We investigated basin-wide distributions of total particulate phosphorus (TPP) and associated biogeochemical parameters in the euphotic zone (EZ) of the subtropical North and South Pacific Ocean. TPP is primarily composed of living microorganisms, and its distribution is likely controlled by nitrogen (N) supply in typical N-limited Pacific waters as well as phosphorus (P) supply in P-limited western North Pacific. TPP concentrations showed a vertically uniform distribution (approximately 15 nM) within the EZ at most stations, but at several northern North Pacific stations, TPP peaks (>20 nM) were observed in the lower EZ where nitrate and nitrite concentrations were high. Relationships between TPP and biogeochemical parameters indicate that a possible factor controlling TPP in the upper EZ was dinitrogen fixation and that in the lower EZ was shoaling of the nitracline. Geographical distribution of TPP was relatively uniform compared to that of phosphate, which varied from <10 nM in the western North Pacific to >100 nM in the eastern North and South Pacific. The western North Pacific was a domain characterized by lower dissolved organic P (DOP) concentrations, higher alkaline phosphatase activities, and higher particulate polyphosphate (PpolyP):TPP ratios compared to other domains. These characteristics indicated that active DOP utilization and PpolyP accumulation could play important roles in the maintenance of TPP stocks as alternatives to phosphate utilization in this distinctive domain. Our results demonstrated that TPP distributions were vertically and geographically uniform across the subtropical Pacific Ocean, concealing that TPP stocks were likely underpinned by a range of nutrient supply mechanisms.

Aim Biological nitrogen fixation supports primary production in oligotrophic water, but its link to higher trophic levels has not been described fully on a biogeographical basis. Here, we determine the regional patterns of the contribution of the combined nitrogen to biological production within the epipelagic layer of the mid‐Pacific Ocean using the isotopic signatures of nitrogen (δ15N) and carbon (δ13C) in the biological components. Location The mid‐Pacific Ocean along 170° W between the southern subtropical front and the Chukchi Sea. Time period Northern and austral summer in 2013 and 2014. Major taxa studied Planktonic and micronektonic biota in the euphotic layer. Methods We measured the geographical variations in δ15N and δ13C of the suspended particulate organic matter (POM), mesozooplankton assemblage and micronektonic fish. We analysed the relationships among these values and the environmental variables of temperature, nitrate concentration and biological nitrogen fixation activity along a 12,000‐km meridional transect. Results The POM δ15N at 0 m was negatively correlated with in situ N2 fixation activity in the subtropical region, whereas that in the equatorial and high‐latitude regions was correlated with the nitrate concentration at 0 m. We found that the ratios of the increase in δ15N to δ13C along the grazing food chain were consistent throughout the equatorial and subtropical regions. Cluster analyses based on the stable isotopic signatures in the biotic components revealed that the food chains in the stations within the subtropical mid‐Pacific Ocean were separated into three groups based on the differential contributions of biological nitrogen fixation. Main conclusions Distinct food chains from primary to tertiary production sustained by different nitrogen sources, nitrate below the euphotic zone, and diazotrophic nitrogen occur within the same biogeographical provinces in the subtropical mid‐Pacific Ocean. The diazotroph‐dominant community contributes substantially to the apex predators in the central areas of the subtropical gyres.

Bioavailable nitrogen and carbon amount are increasing in the ocean. Their stable isotope ratios are good indicators for understanding what happens in the sea. We measured the nitrogen and carbon stable isotope ratio of organic particles which is mainly consisted of living phytoplankton but also consisted of zooplankton, bacteria, and so on in the Kuroshio, flowing in the southern part of Japan. The Kuroshio is one of the key areas not only for fisheries in East Asia countries, but also for Pan-Pacific countries, but we do not know the supply source of nitrogen and carbon which is essential for living resources. Our main finding is that nitrogen sources are different between areas and seasons. That is, deep-sea water is an important source of nitrogen between Japan and the Kuroshio throughout the seasons, but nitrogen fixation by plankton and rain is an important source in the offshore area during summer. KODAMA ET AL.

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