribution from the Bay of Bengal reduced by 21.8% as compared with TY-inactive periods, and that from the WNP increased by 62.3%. During the rainy season of North China, the moisture contribution from the Indian Ocean decreased by 7.2%, while that from the WNP increased by 8.7% and from Northeast China increased by 10.0%. In addition, during TY-active periods, moisture contributions with local origins showed a slight increase.

Interannual variation of the onset of the Tibetan Plateau rainy season and its relationship with the sea surface temperature in the North Pacific

AbstractThe onset of the Tibetan Plateau rainy season (OTPRS) marks the transition from dry season to wet season in situ, which has a great impact on the water cycle and atmospheric heating in extensive areas. This analysis found that the earlier OTPRS is controlled by the weakened westerly wind, a dipole vortex pair with an anomalous anti‐cyclone over Lake Baikal but an anomalous cyclone over the Tibetan Plateau, the intensified southerly water vapour transportation on the southern edge of the Tibetan Plateau, the active convection and the increased humidity over the Tibetan Plateau in spring. And the opposite circulation anomalies correspond to the later OTPRS. The interannual variation in the OTPRS is closely related to sea surface temperature anomalies (SSTA) in the North Pacific in the previous winter, with the opposite phase between the ‘tongue’ extending eastward from the China East Sea and the surrounding ‘semi‐circle’ in the east. This pattern features the third leading mode of the winter SSTA in the North Pacific and can persist to spring. The vertical motion, convergence and divergence associated with the direct thermal effect from the significant SSTA centres in the southeastern and northwestern North Pacific form two opposite zonal circulation circles along 30°N. The zonal circulation circles bridge the SSTA in the North Pacific and the circulation anomalies over the Tibetan Plateau and have a delayed impact on convection over the Tibetan Plateau, thereby affecting the OTPRS.

Subarctic-scale transport of 134Cs to ocean surface off northeastern Japan in 2020

We studied the spatiotemporal variations in 134Cs, 137Cs, and 228Ra concentrations at the sea surface off southeastern Hokkaido, Japan (off-Doto region) from 2018 to 2022 using low-background γ-spectrometry. The 134Cs concentrations in the off-Doto region, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, exhibited wide lateral variation each year (e.g., 0.7–1.1 mBq/L in 2020). By studying the 228Ra concentrations and salinity, this variation was explained based on the current mixing patterns. Furthermore, the 134Cs concentrations in the waters highly affected by the Oyashio Current (OYC) gradually increased from 2018 to 2020, and subsequently decreased in 2022. This implies that the water mass maximally contaminated with 134Cs was transported back to the side of the Japanese islands 10 years after the FDNPP accident along with counter-clockwise currents (e.g., the OYC) in the northern North Pacific Ocean. The 134Cs concentrations in the OYC-affected waters in the off-Doto region in 2020 were ~ 1/6 times those in the 134Cs-enriched core of waters off the western American Coast in 2015, which can be ascribed to dilution via spatial dispersion during subarctic current circulation. Overall, we elucidated the ocean-scale subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.

Phylogeography of a salmonid fish, white-spotted charr (Salvelinus leucomaenis), in a historically non-glaciated region in the northwestern North Pacific

Abstract The area around the Sea of Okhotsk and the Sea of Japan in the north-western North Pacific provides opportunities for studying the impact of climate oscillations on the evolutionary history of cold-temperate species independent of direct ice sheet development. We conducted a range-wide collection survey of white-spotted charr, Salvelinus leucomaenis, and integrated nuclear DNA and mitochondrial DNA (mtDNA) data to reveal the evolutionary history of this species. Phylogeographic analyses using genome-wide SNP data revealed the presence of five geographically distinct groups with slightly overlapping ranges. Of these, a northern group was widely distributed along the coast of the Sea of Okhotsk, the Sea of Japan and the northern region of Honshu Island, whereas the other four groups were only found on Honshu Island. The population structure of Honshu Island was largely divided into the Sea of Japan and Pacific Ocean groups by the mountain ranges running through the centre of Honshu Island, and was further subdivided by many geological factors, such as river capture and other vicariance processes. There was a discrepancy in the phylogeographic patterns between nuclear DNA and mtDNA, which was likely influenced by a complex population history that involved multiple dispersal and secondary contact events during several glacial-interglacial cycles.

Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton

AbstractIn the high‐productivity subarctic ocean, phytoplankton growth is assumed to rely mainly on nitrate, based on snapshot observations conducted in spring or summer. However, the year‐round utilization of nitrate and ammonium by phytoplankton remains unexplored. Here, we show the seasonal transition of the dominant nitrogen source for phytoplankton by using the nitrogen isotopic composition (δ15N) of nitrate, ammonium, and sinking particles in the northwestern North Pacific. To decouple the combined effects of nitrogen sources for phytoplankton and subsequent trophic transfers in δ15N of sinking particles, we used compound‐specific δ15N analysis of amino acids (δ15NAA) in sinking particles. The seasonal change in δ15NAA revealed that most of the seasonal variation in δ15N of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The δ15N of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn.

Wintertime marine extreme temperature events modulate phytoplankton blooms in the North Pacific through subtropical mode water

Marine extreme temperature events (METs), including marine heatwaves (MHWs) and cold spells, have recently gained much attention owing to their vital influence on the marine ecosystem and social economy. Since METs can alter the upper ocean stratification and wintertime convective mixing in the northwestern North Pacific subtropical gyre (NPSG), their activities may modulate phytoplankton blooms by regulating entrainment of the subtropical mode water (STMW) with high NO. Furthermore, because STMW formed in the previous winter reemerges east of its formation site in the following winter, the METs activities imprinted in STMW affect phytoplankton blooms remote from its formation site. Here, we examined the relationship between the MET activities, STMW volume, and phytoplankton blooms using satellite observations and a data-assimilative coupled physical-biogeochemical model dataset. MET activities appearing in the STMW formation region during winter regulate the formation of STMW and the supply of NO from the subsurface, with the latter controlling the spring/autumn blooms in that region under NO-limited conditions. Subsequently, this water mass is transported eastward in the subsurface within the northern flank of the NPSG before reemerging east of the STMW formation site the following spring. This process results in a negative lag-correlation between MET activities and surface chlorophyll in the reemergence region; for example, MHWs in winter at the STMW formation site tend to lower the surface chlorophyll concentration one year later in the reemergence region. Our study suggests that the oceanic processes allow one year of predictability of the marine ecosystems by monitoring METs in the STMW formation site.

Unveiling controls of the latitudinal gradient of surface pCO2 in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring

Unveiling controls of the latitudinal gradient of surface pCO2 in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring

Decreasing Wintertime Mixed‐Layer Depth in the Northwestern North Pacific Subtropical Gyre

AbstractThe deep winter mixed layer (ML), ∼240 m, in the northwestern North Pacific subtropical gyre exchanges heat and absorbs carbon dioxide from the atmosphere, and receives nutrients through the Kuroshio. The ML water spreads broadly in the subsurface ocean over the subtropical gyre. Compiling historical temperature measurements, we show that the winter ML depth (MLD) has decreased by ∼6% over the last six decades. Observations, atmospheric reanalyzes, simulation outputs from a one‐dimensional turbulent closure model, and climate projections reveal that the strengthening of the upper‐ocean stratification due to surface warming under global warming is responsible for the MLD decrease. Climate models project further decrease of winter MLD attributable to the marked strengthening of ocean stratification with climate warming, of up to 27%–40% by 2100 under the highest emission scenario. A monitoring system for winter ML should be established so that important changes for marine ecosystems can be foreseen.

Nonstationary Responses of Demersal Fishes to Environmental Variations in Temperate Waters of the Northwestern North Pacific under a Changing Climate

Although nonstationarity in marine ecosystems has attracted great attention, the nonstationary responses of demersal fishes to environmental variations induced by the changing climate are still not well understood. Here, we collected 21 time series of demersal fishes from 1956 to 2015 to investigate the climate-induced nonstationary responses in temperate waters of the northwestern North Pacific (NWP). We showed that these demersal fishes experienced state shifts in approximately 1986/87, accompanied by abrupt warming over this region. Moreover, the relationships between demersal fishes and sea surface temperature (SST) were found to change between the two identified eras (i.e., a weak relationship before 1986 and a strong negative relationship after 1986), which may be primarily caused by the alternating dominance of the East Asian winter monsoon and mega-ENSO on SST in temperate waters of the NWP. The identified climate-induced nonstationary responses of demersal fishes to SST variability in this study may provide implications for understanding climate-induced biological dynamics, predicting demersal fish fluctuations, coping with potential ecological risks, and the sustainable exploitation of fishery resources in the future climate. Note that the impact of fishing on the demersal fishes in temperate waters of the NWP was not assessed in this study due to the lack of fishing effort data and therefore the conclusions of our research should be approached with caution.

Interannual variability of summer tropical cyclone activity in the northwestern North Pacific modulated by El Niño‐Southern oscillation and intraseasonal oscillation

AbstractInterannual variability of summer (June–August) tropical cyclones (TCs) in the northwestern North Pacific (NP) region (120°–135°E, 20°–35°N) and El Niño‐Southern Oscillation (ENSO) exhibit four asymmetric ENSO‐TC relationships. The major relationship types are El Niño‐enhanced and La Niña‐suppressed, while the minor relationship types are El Niño‐suppressed and La Niña‐enhanced. The major modulatory processes for the El Niño‐enhanced type feature an elongated anomalous cyclone extending from the tropical western NP (WNP) northwestward toward the northwestern NP. This feature intensifies TC formation in the tropical WNP and guides them northward/northwestward increasing TC frequency in the northwestern NP. The El Niño‐suppressed type has its anomalous cyclone displacing eastward in the tropical WNP with an accompanying anomalous anticyclone across the northwestern NP. TC formation reduces in the tropical WNP and TC movement is blocked toward the northwestern NP, resulting in decreased TC frequency. The La Niña‐suppressed type features a dominant elongated anomalous anticyclone in the WNP to suppress TC formation and movement toward the northwestern NP, yielding reduced TC frequency. The La Niña‐enhanced type contains an anomalous anticyclone south of 20°N in the tropical WNP in company with an anomalous cyclone over the northwestern NP. Under this anomalous cyclone, enhanced TC formation and movement in the northwestern NP result in increased TC frequency. For all relationship types, 30–60‐day intraseasonal oscillation (ISO) anomalies feature a dominant anomalous cyclone over the WNP providing favourable conditions that guide TC movement toward its central region over the northwestern NP. Both ENSO and ISO make positive contributions that increase TC frequency in the northwestern NP for the El Niño‐enhanced and La Niña‐enhanced types. ENSO is the major factor reducing TC frequency in the El Niño‐suppressed and La Niña‐suppressed types.

The Formation of Subtropical Phytoplankton Blooms Is Dictated by Water Column Stability During Winter and Spring in the Oligotrophic Northwestern North Pacific

AbstractSubtropical phytoplankton blooms were observed in winter and late spring (rather than in early spring, as is typical) via shipboard observations in an area south of the Kuroshio Extension in the northwestern North Pacific subtropical gyre. Satellite‐based observations revealed that these submesoscale blooms occurred in warmer water masses in winter and in cooler water masses in late spring. The fact that winter blooms occurred in warmer areas suggests that they depend on water column stratification caused by solar heating. In contrast, the fact that the late‐spring blooms occurred in cooler areas suggests a breakdown of stratification due to a recurrence of convective mixing. Mesoscale blooms occurred at intermediate water temperatures in early spring, suggesting a repeating sequence of stratification and mixing during this period. Wintertime deep convective mixing in the northwestern North Pacific subtropical gyre creates Subtropical Mode Water. The behavior of the wintertime mixed‐layer depth, which determines the thickness of Subtropical Mode Water, also dictates the characteristics of subtropical blooms such as their magnitude and the timing of onset and demise. Based on in situ observations and a model analysis, we conclude that deeper winter mixing, which increases nutrient concentrations, will intensify early‐spring blooms and facilitate the formation of episodic blooms in winter and late spring. On the other hand, shallower winter mixing should increase stratification and thus facilitate the formation of smaller blooms, even in winter.

Climate Variability Patterns and Their Ecological Effects on Ecosystems in the Northwestern North Pacific

Climate-induced ecosystem variability is increasingly concerning in recent years. Integrated researches in the northeastern North Pacific have proved the ecological importance of the Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation (NPGO) and El Nino-Southern Oscillation (ENSO) to ecosystem variability. While in the northwestern North Pacific, researches have been independent of each other over different regional ecosystems, and identified relatively weak linkages between these climatic indices (e.g. PDO, NPGO, and ENSO) and variations in the regional ecosystems. Such disassociated researches with unidentified important climate variability patterns may have hampered a holistic understanding of climate-induced ecosystem variability in the northwestern North Pacific. Furthermore, non-stationarity in climate-biology relationships has been proven to be important for ecosystems in the northeastern North Pacific but has not yet been studied in the northwestern North Pacific. Therefore, this research compiles biological, environmental and climatic data in ecosystems in the northwestern North Pacific and employs a suite of analytical techniques, aiming to provide a holistic understanding of the climate-induced ecosystem variability. It shows that ecosystems in the northwestern North Pacific had a leading regime shift in the late 1980s in response to climate variability. The Siberian High, Arctic Oscillation and East Asian Monsoon exhibit greater ecological importance to ecosystem variability than the PDO, NPGO and ENSO. Their variations contribute greatly to sea surface temperature changes and thus variations in ecosystems. Furthermore, modified models considering non-stationary relationships achieve better performances than stationary models, suggesting the existence of non-stationarity in climate-biology relationships in the northwestern North Pacific. This non-stationarity resulted from the decline in variance of the sea level pressure in Siberian High rather than the Aleutian Low as suggested by previous studies in the northeastern North Pacific. Our research provides an improved understanding of the climate-induced ecosystem variability in the northwestern North Pacific, offering implications for furthering research on the entire North Pacific.

Climate‐induced nonlinearity in pelagic communities and non‐stationary relationships with physical drivers in the Kuroshio ecosystem

AbstractClimate‐induced nonlinearity in biological variability and non‐stationary relationships with physical drivers are crucial to understand responses of marine organisms to climate variability. These phenomena have raised concerns in the northeastern North Pacific, but are out of the spotlight in the northwestern North Pacific in spite of potential implications for this productive system under increased climate variability. Pelagic communities in the Kuroshio ecosystem have both ecological and economic importance. However, patterns of climate‐induced nonlinearity in pelagic communities are not well understood, and existence of non‐stationarity in their relationships with physical drivers remains obscure. Here, we compile large numbers of climatic, oceanic and biological long‐term time‐series data and employ diverse statistical techniques to reveal such climate‐induced nonlinearity and non‐stationarity. Results show that pelagic communities in the Tsushima and Pacific areas (major areas in the Kuroshio ecosystem) had regime shifts in the late 1990s and late 1980s, respectively. Winter sea surface temperatures in the Kuroshio Current path and in the eastern part of East China Sea, which are respectively affected by the Kuroshio Current and Siberian High, correlate with dominant variability patterns in their pelagic communities. Furthermore, non‐stationarity was identified with threshold years in the 1990s in the Tsushima area and in the 1980s in the Pacific area as a possible result of the declined variances in the Siberian High and Aleutian Low, respectively. Our findings provide insights on spatial differentiation of climate‐induced nonlinearity and non‐stationarity, which are valuable for the management of pelagic communities in the northwestern North Pacific under changing climatic conditions.

An Expanded Batch-to-Batch Correction for IAPSO Standard Seawater

AbstractWe expanded the batch-to-batch offsets of The International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater (SSW) batches P145–P163 by intercomparison measurements using salinometers. On the basis of our results, we recommend using the correction factors instead of the offsets to correct the batch-to-batch differences, especially for salinity data outside the range of 30–40 g kg−1. We evaluated the expanded batch-to-batch correction factors by applying them to time series salinity data collected in the northwestern North Pacific Ocean and found that they are effective for detecting recent freshening (−0.6 ± 0.1 × 10−3 g kg−1 decade−1) in the deep North Pacific, which might be related to a reduction of the formation rate of Antarctic Bottom Water. We also evaluated the SSW linearity pack by applying the batch-to-batch correction factors. Linearity errors of the salinometers estimated from decade resistance substituters were consistent with the results of the linearity pack measurements. To correct the linearity errors of a salinometer, it might be suitable to use the more detailed distribution of those estimated from the decade resistance substituter than the linearity pack measurements. Since the cause of large batch-to-batch differences is still unclear, a reference seawater that is more robust and stable than SSW might be necessary to establish a high-level of international comparability of salinity measurements; the Multiparametric Standard Seawater (MSSW) currently under development might be a candidate for such reference seawater, because MSSW is expected to be more stable than SSW not only in practical salinity but also in absolute salinity.

Primary production and nitrogen assimilation rates from bay to offshore waters in the oyashio-kuroshio-Tsugaru Warm Current interfrontal region of the northwestern north Pacific Ocean

We examined primary production (PP) and the assimilation rates of nitrate (ρNO3-), ammonium (ρNH4+) and dinitrogen (ρN2) using 13C and 15N tracer techniques along bay to offshore transects in the Oyashio-Kuroshio-Tsugaru Warm Current interfrontal region in the northwestern North Pacific Ocean, during five research cruises covering a full seasonal cycle. Surface inorganic N was depleted in the bays and the offshore region from summer to fall, but the higher particulate organic carbon (POC)/particulate nitrogen (PN) ratio and higher maximum N assimilation rate observed kinetics experiments in the bays suggest that N limitation was stronger in bays than offshore. PP (295–19,200 nmol C L−1 d−1), ρNO3- (9.45–650 nmol N L−1 d−1) and ρNH4+ (11.6–494 nmol N L−1 d−1) in the surface waters were generally low in summer and high in spring. Different from the cases of ρNO3- and ρNH4+, ρN2 was high (up to 12 nmol N L−1 d−1) in mid-summer, especially in offshore regions, and moderate or low (≤2.26 nmol N L−1 d−1) during other seasons. N2 fixation largely contributed to total new production (up to 29%) in mid-summer. The mean f-ratio, estimated as the ratio of (ρNO3- + ρN2) to (ρNO3- + ρN2 + ρNH4+) at two offshore stations varied within the range of 0.29–0.83 (mean 0.54 ± 0.20). Comparison of the PP and total N assimilation (ρNO3- + ρN2 + ρNH4+) indicated that the ratio of C to N assimilation rates (max. 95) far exceeded the Redfield ratio (6.6). This finding suggests that other sources of N (e.g., dissolved organic nitrogen) contributed significantly to total N assimilation; if this is correct, the estimated f-ratio may be too high. Alternatively, the fC-ratio was estimated from the ratio of (ρNO3- + ρN2) to PP by assuming the Redfieldian assimilation ratio, and it ranged from 0.06 to 1.10 (mean 0.45 ± 0.31). Despite uncertainties in both the f- and fC-ratio estimation, our data provide evidence that a large fraction of PP is potentially available for export to deep waters and to higher trophic levels in the interfrontal region of the western North Pacific.

Seasonal and Interannual Variations in Nitrogen Availability and Particle Export in the Northwestern North Pacific Subtropical Gyre

AbstractTime series of particulate nitrogen isotope delta (PN‐δ15N) and flux of trapped particles at 200 m were determined in the northwestern North Pacific subtropical gyre between 2010 and 2014 in order to examine temporal variations in mixed layer nitrate availability and downward particle export. Lower PN‐δ15N (&lt;4‰) seen every winter corresponded to convective nitrate supply triggering phytoplankton blooms. Greater particle fluxes were associated with higher CaCO3 concentrations (mass fraction &gt;50%), implying that new nitrogen supply stimulated predominantly coccolithophore growth and downward transport of coccoliths. Even in the stratified summer of 2011, a δ15N drop associated with a spike in particulate flux was found and was attributed to the uplift of nitrate‐rich deep waters due to a mesoscale cyclonic eddy passing the mooring station. Total nitrate input sustains 86–93% of annual new production in this region while nitrogen fixation accounts for the rest. Trapped particles also showed that the winter δ15N decrease appeared earlier in the 2011–2012 blooms than those in 2013–2014, coinciding with 8% higher CaCO3 concentrations and a 40% lower particulate organic carbon to inorganic carbon export ratio, R(POC:PIC). This reflected stronger convective mixing and hence larger nutrient supply in 2011–2012, caused by larger ocean heat loss related to winter monsoon intensity. Such interannual change of winter R(POC:PIC) can affect CO2 uptake rate in the northwestern North Pacific subtropical gyre where anthropogenic CO2 accumulates in subtropical mode waters.