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

Abstract

[1] The distribution of N 2 fixation was examined using a 15 N 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 >∼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 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 N 2 fixation rate in the KECS of 232 ± 54.8 (±SE, n = 13) mmol N m , 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.

Seasonal shifts in the contributions of the Changjiang River and the Kuroshio Current to nitrate dynamics in the continental shelf of the northern East China Sea based on a nitrate dual isotopic composition approach

“…4.2), as is N 2 -fixation-derived nitrogen; however, δ 15 N NO 3 lower than that of the deep layer was not observed in shallow layers throughout the northern ECS, including the Okinawa Trough, during our survey periods. This is probably because there was no rainfall around the transects before and during our surveys, and N 2 fixation occurs only occasionally in time and space in response to sporadic supply of the limiting nutrients (Fe and/or P) necessary for N 2 fixation to occur (Shiozaki et al, 2010). Moreover, probable contribution of atmospheric nitrogen to NO 3 dynamics in the oligotrophic surface and subsurface layer could be masked by a methodological limitation (i.e., δ 15 N NO 3 was not measured for water samples with [NO 3 ] less than 1.5 µM).…”

“…Shiozaki et al (2010) also estimated that the mean bulk water N 2 fixation by pico-, nano-, and microplanktonic diazotrophs was 232 ± 54.8 µmol N m −2 day −1 in Kuroshio and ECS areas based on a 15 N 2 -tracer method. Relatively higher N * values in the water column on the continental shelf rather than that in deep water at the Okinawa Trough also corresponded with the contribution of N 2 fixation (Tables 1-3), although N * may be also enhanced due to river-derived nutrients having higher N / P ratios (Wang et al, 2006).…”

Effect of light on N&lt;sub&gt;2&lt;/sub&gt; fixation and net nitrogen release of &lt;i&gt;Trichodesmium&lt;/i&gt; in a field study

“…The known uncertainty of the conversion factor for acetylene to N 2 for the ARA method (Montoya et al, 1996;Shiozaki et al, 2010) may bias the DDN release estimate, while potential underestimation of net NF from the 15 N 2 bubble method may result in a higher DDN net release. In this study the direct measurement of the DD 15 N in the dissolved pool using the improved dissolution 15 N 2 -enriched seawater method (Mohr et al, 2010) was applied to assess the DDN net release; thus, our data were quite reliable.…”

Phosphate release due to excess alkaline phosphatase activity in <i>Trichodesmium erythraeum</i>

“…The organism was isolated in 2003 from the East China Sea (Kitajima 2009), where high nitrogen fixation rates and Trichodesmium blooms are frequently observed (Shiozaki et al 2010). Bacteria co-existed with the Trichodesmium even at low density in this culture as detected under fluorescence microscopy, and they were presumed to be superficial to the Trichodesmium trichomes.…”