Nitrogen fixation under the interaction of Kuroshio and upwelling in the northeastern South China Sea

Diazotrophic organisms are vulnerable to physical dynamic processes. However, the interactions and mechanisms of multiple dynamic processes on nitrogen fixation remain unclear. This study aimed to investigate one of these interactions; we conducted field investigations and bioassay experiments in the northeastern South China Sea, which faces the Kuroshio current and coastal upwelling. Using 15N tracer and molecular biology methods, we found that the eastern Guangdong upwelling, having high phosphorus and a lower N:P ratio, had higher nitrogen fixation rates as well as a higher abundance of diazotrophic cyanobacteria Trichodesmium spp. and UCYN-A compared with nonupwelling area. Additionally, a higher abundance of diazotrophic cyanobacteria occurred near the bottom of the euphotic zone along the shelf break that was affected by the Kuroshio current. Field incubation experiments confirmed that phosphorus increased the nitrogen fixation rates and stimulated the growth of cyanobacteria in the area affected by upwelling. Taken together, our results indicated that warmer surface water with a high N:P ratio likely mixed with upwelled phosphorus-rich water containing diazotrophic cyanobacteria, which were transported by the intensified Kuroshio current under the influence of the 2015/16 El Nin similar to o. This mixing then triggered a high abundance of diazotrophic cyanobacteria. These findings provide new insights into how the interaction of multiple dynamic processes can regulate nitrogen fixation.

Automated summary

This study investigated the interaction of multiple dynamic processes on nitrogen fixation and found that the upwelling and Kuroshio current play important roles in nitrogen fixation. The high phosphorus content and low N:P ratio in the upwelling area led to higher nitrogen fixation rates and abundance of diazotrophic cyanobacteria. Additionally, the shelf break affected by the Kuroshio current had higher abundance of diazotrophic cyanobacteria. Adddition of phosphorus increased nitrogen fixation rates and stimulated cyanobacteria growth.