Unicellular diazotrophy as a source of nitrogen to Leeuwin Current coastal eddies

N2-fixation (15N2 uptake) and C-fixation (H213CO3 uptake) rates of the natural phytoplankton assemblage were measured in a pair of counter-rotating mesoscale eddies in the Leeuwin Current, off the coast of Western Australia in October 2003. Day and night incubations of samples from both the surface and the deep chlorophyll maximum (DCM) were performed at nine stations across the warm core (WC) eddy and ten stations across the cold-core (CC) eddy with focal points in the center, the body, and periphery of each eddy. The mean daytime N2-fixation rate was 33±16 ρmole N L−1 h−1 (mean±SD, N=18) in the WC eddy and 21±12 ρmole N L−1 h−1 (mean±SD, N=20) in the CC eddy. We found significant time of day and depth effects, with higher N2-fixation during the day and at the surface in the WC and the CC eddy, respectively. The depth integrated mean a real N2-fixation rate in the WC eddy was 42.9±14.1 μmol N m−2 d−1(mean±SD, N=9), which was significantly greater than that of the CC eddy, 17.1±8.1 μmol N m−2 d−1 (mean±SD, N=10). Vertical profiles show measurable but highly variable volumetric rates of N2-fixation in the upper 100 m of the water column ranging from 0.13 to 9.6 ρmole N L−1 h−1, with a mean of 4.3±1.6 ρmole N L−1 h−1 (mean±SD, N=10), without any obvious vertical structure. Size-fractionated experiments generally showed no difference in N2-fixation rates in whole-water samples as compared to <100-μm and <10-μm size-fractioned samples, suggesting that the diazotrophic assemblage was <10 μm. We found no significant differences among size fractions in volumetric C-fixation rates. Simultaneous N2- and C-fixation rate measurements suggest that N2-fixation could support only about 2% of total production in the WC eddy and only 0.6% in the CC eddy. Though measurable, N2-fixation in these eddies does not appear to be the primary source of nitrogenous nutrients to the phytoplankton.