Biogeography of N2 fixation influenced by the western boundary current intrusion in the South China Sea

The N 2 fixation and primary production rates were measured simultaneously using 15 N 2 and 13 C incubation assays in the northern South China Sea influenced by the Kuroshio intrusion (KI) seasonally. The degree of KI (KI index, range from 0 to 1) was assessed by applying an isopycnal mixing model. The water column integrated N 2 fixation and primary production for stations with KI index larger than 0.5 were 463 ± 260 μmol N·m −2 ·day −1 and 62 ± 19 mmol C·m −2 ·day −1 , respectively, significantly higher than those for stations with KI index lower than 0.5 (50 ± 10 μmol N·m −2 ·day −1 and 28 ± 10 mmol C·m −2 ·day −1 , respectively). Trichodesmium was the dominant diazotroph at stations with KI index larger than 0.5, with 2 orders of magnitude higher nifH gene abundance than that at stations with KI index lower than 0.5. However, the highest N 2 fixation rates were found in waters with moderate KI index around 0.6, suggesting that frontal zone mixing might stimulate N 2 fixation. Our results demonstrated that diazotrophs (mainly Trichodesmium) were tightly associated with the KI, which modulated the biogeographic distribution of N 2 fixers. In summary, we found the transportation of Trichodesmium by KI, then, we quantified the fraction of KI and N 2 fixation rates in the northern South China Sea. The results suggested that KI generated a new biogeographic regime which could significantly influence the carbon and nitrogen cycles far away from the main stream.