Nitrogen fixation rates and controls at Stn ALOHA

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 N-15-labeled N-2 under simulated in Situ conditions for 24 h prior to filtration of either whole water or <10 mu m filtrate on microfine (0.7 mu m 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 mu m 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 mu mol N m(-3) d(-1)) and lowest N-2 fixation rates occurred in September to November (0.38 to 0.68 mu mol N m(-3) d(-1)). Vertically integrated rates of whole community N-2 fixation (measured in November, February and March) varied 5-fold (20.2 to 109 mu mol N m(-2) d(-1)). 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.