Mixed Water Region Research Articles

Steady nutrient upwelling around a biological hotspot of the confluence between the quasi-stationary jet and the Oyashio in the western North Pacific

The quasi-stationary jet, a branch of the Kuroshio Extension, transports warm saline water in the mixed water region of the western North Pacific. Around the subarctic front between the quasi-stationary jet and Oyashio and its downstream area is a biologically productive area including small pelagic fishes. However, how nutrient is supplied to the euphotic zone in this region remains elusive, especially into the quasi-stationary jet. Using high-resolution hydrography sections across the jet, we showed that Oyashio water isopycnally intrudes under the jet around 26.5–26.8 σθ and forms nutrient-rich intermediate water. Upwelling associated with ageostrophic secondary circulation across the front, caused by confluence, uplifts the intermediate water. A local nitrate maximum was also identified inside the jet by the hydrographic observation. Upwelling has been suggested as a precondition for nutrient supply from nutrient-rich intermediate water to the jet through water mixing which potentially sustains high biological production in the downstream.

Image fusion based approach of water extraction from spectrally mixed water regions belonging to the sources of varying nature

Image fusion based approach of water extraction from spectrally mixed water regions belonging to the sources of varying nature

Dissolved rare earth elements in the Northwest Pacific: Sources, water mass tracing, and cross‐shelf fluxes

In the Northwest Pacific, a key area for understanding the sources and transport of materials in the ocean, knowledge of the sources, transport, and biogeochemical cycling of trace elements is limited. Trace elements such as the rare earth elements (REEs) can trace the sources and transport of water masses. Here we present dissolved REE concentrations along a longitudinal transect (150 oE) from 13°N to 40°N in the Northwest Pacific (≤2000 m). We divided the transect into two subregions: a mixed water region (MWR; 37~40 °N, where the Oyashio and Kuroshio currents mix) and a subtropical region (13~34 °N). In the MWR, REEs were strongly positively correlated with apparent oxygen utilization in subsurface water (depth > the chlorophyll maximum layer, potential density <26.6 kg/m3), with about a 4-fold higher slope (0.15±0.06) than in the subtropical region in subsurface and intermediate waters (0.04±0.003, potential density <27.5 kg/m3). This suggests that REEs are released by organic matter remineralization at a higher efficiency in the MWR vs. in the subtropical region, which can be explained by different water masses and plankton community structures. In addition, we observed a lithogenic input signal of REEs from the Aleutian Islands based on the high La/Yb ratio (>0.35). This ratio was controlled by lateral transport and showed a good agreement with salinity, indicating that it is a useful tracer of low salinity water originating from the subarctic region. Furthermore, we estimated the cross-shelf fluxes of Nd in the Northwest Pacific. The estimated Nd fluxes from the Sea of Okhotsk, the Sea of Japan, the East China Sea, and the South China Sea into the Northwest Pacific were 29~32 t/y, 159~302 t/y, 142~616 t/y, and -298~34 t/y, respectively. This study highlights the importance of considering the cross-shelf REE fluxes in the Northwest Pacific when constructing the oceanic REE budgets.

Estimating abundance of Risso's dolphins using a hierarchical Bayesian habitat model: A framework for monitoring stocks of animals inhabiting a dynamic ocean environment

Cetaceans often inhabit a dynamic ocean environment; therefore, a standard approach to estimating abundance faces the difficult task of discriminating between actual population trends and habitat shifts. In addition, because of the wide distribution of many cetaceans, it is often difficult to survey their entire habitat at one time, which makes it difficult to compare abundance estimates between years and to detect a trend. In this study, we used a hierarchical Bayesian habitat model to estimate the abundance and habitat distribution of Risso's dolphins in the western North Pacific. Based on information criteria, a model including depth, temperature, and yearly trends performed better than a model including only depth and temperature. The medians (with 95% credible intervals) of total abundance estimates in June were 54,479 dolphins (25,579–102,086) in 2006, 54,737 dolphins (26,925–103,932) in 2007, and 146,179 dolphins (75,352–264,115) in 2014. Abundance was also estimated in August, but the seasonal difference between June and August was minor. Our models estimated that high densities of Risso's dolphins occurred in the mixed-water region off northern Japan and the cold-water mass off central Japan. These are productive waters because of their complex hydrographic features. Our results showed a probable increase in abundance and not simply a shift in habitat conditions or seasonal migration patterns. This study provides a framework for monitoring widely dispersed species in a dynamic environment, which can improve management and conservation in terms of more reliable trend estimation.

Dietary overlap and optimal prey environments of larval and juvenile sardine and anchovy in the mixed water region of the western North Pacific

Dietary overlap and optimal prey environments of larval and juvenile sardine and anchovy in the mixed water region of the western North Pacific

Spatio-temporal changes in catch rates of pelagic sharks caught by Japanese research and training vessels in the western and central North Pacific

Blue shark and shortfin mako are key pelagic shark species distributed throughout the temperate and tropical waters of the world’s oceans. These species are caught in the North Pacific by Japanese longline fisheries as bycatch and occasionally targeting blue shark. Japanese research and training vessels (JRTVs) have been collecting fishery-independent data on these sharks in the western and central North Pacific since 1992. However, the catch per unit of effort (CPUE) estimated from the fishery-independent data had not been used for stock assessments due to an issue concerning reporting after the 2000s. In this study, a statistical filtering method was used to remove unreliable set-by-set data collected by JRTVs. The nominal CPUE of the JRTVs was then standardized using the spatio-temporal generalized linear mixed model to provide the spatial and temporal changes in the CPUEs of these sharks in the North Pacific. The hotspots of blue shark appeared in temperate waters of the western and central North Pacific, whereas those of shortfin mako were distributed in coastal and offshore waters along the Kuroshio-Oyashio transition zone and the Mixed Water Region. The yearly changes in the CPUE of blue shark illustrated a downward trend until 2008 and an upward trend thereafter, whereas those of shortfin mako exhibited a flat trend with large fluctuations from 1994 to 2008, and a down-and-up trend thereafter. The information predicted from the fishery-independent data is very useful for improving the stock assessment and management of these sharks as it provides an understanding of spatio-temporal changes in the hotspots and temporal changes in the catch rates.

The Oyashio Nutrient Stream and Its Nutrient Transport to the Mixed Water Region

AbstractThe Oyashio, a subarctic western boundary current in the North Pacific, transports cool, fresh, and nutrient‐rich water equatorward. The Oyashio nutrient stream was assessed using long‐term mean hydrographic observation data. Its nitrate transport (about 350 kmol/s) is comparable to that of the Kuroshio but has a much shallower subsurface core at 250‐m depth. In the layer ranging from 26.6 to 27.4 σθ, the Oyashio nitrate transport to the mixed water region is about 110 kmol/s. This corresponds to the nitrate transport of the Oyashio to the North Pacific Intermediate Water in the mixed water region. Along with this process, the subarctic gyre nitrate export to the subtropic gyre reaches 24 kmol/s. This work first quantifies the Oyashio nutrient transport and its intrusion into the mixed water region and suggests its importance in local/basin‐scale nutrient cycles.

Assimilation of high-resolution sea surface temperature data into an operational nowcast/forecast system around Japan using a multi-scale three-dimensional variational scheme

A multi-scale three-dimensional variational (MS-3DVAR) scheme is developed to assimilate high-resolution Himawari-8 sea surface temperature (SST) data for the first time into an operational ocean nowcast/forecast system targeting the North Western Pacific, JCOPE2. MS-3DVAR improves representation of the Kuroshio path south of Japan, its associated sea level variations, and temperature/salinity profiles south of Japan, the Kuroshio/Oyashio mixed water region, and the Japan Sea as compared to those of the products by the traditional single-scale 3DVAR. Validation results demonstrate that MS-3DVAR well assimilates the sparsely distributed in situ temperature and salinity profiles data by spreading the information over the large scale and by representing the detailed information near the measurement points. MS-3DVAR succeeds to assimilate the Himawari-8 SST product without noisy features caused by the cloud effects. We also find that MS-3DVAR is more effective for estimating oceanic conditions in regions with smaller mesoscale variability including the mixed water region and Japan Sea than in south of Japan.

Estimating surface water mixing ratios using salinity and potential alkalinity in the Kuroshio‐Oyashio mixed water regions

AbstractWe used salinity and potential alkalinity data from hydrographic observations to investigate surface mixing ratios in the Kuroshio‐Oyashio mixed water region in the western North Pacific. In addition to mixing between the Kuroshio Extension (KE) and Oyashio, we assessed freshwater input/removal. A mixing scenario with three end‐members was assumed in the surface layer ≤100 m. The results indicate that water masses near the sea surface in the Kuroshio‐Oyashio mixed water region were mainly the result of mixing between the KE and Oyashio. The freshwater contribution was approximately 2.2% at depth 10 m. The volume of freshwater estimated from this percentage was consistent with surface water budgets estimated from reanalysis precipitation and evaporation data. The estimated mixing ratio of the KE (rk) along the quasi‐stationary jet in the western North Pacific, which splits from the KE and flows northeastward toward the subarctic region, decreased downstream from 95% to 27% in only 42 days, suggesting that water properties were changed rapidly by mixing. Correlation between rk around the quasi‐stationary jet and nutrients concentration was significantly negative in the layer where photosynthesis was negligible, indicating that the mixing between the KE and Oyashio is an important determinant of the horizontal distribution of nutrients in this area.

Float observations of an anticyclonic eddy off Hokkaido

AbstractTo clarify the formation process of the salinity minimum in the Kuroshio–Oyashio mixed water region and understand the mechanism of meridional heat transport between the subtropical and subpolar gyres, 16 profiling floats were deployed within a warm‐core anticyclonic eddy off Hokkaido from June 2012 to December 2013. Then, the evolution of an anticyclonic eddy was examined using time series of the water properties. The largest fluctuations in water properties were observed in April and May 2013, when the anticyclonic eddy first moved south to interact with a warm front, then back north. Salinity in the salinity minimum layer increased during the interaction. After the eddy detached from the frontal structure, low‐salinity water was again observed with small intrusive structures, which eventually converged to a smooth zigzag structure in the potential temperature‐salinity diagram, suggesting that both vertical mixing and vertical heaving played a role in the temporal changes observed after the eddy detached from the front. Since the salinity variation during the interaction event was about half the total salinity change during the whole experimental period, the interaction of an eddy with a front might be important for modifying the water properties of the eddy, and, therefore, for the meridional transport of heat and fresh water.

Intensification of North Pacific intermediate water ventilation during the Younger Dryas

Modern North Pacific intermediate water (NPIW) is formed in the mixed water region where the Oyashio and Kuroshio currents meet. The source for cooling and freshening of NPIW is intermediate water in the Okhotsk Sea. The Okhotsk intermediate water outflows to the open Pacific, forming the Oyashio intermediate water by mixing with the subarctic gyre water. In the Oyashio region, the intermediate water originating from the Okhotsk Sea flows mainly at depths shallower than 500 m. On the other hand, ventilation of intermediate water in the subarctic Pacific during the deglaciation remains a topic of debate. In this study, foraminiferal δ18O and δ13C signatures were determined in a dated sediment core collected at 777 m water depth to evaluate the intensity and depth distribution of the source of NPIW since the last deglaciation in the Oyashio region. Benthic foraminiferal δ18O increased by 0.3–0.4‰ from the end of the Bolling/Allerod warm episode to the Younger Dryas cold episode, suggesting intermediate water cooling. Consistent with this trend, benthic δ13C values point to decreased nutrient contents during the cold event. Conversely, benthic δ18O signatures from a nearby core site at a water depth of 1,366 m did not show such cooling. These results suggest that cold intermediate water originating from the north was actively ventilated at depths of at least 700–800 m, and possibly even 1,200 m during the Younger Dryas, implying that NPIW ventilation was thicker and deeper than under modern conditions.

Seasonal and regional change in vertical distribution and diel vertical migration of four euphausiid species (Euphausia pacifica, Thysanoessa inspinata, T. longipes, and Tessarabrachion oculatum) in the northwestern Pacific

We studied seasonal and regional change in vertical distribution and DVM patterns of four euphausiid species (Euphausia pacifica, Thysanoessa inspinata, Thysanoessa longipes, and Tessarabrachion oculatum) from two years of surveys using MOCNESS above 1500m depth across a transect in 3 regions of the northwestern (NW) Pacific, off east of Japan; Oyashio, Kuroshio, and Oyashio–Kuroshio Mixed Water Regions (MWR). The four euphausiid species exhibited a regional change in vertical distribution, i.e., slightly deeper in the MWR and much deeper in the Kuroshio region than in the Oyashio region. They found in higher and wider temperature ranges in the MWR than in the Oyashio region, which demonstrated that the four species were able to adapt to different temperatures in different regions. In the MWR and Oyashio regions, E. pacifica is a surface migrant (differences between day and night mean median depths, D-N, were ca. 300m) and T. oculatum is a moderate subsurface migrant that performs short DVM in the upper mesopelagic zone (D-N ca. 100m). The other two morphologically similar Thysanoessa species (T. inspinata and T. longipes) segregated vertically between E. pacifica and T. oculatum at night in the Oyashio region, suggesting vertical habitat partitioning with the former two species but not with themselves. However, a seasonal pattern was observed in the vertical distribution and DVM of T. longipes in the Oyashio region. It behaves as a surface migrant in May, whereas most of individuals were found in the mesopelagic layer in September. In contrast, T. inspinata did not exhibit a clear DVM throughout the year (i.e., a moderate subsurface migrant). This seasonal difference might be a strategy to minimize competition between related species. Among the four species, only E. pacifica was found in higher temperatures at night than during the daytime, and the highest temperatures at the median depth varied among species (from 7.5°C to 13.7°C) although the lowest temperature did not vary greatly (from 1.0°C to 1.8°C), which indicates high temperatures act as a limiting factor as opposed to low temperatures. Furthermore, the integrated chlorophyll a values exhibited significant negative correlation with median depths of only E. pacifica at night. These results indicate a strategy which makes E. pacifica the dominant species in the area, that is, it has a trade-off of long migrations and a warmer environment that accelerates metabolism, in return for obtaining a food-rich environment.

Effect of seasonal change in gas transfer coefficient on air–sea CO2 flux in the western North Pacific

We used an eddy-permitting three-dimensional ocean ecosystem model and applied it in the western North Pacific to understand the seasonal variations and horizontal distributions of the air–sea CO2 flux and difference in the partial pressure between sea water and the atmosphere (∆pCO2). The high-resolution model reproduced the observed zonal belt of strong CO2 uptake in the mid-latitude (30–45°N) western North Pacific including the Kuroshio extension and mixed water regions, which was difficult to show in previous coarse-resolution models. The East Asian winter monsoon, an important phenomenon in the western North Pacific, affects the seasonal CO2 air–sea gas exchange with a high (low) gas transfer coefficient in winter (summer). In the subtropical region, ∆pCO2 is negative in winter and positive in summer as a result of the temperature effect. Combination of seasonal change in gas transfer coefficient with ∆pCO2 suppresses CO2 release in the subtropical region, and vice versa in the subarctic region (i.e., suppresses CO2 uptake). That is, the East Asian winter monsoon in the western North Pacific contributes to the reduction of the annual CO2 flux through the seasonal change in the gas transfer coefficient, leading to an overall annual CO2 uptake in the subtropical region and CO2 release in the subarctic region.

In situ observations of a doliolid bloom in a warm water filament using a video plankton recorder: Bloom development, fate, and effect on biogeochemical cycles and planktonic food webs

AbstractWe investigated distribution patterns of a doliolid (Dolioletta gegenbauri) bloom in relation to the physical environment using a video plankton recorder in the Oyashio–Kuroshio mixed water region. Using 12 km transects, doliolid blooms were encountered at a horizontal scale of about 2–3 km, which corresponds to submesoscale physical events. Doliolids were also consistently encountered in the subsurface layer above the pycnocline in warmer (> 14ºC) and higher‐salinity (> 34) water masses, and seawater density was the most critical factor affecting distribution depth. Compared to previous studies, the density and biomass of the blooms observed in this study (77 mgC m−3and 4600 inds m−3) were highest in the open ocean. Bloom formation consisted of two phases; first, the seeding population of a nurse stage increased rapidly to 2000 inds m−3by asexual reproduction, followed by asexual production of phorozooids. Estimated population clearance rates revealed that these dense patches could potentially sweep the surrounding water within 2–3 d. The incidence of exhausted and shrunken zooids was significantly correlated with patch density, suggesting that mortality was due to overgrazing. Shrunken doliolids appeared to sink below the pycnocline, corresponding to 8–17% of the particulate organic carbon flux at 150 m. Hydromedusae, pelagic polycheates, and sapphirinid copepods preyed on the doliolids. These results indicate that doliolids, which were seeded by populations originating from the Kuroshio, formed dense blooms in response to submesoscale physical events and would alter the sinking particle properties (i.e., biological pump) and the epipelagic food web structure through their grazing and mortality.

Multidecadal-Scale Freshening at the Salinity Minimum in the Western Part of North Pacific: Importance of Wind-Driven Cross-Gyre Transport of Subarctic Water to the Subtropical Gyre

AbstractUsing oceanographic observations and an eddy-resolving ice–ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (~26.8 σθ) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6–26.8 σθ in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre.

Estimating the nitrate concentration from the dissolved oxygen concentration and seawater temperature in the Kuroshio extension, Oyashio, and mixed water regions

For the purpose of estimating nitrate concentration using the dissolved oxygen concentration and seawater temperature recorded in an instrument such as a profiling float with an oxygen sensor, we examined the relationship between oxygen and nitrate concentrations in the Kuroshio Extension, Oyashio, and mixed water regions (34–43oN, 140–150oE) using vertical profile data, which includes temperature, salinity, oxygen, and nitrate, from the World Ocean Database 2005. Examining the NO3/O2 ratio along isotherms, we found a clear linear regression between the oxygen and nitrate concentrations in each potential temperature range, irrespective of water mass and seasonal variation. The slopes represented by the NO3/O2 ratio in each temperature range varied from −0.07 to −0.12. The benefit of analyzing data on constant potential temperature surfaces arises because of the strong dependence of the oxygen saturation of surface waters on temperature. This sets the end point of subducted waters and helps distinguish waters that may have similar preformed nitrate values. Our results suggest that subsequent reminearlization in the subducted water retained the Redfield ratio between N and O2. The estimate of nitrate concentration, by using the oxygen–nitrate relationship discovered in the present study, showed good accuracy with in general a mean error of less than 2.0 μmol/kg. The present method has been demonstrated to be a useful tool for estimating the nitrate concentration from temperature and oxygen.

Shipboard determination of radiocesium in seawater after the Fukushima accident: results from the 2011–2012 Russian expeditions to the Sea of Japan and western North Pacific Ocean

A total of 88 seawater samples were collected during two Russian research expeditions (April–May 2011 and August–September 2012) to the Sea of Japan, the Oyashio Current region near Kuril Islands and the Kuroshio–Oyashio transition area in the western North Pacific Ocean. The observations were made aboard the R/V Pavel Gordienko and Akademik Shokalsky in order to study the impact of the Fukushima accident on radioactive contamination of the marine environment. On the board of a ship, the water samples were passed through filters to retain particles with the size of >1 micron. Cesium was extracted from the large volumes (100–3000 L) of the filtrated water using a selective fiber chemisorbent impregnated with copper ferrocyanide. Measurements of 134Cs and 137Cs activities in 83 samples of sorbents and 21 samples of filters were performed in the ship-based laboratory with a semiconductor HP-Ge detector. The quantified activity concentrations of dissolved radiocesium ranged from 1 Bq m−3 to 34 Bq m−3 for 137Cs and from 0.2 Bq m−3 to 29 Bq m−3 for 134Cs. Activity concentrations of 137Cs and 134Cs were strongly correlated with each other (r = 0.993, n = 59). The 137Cs/134Cs activities ratio in the Fukushima-derived radiocesium inventory for the study areas was deduced to be 0.99 ± 0.03 (on 15 March 2011) and the pre-Fukushima background level of 137Cs in seawater was estimated as 1.3 ± 0.3 Bq m−3. The lowest activities of both isotopes were determined in the western part of the Sea of Japan near the Russian coast, while the maximal levels were observed in the open Pacific Ocean, some 500–800 km offshore the Fukushima Dai-ichi Nuclear Power Plant. Contamination with 134Cs at a level of 0.3–2.6 Bq m−3 was registered in seawater samples collected in 2011 near the Kuril Islands and Kamchatka in the Oyashio Current region. During the period from April–May 2011 to August–September 2012, activity concentrations of 137Cs and 134Cs in surface waters had decreased for all seven stations repeatedly sampled in the study. A detailed observation of radiocesium distribution within the water column down to the depth of 200 m at nine stations from the Kuroshio–Oyashio Interfrontal Zone and Kuroshio Extension in 2012 revealed maximal activity concentrations of both cesium radionuclides in the 100–200 m depth layer. The average inventory of Fukushima-derived 137Cs in the top 200 m of the water column for the nine stations was estimated as 1.19 kBq m−2 (decay corrected to 15 March 2011) which is 4.6 times higher than the background value of 0.26 kBq m−2 expected for this depth. The monitoring results obtained in the study and relevant data published by others show that following the Fukushima accident, the Oyashio current acts as a provider of low-contaminated subarctic waters to the heavily contaminated Kuroshio–Oyashio mixed water region.

Tracer transport in cold-core rings pinched off from the Kuroshio Extension in an eddy-resolving ocean general circulation model

[1] Using an eddy-resolving ocean general circulation model, we examine the cold-core rings pinched off from the Kuroshio Extension (KE), focusing on their tracer transport. The origin of the tracers in the rings and how they are trapped are directly investigated by releasing two passive tracers, corresponding to the KE water and the water in the Mixed-Water Region (MWR) to the north of the KE. The cold-core rings carry the tracer in the MWR wrapped by the KE water from the surface to the bottom at the pinch-off. In and above the upper thermocline ( σθ < 26.4), these tracers are trapped in the ring and move along the ring until the rings rejoin the Kuroshio. Even in the lower thermocline ( σθ ∼ 26.4–27.2), a substantial part of the tracers moves along the ring, while gradual leakage occurs. Vorticity balance in the rings is consistent with the passive tracer experiment, indicating that water in the upper thermocline moves along the rings, while the synchronous movement in and below the lower thermocline is severely contaminated by baroclinic Rossby waves. These direct estimations are compared with the indirect diagnoses using PV, the ratio of drift velocity of ring to its tangential velocity (nonlinear parameter), and the largest closed contour of stream function in the comoving frame of the ring. These indirect diagnoses are consistent with the direct estimation in the upper thermocline, but are not good indicators below the lower thermocline. The cause of their insufficient performance is discussed.

Pathways of the North Pacific Intermediate Water identified through the tangent linear and adjoint models of an ocean general circulation model

This study develops a strategy for tracing a target water mass, and applies it to analyzing the pathway of the North Pacific Intermediate Water (NPIW) from the subarctic gyre to the northwestern part of the subtropical gyre south of Japan in a simulation of an ocean general circulation model. This strategy estimates the pathway of the water mass that travels from an origin to a destination area during a specific period using a conservation property concerning tangent linear and adjoint models. In our analysis, a large fraction of the low salinity origin water mass of NPIW initially comes from the Okhotsk or Bering Sea, flows through the southeastern side of the Kuril Islands, and is advected to the Mixed Water Region (MWR) by the Oyashio current. It then enters the Kuroshio Extension (KE) at the first KE ridge, and is advected eastward by the KE current. However, it deviates southward from the KE axis around 158°E over the Shatsky Rise, or around 170°E on the western side of the Emperor Seamount Chain, and enters the subtropical gyre. It is finally transported westward by the recirculation flow. This pathway corresponds well to the shortcut route of NPIW from MWR to the region south of Japan inferred from analysis of the long‐term freshening trend of NPIW observation. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Congruence between euphausiid community and water region in the northwestern Pacific: particularly in the Oyashio–Kuroshio Mixed Water Region

The relationship between euphausiid community structure and water region was studied during a 2-year seasonal survey in the northwestern (NW) Pacific Ocean. The euphausiid community structure and its associated species were analyzed from 38 micronekton samples collected during eight cruises. The euphausiid community structure and its distribution patterns clearly corresponded to physical oceanographic features in the Oyashio region, Oyashio–Kuroshio Mixed Water Region (OKMWR), and Kuroshio region. In contrast, community structure was unrelated to seasonality. The 19 species out of 40 identified in this area were grouped and named after their habitats. The six cold-water species were grouped into three regional types: two coastal Oyashio species, three Oyashio–OKMWR species, and one Oyashio–Kuroshio species. The four species dominating in the OKMWR were categorized into each specific types: Nematoscelis difficilis as OKMWR–Oyashio species, Euphausia gibboides as OKMWR species, Euphausia similis as OKMWR–Oyashio & OKMWR–Kuroshio species, and Euphausia recurva as OKMWR–Kuroshio species. The seven warm-water species were categorized as Kuroshio–OKMWR species or Kuroshio species. The other two species were categorized as cosmopolitan species. In particular, regarding the result in the OKMWR, our study suggest that (1) the OKMWR has high species diversity, and (2) the dominant species, such as Euphausia pacifica, N. difficilis, E. similis, and E. gibboides, are considered to be key species in the food webs in this region.