New estimation of N₂ fixation in the western and central Pacific Ocean and its marginal seas

The distribution of N₂ fixation was examined using a ¹⁵N₂ 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. N₂ 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 N₂ 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, N₂ 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 N₂ fixation "hot spots" because of the occurrence of Trichodesmium spp. The average N₂ fixation rate in the KECS of 232 ± 54.8 (±SE, n = 13) µmol N m^-2 d^-1 was almost 1 order of magnitude higher than that in theWCONP of 39.2 ± 7.51 (n = 26) µmol N m^-2 d^-1. On the basis of these estimates and reported values obtained using ¹⁵N₂, depth-integrated N₂ fixation in the North Pacific was estimated to be 2.6 ± 0.3 × 109 (n = 63) mol N d^-1, which is less than half of previous estimates. This difference was ascribed primarily to the unavailability of N₂ 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.