Latitudinal distribution of diazotrophs and their nitrogen fixation in the tropical and subtropical western North Pacific

Abstract

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.

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

“…Due to the sparse information available in the literature, the notable lower salinity measured at station 8 between 100 m and 130 m depths remains difficult to link to a particular seawater mass. However, several studies reported a slightly lower salinity in the upper layer around 21 • N ( Kitajima et al, 2009;Xie et al, 2009). In the present study, the lower salinity recorded in the subsurface layer can be explained in part by the vicinity of the subtropical front (STF).…”

“…However, only a few reports, mainly conducted under the warm phase of the ENSO (El Niño), have addressed the distribution of phytoplankton in the western part of the NPSG (Zhang et al, 2008;Kitajima et al, 2009). Nutrients in this area were reported to be depleted in the surface layer ).…”

“…Suzuki et al (1997) conducted experiments in August-September 1992. Horimoto et al (2001 collected samples during 3 cruises (October 1994, June 1995and March 1998 and Kitajima et al (2009) 545 nm), orange (564-606 nm) and red (653-669 nm) wavelength ranges. Data were collected using the CellQuest software (BD Biosciences ® ).…”

“…The flow cytometer is equipped with a blue (488 nm) air-cooled argon laser and a red (634 nm) diode laser. For each particle (cell), five optical parameters were recorded: two light scatter signals, namely forward and right angle light scatter, and three fluorescences corresponding to emissions in green (515- et al (1993), Suzuki et al (1997), Horimoto et al (2001) and Kitajima et al (2009), respectively. Shimada et al (1993 sampled seawater during two cruises (from August to September and from November to December).…”

Determination of total N <sub>2</sub> fixation rates in the ocean taking into account both the particulate and filtrate fractions

“…Using the C 2 H 2 reduction method, Kitajima et al (2009) found high N 2 fixation rates of 0.5-12 nmol N L −1 d −1 in the western North Pacific region; this range is more than twice as high as that estimated for the particulate fraction in this study (0.4-3.2 nmol N L −1 d −1 during the KH06-2 expedition) based on the 15 N 2 tracer method. It is difficult to attribute the differences in the estimations to the seasonal variations because both experiments were performed during the same early summer season (May to June).…”