Rates of dinitrogen (N 2 ) fixation were measured at Stn ALOHA in the North Pacific Subtropical Gyre (NPSG) on 9 cruises during the period April 2004 to March 2005. On each cruise, a near-surface (5 m) seawater sample was incubated with 15 N-labeled N 2 under simulated in situ conditions for 24 h prior to filtration of either whole water or <10 μm filtrate on microfine (0.7 μm nominal porosity) glass fiber filters; on 3 cruises, surface to 125 m depth profiles of size-fractionated N 2 fixation rates were also obtained. Nearly all (on average 95%) of the net N 2 fixation in the euphotic zone occurred in the upper 75 m, and was mostly (64 ± 5 [SE]%) contained in the <10 μm size fraction following a 24 h incubation period. Highest surface diazotroph activity at this site was observed in July to August (1.63 to 1.68 μmol N m -3 d ). The short-term response of the microbial community to the addition of iron (Fe) and/or phosphorus (P) was variable, suggesting that contemporaneous N 2 fixation at Stn ALOHA may be controlled by the population dynamics of the various diazotroph species rather than by instantaneous resource limitation.

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