Seasonal dynamics of new production from Trichodesmium N2 fixation and nitrate uptake in the upstream Kuroshio and South China Sea basin

The seasonal dynamics of primary production in the upstream Kuroshio was studied to understand why pelagic fish are relatively populous in this oligotrophic tropical ocean region. The relative importance of new production from nitrate uptake vs. Trichodesmium N-2 fixation was compared to differentiate the seasonal production dynamics in the upstream Kuroshio from that in the neighboring and interacting northern South China Sea (SCS). Unlike the SCS, which was most productive in the winter, seasonal fluctuation of primary production (0.51-0.53 g C m(-2) d(-1)) was not apparent in the upstream Kuroshio. The seasonality in new production in the Kuroshio, in contrast, was pronounced. Its nitrate-uptake-based new production was high in the winter (0.27 g C m(-2) d(-1)) and low in the summer (0.16 g C m(-2) d(-1)); conversely, Trichodesmium N-2-fixation-based new production was low in the winter (2.4 mu mol N m(-2) d(-1)) and high in the summer (168.1 mu mol N m(-2) d(-1)). In the summer when nitrate-uptake-based new production was low due to nitrate depletion, the Kuroshio remained productive because of enhanced Trichodesmium N-2 fixation from strong stratification and a deepened nitracline. The standing crop of Trichodesmium was consistently much higher in the Kuroshio (34.61 x 10(6) and 0.49 x 10(6) trichomes m(-2) in the summer and winter, respectively) than in the SCS (4.87 x 10(6) and 0.15 x 10(6) trichomes m(-2), respectively). In both regions, surface water temperature correlated significantly with increased Trichodesmium stocks. At a given temperature, higher Trichodesmium standing crop in the Kuroshio than the SCS was related to the deeper nitracline and ensuing decrease in nitrate availability in the Kuroshio.