ASLO/TOS/AGU Ocean Sciences Meeting, Honolulu, HI, February 2006.

Iron and phosphorus as controls on nitrogen fixation at Station ALOHA

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

Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Temporal and depth variability in rates of dinitrogen (N₂) fixation at Stn. ALOHA, an oligotrophic site in the North Pacific Subtropical Gyre (NPSG), were assessed with direct measurements using ¹⁵N₂ tracer technique during the period April 2004 - March 2005. Measured N₂ fixation rates and the implied large fraction of new production supported by diazotrophy emphasize the importance of N₂ fixation in the NPSG. Nearly all (95%) N₂ fixation in the euphotic zone (0- 125 m) occurred in the upper 75 m and was mostly supported (57 - 76%) by diazotrophs less than 10 µm in diameter. Over the year long period of observation, N₂ fixation varied with measured soluble reactive phosphorus inventories and previously documented seasonality of iron (Fe) delivery to the North Pacific. High surface diazotroph activity was observed in July - August 2004 (1.63 - 1.68 µmol N m^-3 d^-1) and low N₂ fixation rates occurred September - November 2004 (0.38 - 0.68 µmol N m^-3 d^-1). The dependence of diazotrophy on available Fe and phosphorus (P) was further clarified in manipulation experiments which resulted in enhanced N₂ fixation rates upon addition of Fe and/or P. Occasional Fe and/or P limitation on diazotroph activity may result in a net decrease in N₂ fixation at Stn. ALOHA, thereby potentially restricting carbon export from the surface ocean to the deep sea.