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

Abstract

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 the WCONP 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.