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

The distribution of N-2 fixation was examined using a N-15(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 >similar to 20 degrees 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 N2 fixation rate in the KECS of 232 +/- 54.8 (+/- SE, n=13) mu mol N m(-2) d(-1) was almost 1 order of magnitude higher than that in the WCONP of 39.2 +/- 7.51 +/- (n = 26) mu mol N m(-2) d(-1). On the basis of these estimates and reported values obtained using N-15(2), depth-integrated N-2 fixation in the North Pacific was estimated to be 2.6 +/- 0.3 x 109 (n = 63) mol N d(-1), 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.