The relative contributions of unicellular and filamentous diazotrophs to N2 fixation in the South China Sea and the upstream Kuroshio

We studied the seasonal, diel, and vertical distribution of phytoplankton N-2 fixation to understand the relative contributions of unicellular and filamentous nitrogen fixers (diazotrophs) to N-2 fixation and nitrogen recycling in the northern South China Sea (SCS) and the neighboring upstream Kuroshio. N-2-fixation rates were measured by the N-15(2) tracer technique (addition by bubble) on unicellular ( < 10 or 20 mu m) and the filamentous diazotrophs ( > 10 or 20 mu m, mostly Trichodesmium and Richelia) fractionated by 10- or 20-mu m mesh sizes. The mean depth-integrated total (unicellular + filamentous) N-2-fixation rates in the SCS (51.7 +/- 6.2 mu mol N m(-2) d(-1)) averaged 1/3 of that in the Kuroshio (142.7 +/- 29.6 mu mol N m(-2) d(-1)), with higher rates in the winter than in other seasons in the SCS and the opposite seasonal pattern in the Kuroshio. Unicellular diazotrophs contributed 65% of the total N-2 fixation in the SCS, which were negatively correlated with surface temperature and, as for total N-2 fixation, were higher in the winter when Trichodesmium spp. were scarce. In comparison, the unicellular diazotrophs contributed 50% of total N-2 fixation in the Kuroshio, and their contributions were not significantly correlated with surface temperature. In both the SCS and the Kuroshio, the unicellular N-2 fixation was more important during the night than during the day, and in the deep euphotic layer than in the surface layer, even in the daytime. Our results show that the unicellular diazotrophs were important N-2 fixers and contributed significantly to N-2 fixation in the tropical marginal seas, more so in the SCS than the Kuroshio. (C) 2013 Elsevier Ltd. All rights reserved.