Nitrogen fixation rates and controls at Stn ALOHA

M. N.W. Grabowski, M. J. Church, D. M. Karl

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Rates of dinitrogen (N2) 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 15N-labeled N2 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 N2 fixation rates were also obtained. Nearly all (on average 95 %) of the net N2 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-1) and lowest N2 fixation rates occurred in September to November (0.38 to 0.68 μmol N m-3 d-3). Vertically integrated rates of whole community N2 fixation (measured in November, February and March) varied 5-fold (20.2 to 109 μmol 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 N2 fixation at Stn ALOHA may be controlled by the population dynamics of the various diazotroph species rather than by instantaneous resource limitation.

Original languageEnglish
Pages (from-to)175-183
Number of pages9
JournalAquatic Microbial Ecology
Issue number2
StatePublished - Aug 2008


  • Diazotroph
  • Nitrogen fixation
  • North pacific gyre
  • Stn ALOHA


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