Seasonal variations of unicellular diazotroph groups A and B, and Trichodesmium in the northern South China Sea and neighboring upstream Kuroshio Current

• Seawater temperatures are correlated with abundances of the three diazotrophs.
• Trichodesmium increased and group B decreased where internal waves occurred.
• Trichodesmium and group B abundances were positively related with nitracline depth.

Seasonal changes in abundances of Trichodesmium spp. and unicellular cyanobacterial phylotype groups A and B in the northern South China Sea (SCS) and upstream Kuroshio Current were investigated using qPCR techniques targeting the nifH gene that encodes nitrogenase for N2 fixation. The aim was to understand the community structures and factors associated with the abundances of the diazotrophs (nitrogen-fixers) in this important marginal sea. The results from 3 cruises covering 9 sampling stations revealed a close correlation between seawater temperature and gene abundances of the three nifH phylotypes. Trichodesmium spp. and group B were more abundant in the Kuroshio than the SCS, whereas group A was equally distributed. The abundances of Trichodesmium spp. and group B were positively correlated with nitracline depths. As nitracline is consistently deeper in the Kuroshio than in the SCS, nitracline depth could be used to track the abundance dynamics of the two phylotypes. In the station with prevailing internal waves, we observed unusually high abundances of Trichodesmium spp. but markedly low abundances of group B. Distributions of Trichodesmium spp. and group B were not only influenced by temperature and nitracline depth, but also occurrence of internal waves.