[1] The distribution of N 2 fixation was examined using a 15 N 2 tracer with accompanying measurements of abundance of Trichodesmium spp. and Richelia intracellularis, nitrate plus nitrite (N+N) and soluble reactive phosphorus at the nanomolar level, and primary production in the western and central Pacific Ocean. N 2 fixation occurred only in >∼20°C oligotrophic (i.e., N+N < 100 nM) waters except at a station in the equatorial upwelling zone where N+N was 1880 nM. High N 2 fixation rates were observed in the Kuroshio and East China Sea (KECS) and near Fiji and other isolated islands with concomitant high abundance of Trichodesmium spp. In contrast, N 2 fixation in the western and central oligotrophic North Pacific (WCONP) was significantly lower, and Trichodesmium spp. were rarely observed. These observations hint that KECS and waters around isolated islands are N 2 fixation "hot spots" because of the occurrence of Trichodesmium spp. The average N 2 fixation rate in the KECS of 232 ± 54.8 (±SE, n = 13) mmol N m , which is less than half of previous estimates. This difference was ascribed primarily to the unavailability of N 2 fixation rates in the WCONP, which occupies a vast area of the subtropical North Pacific, and the use of data obtained in the hot spots which represent small areas that likely led to the previous overestimation.

Ongoing radionuclide monitoring and tracking efforts are required following the nuclear accident at the Fukushima Daiichi Nuclear Power Plant.

In the subtropical oceans, nutrient concentrations are frequently below the detection limits of standard analytical methods. We applied a highly sensitive method to the surface water of the western and central Pacific between 42°N and 40°S and between 141°E and 158°W except in the equatorial zone, and detected overall depletion of nitrate + nitrite and an excess of SRP. However, a remarkable exception was found: an almost complete exhaustion of SRP (<10 nM) existed at a horizontal scale of >2000 km in the western subtropical North Pacific in both summer and winter. The SRP exhaustion was a consequence of an elevated dinitrogen fixation, which occurred in areas with high dust deposition from the Asian continent that likely enhanced SRP consumption. A coupling among nutrient dynamics, dinitrogen fixation and dust deposition produces the extremely low P availability spanning a large area, which appears to be unique to the western North Pacific.

Latitudinal distribution of diazotrophs and their nitrogen (N 2 ) fixation activity were investigated in the western North Pacific in winter (Nov to Dec 2004) and summer (May to Jun 2005) along meridional transects from 37uN to the equator. N 2 fixation activity in whole seawater and seawater passed through a 10-mm filter was assayed by acetylene reduction. The whole-water N 2 fixation was markedly elevated in winter throughout the study area compared to that in summer, probably due to the increased upward supply of phosphate as a result of deeper mixed layer in winter. During both periods a distinct latitudinal variation was observed in N 2 fixation of the whole-water samples at the surface; further, higher activity was observed between the Kuroshio Extension and the salinity front in the North Equatorial Current than in the neighboring areas. The elevated N 2 fixation was primarily ascribed to ,10-mm diazotrophs during both seasons. Flow cytometry conducted in summer revealed that distribution of nanoplanktonic cyanobacteria was closely correlated with that of N 2 fixation activity in the ,10-mm fraction, indicating that nanoplanktonic cyanobacteria were the major diazotrophs in that area. In contrast, microplanktonic diazotrophs, Trichodesmium spp. and Richelia intracellularis exhibited different latitudinal distributions from that of nanoplanktonic cyanobacteria, with maximum numerical abundance of R. intracellularis around 8uN and 30uN, and that of Trichodesmium spp. at 26.5uN. Few microplanktonic diazotrophs occurred in the winter. The distribution of the diazotrophs and their N 2 fixation activity may be controlled by the supply of phosphate and aeolian dust deposition.

Surface ocean phosphate is commonly below the standard analytical detection limits, leading to an incomplete picture of the global variation and biogeochemical role of phosphate. A global compilation of phosphate measured using high-sensitivity methods revealed several previously unrecognized low-phosphate areas and clear regional differences. Both observational climatologies and Earth system models (ESMs) systematically overestimated surface phosphate. Furthermore, ESMs misrepresented the relationships between phosphate, phytoplankton biomass, and primary productivity. Atmospheric iron input and nitrogen fixation are known important controls on surface phosphate, but model simulations showed that differences in the iron-to-macronutrient ratio in the vertical nutrient supply and surface lateral transport are additional drivers of phosphate concentrations. Our study demonstrates the importance of accurately quantifying nutrients for understanding the regulation of ocean ecosystems and biogeochemistry now and under future climate conditions.