Latitudinal distribution of diazotrophs and their nitrogen fixation in the tropical and subtropical 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 37 degrees N to the equator. N-2 fixation activity in whole seawater and seawater passed through a 10-mu m 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-mu m 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-mu m 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 8 degrees N and 30 degrees N, and that of Trichodesmium spp. at 26.5 degrees N. 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.