Western North Pacific Subtropical Gyre Research Articles

Assessing N2 fixation flux and its controlling factors in the (sub)tropical western North Pacific through high‐resolution observations

AbstractThe (sub)tropical western North Pacific is potentially an area of intense nitrogen (N2) fixation in the global ocean, despite limited understanding of the flux and controlling factors. We conducted high‐resolution observations from 2016 to 2021 in this region and used machine learning algorithms to simulate N2 fixation flux. Models estimated an N2 fixation flux from 5.72 to 6.45 Tg N yr−1, with strong seasonal variation and peak rates in summer. The western North Pacific Subtropical Gyre and the Kuroshio Current contributed more to N2 fixation flux than did the adjacent areas. Models suggested that sea surface temperature, photosynthetically available radiation, and nutrient supply were most strongly correlated with seasonal and spatial variations in N2 fixation. This study provides an improved estimation of N2 fixation in the western North Pacific and advances our understanding of its role in ocean productivity.

Reduced nitrite accumulation at the primary nitrite maximum in the cyclonic eddies in the western North Pacific subtropical gyre.

Nitrite, an intermediate product of the oxidation of ammonia to nitrate (nitrification), accumulates in upper oceans, forming the primary nitrite maximum (PNM). Nitrite concentrations in the PNM are relatively low in the western North Pacific subtropical gyre (wNPSG), where eddies are frequent and intense. To explain these low nitrite concentrations, we investigated nitrification in cyclonic eddies in the wNPSG. We detected relatively low half-saturation constants (i.e., high substrate affinities) for ammonia and nitrite oxidation at 150 to 200 meter water depth. Eddy-induced displacement of high-affinity nitrifiers and increased substrate supply enhanced ammonia and nitrite oxidation, depleting ambient substrate concentrations in the euphotic zone. Nitrite oxidation is more strongly enhanced by the cyclonic eddies than ammonia oxidation, reducing concentrations and accelerating the turnover of nitrite in the PNM. These findings demonstrate a spatial decoupling of the two steps of nitrification in response to mesoscale processes and provide insights into physical-ecological controls on the PNM.

Anticyclonic mesoscale eddy induced mesopelagic biomass hotspot in the oligotrophic ocean

Increasing evidence suggests that mesoscale eddies have a strong influence on the distribution and behavior of zooplankton and micronekton. However, the mechanisms driving their responses to mesoscale eddies are not yet fully understood, largely due to the low resolution of previous observations. Underwater gliders equipped with acoustic equipment and environmental sensors provide an ideal means to obtain observations with high temporal and spatial resolutions. In this study, two underwater gliders were used to record environmental parameters and acoustic intensities across an anticyclonic eddy in the oligotrophic western North Pacific subtropical gyre in September 2019. Based on the variations in the sea surface current and sea surface height anomaly (SSHA) along the underwater glider tracks, the study area was divided into three subregions: eddy core (SSHA >27 cm), eddy periphery (27 cm > SSHA >20 cm), and ambient water (SSHA <20 cm). More intense backscattering occurred in the upper 800 m of the eddy core with mean area backscattering coefficient values that were 35.8% higher than those of the eddy periphery and 19.8% higher than those of the ambient water. The convergence associated with the eddy could explain that the largest animal congregations occurred in the upper 100 m of the eddy core at night. Downwelling of warmer and oxygen-rich water in the eddy core induced animals to accumulate toward the deep of the eddy core, while migrating downwards during the day. Both these results and the lack of spatial heterogeneity of chlorophyll -a in and around the anticyclonic eddy indicated that horizontal and vertical distributions of zooplankton and micronekton in the anticyclonic eddy in the oligotrophic ocean were less related to spatial variations in the phytoplankton, but might be associated with the combined effect of the physical process in the anticyclonic eddy and the diel vertical migration behavior of animals. Our results suggest that anticyclonic eddies can dynamically modulate the spatial distribution of biomass, leading to the formation of mesopelagic biomass hotspots in the oligotrophic oceans and strongly affecting the ecology and behaviors of top predators that prey on small animals. • Spatial heterogeneity in biomass distribution was observed in an anticyclonic eddy. • Redistribution of biomass occurs within anticyclonic eddies. • Anticyclonic eddies in oligotrophic oceans support mesopelagic biomass hotspots.

Distribution and geographic differences in stable isotope ratios of diamond squid Thysanoteuthis rhombus (Cephalopoda: Thysanoteuthidae) in Japanese waters

AbstractDiamond squid Thysanoteuthis rhombus (Troschel, 1857), which can grow to a dorsal mantle length of ~100 cm in 1 year, is an important fisheries resource in subtropical and temperate waters worldwide. Around Japan, it is an important resource in the East China Sea, Sea of Japan, and western North Pacific Subtropical Gyre, including the Kuroshio area. It is not known if linkages occur between these habitats. To clarify the species distribution, we estimated the habitat of young squid using a generalized additive model based on trawl surveys and analyzed carbon and nitrogen stable isotope ratios (δ13C and δ15N) in tissue samples. Based on trawl observations, young squid were distributed around northeastern Taiwan and the Okinawa Islands in June–July and in the Sea of Japan and the Kuroshio extension areas in August–September. We observed ontogenetic changes in the stable isotope ratios. The δ15N values in large diamond squid (dorsal mantle length &gt; 400 mm) were significantly lower in the Pacific than those in other areas. Considering that the δ15N of forage fish is low in the Pacific and high in the East China Sea and Sea of Japan, large diamond squid in the northwest Pacific presumably have two distinct habitats.

Regionally Variable Responses of Nitrogen Fixation to Iron and Phosphorus Enrichment in the Pacific Ocean

AbstractIron and phosphorus are major nutrients limiting marine nitrogen fixation. We directly assessed iron and phosphate enrichment using the surface water of the western and central North Pacific subtropical gyre (NPSG), central equatorial Pacific, and eastern South Pacific subtropical gyre (SPSG). Diazotrophic phylotypes showed regionally distinct distribution patterns. The abundances of nifH in UCYN‐A, Trichodesmium, Crocosphaera, and γ‐24774A11 and filaments of Richelia were significantly higher in the central and western NPSG than in the eastern SPSG. The surface dissolved iron concentration was &lt;0.2 nmol l−1 in most of the regions; however, nitrogen fixation was enhanced by the addition of iron only in the western NPSG and equatorial upwelling. In the western NPSG, where surface phosphate was almost exhausted, and levels varied below 7 nmol l−1, diazotrophs also exhibited either phosphate limitation or colimitation by iron and phosphate. The present study demonstrated the limitation of nitrogen fixation by iron and phosphorus individually and colimitation by iron and phosphorus in the western NPSG for the first time, and the data showed that the nutrients limiting nitrogen fixation in the Pacific Ocean differed among the regions, indicating that the response of nitrogen fixation to environmental changes is not uniform.

Micro-size plankton abundance and assemblages in the western North Pacific Subtropical Gyre under microscopic observation.

While primary productivity in the oligotrophic North Pacific Subtropical Gyre (NPSG) is changing, the micro-size plankton community has not been evaluated in the last 4 decades, prompting a re-evaluation. We collected samples over three years (2016-2018) from depths of 10 to 200 m (n = 127), and the micro-size plankton were identified and counted to understand the heterogeneity of micro-size plankton community structure. The assemblages were consistent to the those of 4 decades ago. Dinophyceae (dinoflagellates) were the most numerically abundant, followed by Cryptophyceae and Bacillariophyceae (diatoms). The other micro-size plankton classes (Cyanophyceae, Haptophyceae, Dictyochophyceae, Euglenophyceae, and Prasinophyceae) were not always detected, whereas only Trichodesmium spp. was counted in the Cyanophyceae. Other unidentified autotrophic and heterotrophic flagellates were also significantly present, and their numeric abundance was higher than or at the same level as was that of the Dinophyceae. In the Dinophyceae, Gymnodiniaceae and Peridiniales were abundant. The chlorophyll a concentration and these class-level assemblages suggested micro-size plankton is not a major primary producer in this area. We applied generalized additive models (GAMs) and principal coordination analyses (PCoAs) to evaluate the habitats of every plankton group and the heterogeneity of the assemblages. The GAMs suggested that every classified plankton abundance showed a similar response to salinity, and we observed differences in habitats in terms of temperature and nitrate concentrations. Based on the PCoAs, we observed unique communities at the 200 m depth layer compared with those at the other sampling layers. The site scores of PCoAs indicated that the micro-size plankton assemblages are most heterogeneous at the 10 m depth layer. At such depth, diazotrophic Cyanophyceae (Trichodesmium spp.) are abundant, particularly in less-saline water. Therefore, nitrogen fixation may contribute to the heterogeneity in the abundance and assemblages in the western NPSG.

Late Summer Peak and Scavenging-Dominant Metal Fluxes in Particulate Export Near a Seamount in the Western North Pacific Subtropical Gyre

We measured sedimentation fluxes around JA06 Seamount (Xufu Guyot; 19°30′N, 158°00′E) as part of an environmental baseline survey in Japan’s exploration area for cobalt-rich crusts in the subtropical Northwest Pacific. Sinking particles were collected at the flat top (sediment trap depths: 900 and 1000 m) and northeastern base (sediment trap depths: 1000 and 4720 m) of the seamount from June 2016 to April 2017. Total mass fluxes were very low, with average values of 4.3–4.9 mg m–2 d–1 and 9.3 mg m–2 d–1 in the shallow traps at the flat top and base, respectively, which is consistent with an oligotrophic system. The lower fluxes at the flat top probably reflect lower productivity of siliceous microplankton, such as diatoms. However, we were unable to substantiate any potential mechanisms for this difference in productivity and cannot evaluate whether this is representative of typical conditions. When combined with previous observations at two adjacent seamounts, our results indicate widespread seasonality in sediment fluxes with a peak in late summer (August–September). However, satellite data indicate that summer is the season with the lowest primary production. This discrepancy could be explained either by phytoplankton blooms fueled by symbiotic nitrogen fixation that only cause minor increases in surface-ocean chlorophyll or short-lived blooms induced by passing typhoons under thick cloud cover. At the base site, we also analyzed material and element transport rates from shallow to deep waters. Half of the organic matter and >80% of the carbonate in sinking particles was not degraded in the water column, suggesting that most of the regeneration of these materials occurs near or on the sediment surface. Furthermore, four major processes appeared to control elemental fluxes in the area: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, rare earth elements) processes. The estimation of excess flux based on the composition of upper continental crust demonstrated that >85% of total Mn, Co, Ni, Cu, Zn, Cd, and Pb fluxes were attributable to scavenging (+biogenic uptake). Scavenging-dominant metal fluxes are likely ubiquitous in the oligotrophic open ocean.

Effect of larval swimming in the western North Pacific subtropical gyre on the recruitment success of the Japanese eel.

The possible effect of directional larval swimming on the recruitment success of the Japanese eel, Anguilla japonica, was examined with a three-dimensional particle-tracking ocean circulation model using horizontal northwestward swimming and diel vertical migration (DVM). Four separate experiments included virtual larvae (v-larvae) movement from the spawning area over 290 days (total migration) and 160 days (stage A), from the STCC eddy region in 70 days (stage B), and from the origin of the Kuroshio in 60 days (stage C) to evaluate the effect of directional swimming and DVM compared to simple drifting. Passive or random swimming were not the most effective strategies for larvae dispersing from the spawning area because most v-larvae remained south of 20°N without entering the Kuroshio. Northwestward swimming resulted in wider dispersion and a better chance of successful recruitment, with v-larvae becoming widely distributed in the STCC eddy zone, arriving at the east coast of the Philippines (stage A), escaping the STCC eddy area and reaching the Kuroshio (stage B), and crossing the Kuroshio into the East China Sea shelf (stage C). DVM slightly shortened the migration period due to faster shallow layer ocean currents during nighttime. The NEC transported non-swimming v-larvae westward to the Kuroshio and occasionally northward into the Subtropical Countercurrent (STCC) area where eddies transported v-larvae westward into the Kuroshio, but less than with swimming. Directional swimming increased recruitment success, northwestward swimming was more effective than other directions, and a slower swimming speed was still better than no/random swimming in sensitivity tests. The present study demonstrated a first view of the possibility that Japanese eel larvae might be able to use a strategy of single-direction swimming to increase arrival at their recruitment areas.

Rapid decline in pH of coral calcification fluid due to incorporation of anthropogenic CO2

Marine calcifying organisms, such as stony corals, are under threat by rapid ocean acidification (OA) arising from the oceanic uptake of anthropogenic CO2. To better understand how organisms and ecosystems will adapt to or be damaged by the resulting environmental changes, field observations are crucial. Here, we show clear evidence, based on boron isotopic ratio (δ11B) measurements, that OA is affecting the pH of the calcification fluid (pHCF) in Porites corals within the western North Pacific Subtropical Gyre at two separate locations, Chichijima Island (Ogasawara Archipelago) and Kikaijima Island. Corals from each location have displayed a rapid decline in δ11B since 1960. A comparison with the pH of the ambient seawater (pHSW) near these islands, estimated from a large number of shipboard measurements of seawater CO2 chemistry and atmospheric CO2, indicates that pHCF is sensitive to changes in pHSW. This suggests that the calcification fluid of corals will become less supersaturated with respect to aragonite by the middle of this century (pHCF = ~8.3 when pHSW = ~8.0 in 2050), earlier than previously expected, despite the pHCF-upregulating mechanism of corals.

A rare and extensive summer bloom enhanced by ocean eddies in the oligotrophic western North Pacific Subtropical Gyre

The North Pacific Subtropical Gyre (NPSG) is the largest ecosystem on Earth, and it plays a critical role in global ocean productivity and carbon cycling. Here, we report a rare and striking ~2000-km-long phytoplankton bloom that lasted over one month in the western part of the NPSG in summer 2003. The bloom resulted from the co-occurrence of a northward-shifted North Equatorial Current (NEC) supplying additional phosphate, and strong eddy activity that fueled productivity and spread chlorophyll mainly through horizontal stirring. The extensive one-month bloom had a maximum Chl concentration of six times the summer mean value and collectively fixed an additional five teragrams (5 × 1012 g) of carbon above the summer average. An increase in the pCO2 during the bloom suggests that most of the additionally fixed carbon was rapidly consumed.

Long-term change and variation of salinity in the western North Pacific subtropical gyre revealed by 50-year long observations along 137\xb0E

The 137°E repeat hydrographic section for 50 winters during 1967–2016 has been analyzed to examine interannual to interdecadal variations and long-term changes of salinity and temperature in the surface and intermediate layers of the western North Pacific, with a particular focus on freshening in the subtropical gyre. Rapid freshening on both isobars and isopycnals began in the mid-1990s and persisted for the last 20 years in the upper main thermocline/halocline in the western subtropical gyre. In addition, significant decadal variability of salinity existed in the subtropical mode water (STMW), as previously reported for the shallower layers. An analysis of the 144°E repeat hydrographic section during 1984–2013 supplemented by Argo profiling float data in 2014 and 2015 revealed that the freshening trend and decadal variability observed at 137°E originated in the winter mixed layer in the Kuroshio Extension (KE) region and was transmitted southwestward to 137°E 1–2 years later in association with the subduction and advection of STMW. The mechanism of these changes and variations in the source region was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.

Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent

Abstract Located at the center of the western North Pacific Subtropical Gyre, the Subtropical Countercurrent (STCC) is not only abundant in mesoscale eddies, but also exhibits prominent submesoscale eddy features. Output from a ° high-resolution OGCM simulation and a gridded satellite altimetry product are analyzed to contrast the seasonal STCC variability in the mesoscale versus submesoscale ranges. Resolving the eddy scales of &amp;gt;150 km, the altimetry product reveals that the STCC eddy kinetic energy and rms vorticity have a seasonal maximum in May and April, respectively, a weak positive vorticity skewness without seasonal dependence, and an inverse (forward) kinetic energy cascade for wavelengths larger (shorter) than 250 km. In contrast, the submesoscale-resolving OGCM simulation detects that the STCC eddy kinetic energy and rms vorticity both appear in March, a large positive vorticity skewness with strong seasonality, and an intense inverse kinetic energy cascade whose short-wave cutoff migrates seasonally between the 35- and 100-km wavelengths. Using a 2.5-layer, reduced-gravity model with an embedded surface density gradient, the authors show that these differences are due to the seasonal evolution of two concurring baroclinic instabilities. Extracting its energy from the surface density gradient, the frontal instability has a growth time scale of O(7) days, a dominant wavelength of O(50) km, and is responsible for the surface-intensified submesoscale eddy signals. The interior baroclinic instability, on the other hand, extracts energy from the vertically sheared STCC system. It has a slow growth time scale of O(40) days, a dominant wavelength of O(250) km, and, together with the kinetic energy cascaded upscale from the submesoscales, determines the mesoscale eddy modulations.

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

Abstract. The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. Prochlorococcus outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of Synechococcus, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of Prochlorococcus from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths &gt; 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of Synechococcus in the Subtropical gyre and a lower abundance of Synechococcus in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column.

Different mechanisms controlling interannual phytoplankton variation in the South China Sea and the western North Pacific subtropical gyre: A satellite study

A time series of remotely-sensed chlorophyll a (chl a) in 1997–2010 was evaluated to determine mechanisms of phytoplankton variation in recent decade in the South China Sea (SCS) and the western North Pacific subtropical gyre (WNPSG). Satellite-derived sea surface temperature (SST) and aerosol optical thickness (AOT) were used as proxies for vertical nutrient supply and atmospheric aerosol, respectively. Chl a in the WNPSG was not significantly correlated with SST (r=0.18, p>0.05), but was with AOT (r=0.31, p<0.05), indicating the chl a was influenced by atmospheric deposition. Chl a in the SCS was negatively correlated with SST (r=−0.60, p<0.05) and was positively with AOT (r=0.20, p<0.05). The correlation between AOT and chl a in the SCS does not reflect a major contribution from atmospheric deposition to chl a; instead, the relationship resulted from concurrence of the peaks of AOT and wind speed, which drive water mixing and nutrient supply. Consequently, chl a in the SCS would be regulated primarily by the nutrient supply from deep waters. Because SST was controlled by the ENSO teleconnection in the SCS, the chl a was coupled with ENSO events. The present study demonstrated that interannual phytoplankton variation could be controlled by different factors even in neighboring oligotrophic regions.

Concurrent Decadal Mesoscale Eddy Modulations in the Western North Pacific Subtropical Gyre

Abstract Satellite altimeter data of the past two decades are used to investigate the low-frequency mesoscale eddy variability inside the western North Pacific subtropical gyre. Eddy activity modulations with a decadal time scale are detected concurrently within the 18°–28°N band, including the three branches of the Subtropical Countercurrent (STCC) and the Hawaiian Lee Countercurrent (HLCC). Lagging behind the Pacific decadal oscillation (PDO) index by six months, enhanced eddy activities were detected in 1995–98 and 2003–06, whereas the eddy activities were below the average in 1999–2002 and 2009–11. Analysis of the temperature and salinity data that became available after 2001 via the International Argo Program reveals that the modulating eddy activities are due to the decadal change in the upper-ocean eastward shear in the broad-scale STCC–HLCC band. By conducting an upper-ocean temperature budget analysis, the authors found that this observed eastward shear change can be effectively accounted for by the decadal-varying surface heat flux forcing. Using the Argo-based temperature and salinity data, it is further found that the decadal subsurface potential vorticity (PV) signals to the north and beneath the STCC–HLCC were vertically coherent and not confined to the mode water isopycnals. Adjusting to the PDO-related surface forcing, these subsurface PV anomalies lagged behind the upper-ocean eastward shear signals and likely made minor contributions to generate the decadal-varying eddy signals observed in the western North Pacific subtropical gyre.

Decrease of dissolved oxygen after the mid‐1980s in the western North Pacific subtropical gyre along the 137°E repeat section

The Japan Meteorological Agency has acquired dissolved oxygen (DO) concentration data each year since 1967 along the 137°E repeat section in the western North Pacific. In this data set we found significant regional temporal trends of decreasing or increasing DO concentrations on various isopycnal surfaces. DO decreases were particularly significant after the mid‐1980s in the subtropical gyre; mean rates of DO change at 20–25°N for 1985–2010 were −0.28 ± 0.08 μmol kg−1 yr−1 on 25.5 σθ in North Pacific Subtropical Mode Water (NPSTMW), −0.36 ± 0.08 μmol kg−1 yr−1 on 26.8 σθ in North Pacific Intermediate Water (NPIW), and −0.23 ± 0.04 μmol kg−1 yr−1 on 27.3 σθ in the O2 minimum Layer (OML). The cause of DO decrease differed among isopycnal surfaces. On density surfaces shallower than 26.0 σθ (less than about 400 m), the deepening of isopycnal surfaces and decline of oxygen solubility due to ocean warming have had the greatest influence. In particular, between 25.2 σθ and 25.8 σθ near the NPSTMW their combined contributions accounted for &gt;50% of the DO decrease. In the NPIW core at roughly 26.8 σθ (∼700 m), the decline in DO was attributable to the DO decrease in the formation region. In the OML between 27.0 σθ and 27.3 σθ (∼1000 m), the DO decrease likely resulted from an increase in westward transport of low O2 water due to strengthening of the subtropical gyre. The result of this study shows the importance of the long‐term and high‐frequency along the 137°E repeat section.

Fertilization potential of volcanic dust in the low-nutrient low-chlorophyll western North Pacific subtropical gyre: Satellite evidence and laboratory study

volcanic particles and a phytoplankton bloom. FLH was found to be ∼9–17 × 10 −3 mW cm −2 mm −1 sr −1 in the patch and ∼3– 5×1 0 −3 mW cm −2 mm −1 sr −1 in the ambient water, indicating that a 2–5‐fold increase in biological activity occurred during the week following the eruption. Satellite altimetry indicated that the bloom took place in the presence of downwelling and was not a result of upwelled nutrients in this oligotrophic ocean. Analysis of satellite ocean color spectra of the bloom region found similar spectra as the reference Trichodesmium spectra. Laboratory experiments further substantiate the satellite observations which show elevated concentrations of limiting nutrients provided by the Anatahan samples, and the averaged soluble nitrate, phosphate, and Fe were 42, 3.1, and 2.0 nM, respectively. Though it was not possible to obtain in situ observations of the ocean biogeochemical responses that followed the Anatahan eruption, this study provided evidence based on remote sensing data and laboratory experiment that fertilization of volcanic aerosols occurred following this eruption in one of the most oligotrophic low‐nutrient low‐chlorophyll ocean deserts on Earth.

Concentration and UV reactivity of total organic carbon in the upper layer in the oligotrophic subtropical western North Pacific

Characteristics of temporal variation in the vertical profile of total organic carbon (TOC) were investigated in the upper layer of the ocean at various latitudes (10–30°N) of 137°E in the western North Pacific subtropical gyre. Bulk TOC was divided into labile and stable fractions by the specific UV irradiation to evaluate their contributions to the temporal fluctuation of TOC. Authentic specimens and dissolved organic carbon collected from coastal waters were used to examine the correspondence between the photochemical reactivity to the specific UV irradiation and biological reactivity. The concentration of TOC in the upper layer was higher in summer when the surface layer is well stratified and was lower in winter when the mixed layer deepened. Although the winter mixed layer is shallower in lower latitudes, the amplitude of the seasonal variations in the top 50 m weighted mean TOC concentration (19.5–27.7 μM) did not vary considerably over the subtropical zones we studied. A major fraction (51.9–89.8%) of TOC was stable against specific UV irradiation and was thought to be mainly composed of biologically stable organic compounds, but the seasonal variation of TOC at each latitude was mainly attributable to the variation of UV labile fraction. This finding suggests that the seasonal variation of TOC in the upper layer is explained by the biological production of TOC in summer surface water and its vertical mixing with low UV, labile TOC in winter. Upward advection of subsurface water due to the passage of cyclonic eddies and changes in the geostrophic current are also likely to affect the temporal TOC variation, in particular, in the lower latitudes where winter mixed layer and permanent thermocline are shallower.

Changes in water properties around North Pacific intermediate water between the 1980s, 1990s and 2000s

Abstract We detected temperature (salinity) decreases on the neutral density surfaces above the salinity minimum and increases below throughout the western North Pacific subtropical gyre ( 25 ∘ N – 40 ∘ N , west of 170 ∘ W ) by comparing World Ocean Circulation Experiment (WOCE) hydrographic sections from the 1980s, 1990s, and 2000s. The temperature (salinity) changes around the salinity minimum were associated with an apparent oxygen utilization (AOU) increase, and could not be explained by an increase of the subpolar input. These property changes were due to a density decrease of the salinity minimum. Furthermore, similar changes in temperature (salinity) were detected along 165 ∘ E as a 12-year trend from 1996 to 2007. This suggests that the changes evident in the comparison of the WOCE sections are the representative of changes at the intermediate depths in the last decade. This pattern was not detected in the eastern subtropical gyre ( 25 ∘ N – 40 ∘ N , east of 170 ∘ W ) and along 47 ∘ N , where the AOU increase due to ventilation reduction has been reported. Along 47 ∘ N , changes in temperature, salinity and AOU from 1999 to 2007 were opposite to those from 1985 to 1999.

Comparative study of picoplankton biomass and community structure in different provinces from subarctic to subtropical oceans

Picoplankton biomass and community structure in the subtropical and subarctic Pacific Oceans were investigated during November 2003, April-August 2005 and July-August 2005. The sampling covered the subarctic K2 station, the Western North Pacific subtropical gyre (WNPG1 and 2 stations) and the Eastern North Pacific subtropical area (ENP1, 2, 3 and 4 stations). Distinct differences in community structure and autotrophic and heterotrophic picoplankton biomass were observed among the above provinces. In subtropical areas, the picoplankton community comprised Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. While in the subarctic area (K2 station), Prochlorococcus were absent. Prochlorococcus were numerically dominant in the subtropical oceans, their abundance tended to decrease with increasing nutrient levels, which is the opposite of the other picoplankton populations. Although the aerobic anoxygenic phototrophic heterotrophic bacteria (AAPB), accounted for only a small proportion of total heterotrophic bacterial abundance, their potential contribution to carbon export may be important due to their larger cell size and higher cell turnover rates compared with other heterotrophic bacteria. Biomass contribution of the AAPB increased distinctly along the oligotrophic to relatively eutrophic gradient. Vertically, AAPB generally followed the phytoplankton except in the subtropical WNPG. Spatial variability of biomass in the autotrophic picoplankton was distinctly larger than that in the heterotrophic bacteria. Changes in the picoplankton community were more closely associated with latitude while nutrient availability was more important for differences in picoplankton biomass. The biomass of autotrophic picoplankton in the upper mixed layer, and also the depth attenuation, were higher in eutrophic relative to oligotrophic waters. Picoplankton seemed to be an important source of new organic carbon for higher trophic level organisms and for detritus production, especially in the oligotrophic subtropical gyre.