El Niño, characterized by the interaction between tropical atmosphere and ocean, is traditionally associated with the abnormal warming of sea surface temperatures in the equatorial Pacific. Nitrogen fixation contributes approximately half of the nitrogen demand of primary production in some oligotrophic oceans, suggesting that it plays a crucial role in influencing the efficiency of the biological carbon pump. Although El Niño exerts profound impacts on marine ecosystems, its effects on marine nitrogen fixation remain insufficiently understood. This study analyzes Trichodesmium abundance in the coastal waters of the South China Sea over the past two decades, revealing peak abundances during two strong El Niño events in the summers of 2004 and 2015–2016. Additionally, through integrated field observations and in situ enrichment experiments conducted in the northeastern South China Sea from 2014 to 2016, we found that the 2015/2016 El Niño intensified the intrusion of Kuroshio waters into the South China Sea, transporting substantial quantities of nitrogen-fixing cyanobacteria (Trichodesmium and unicellular nitrogen-fixing cyanobacteria) to the shelf region. The intensified Kuroshio intrusion, together with upwelling, eddy, and the Pearl River plume, jointly enhanced nitrogen fixation in coastal waters. Our results highlight the significant regulatory role of climate change on biological nitrogen fixation processes.

Nitrogen fixation, El Niño