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

Size fractionated rates and patterns of nitrogen and carbon fixation in the North Pacific Subtropical Gyre

J. P. Montoya¹, A. Hansen², D. M. Karl², J. P. Zehr³

¹School of Biology, Georgia Institute of Technology 311 Ferst Drive, Atlanta, GA 30332

²Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

³Department of Ocean Sciences, University of California at Santa Cruz 110 High Street, Santa Cruz, CA 95064

We measured the rates of nitrogen and carbon fixation in seawater collected at Station ALOHA and in coastal waters just outside Kaneohe Bay, Oahu, Hawaii. The rate of nitrogen fixation by particles passing a 10 µm filter varied between 0.014 and 0.095 nmol N L^-1 h^-1 with little or no diel variation in natural samples. We found similar rates in most samples amended with phosphate, though two of our five experiments showed signs of enhanced nitrogen fixation in the presence of added P. The rate of carbon fixation by small cells ranged from 3.5 to 16.2 nmol L^-1 h^-1 and showed no effect of added P in any of our experiments. The ratio of biomass specific fixation rates indicated that diazotrophs comprised between roughly 0.2 and 2% of the biomass of photosynthetically active plankton in our incubations. In combination with molecular assays of diazotroph community structure, our results indicate that diazotroph abundance is a major determinant of nitrogen fixation rate, and that different diazotroph groups have very different cell-specific rates of nitrogen fixation. Our data imply a rate of nitrogen fixation by small diazotrophs comparable to geochemically derived estimates of the rate of nitrogen fixation at Station ALOHA. On short time scales (weeks to months), N₂ fixed by small diazotrophs is more likely to be retained in the upper water column, with very different implications for food web structure and vertical export of organic matter than nitrogen fixed by larger, often episodically occurring diazotrophs like Trichodesmium and chain/mat forming diatoms containing endosymbiotic cyanobacteria.