Global warming is profoundly influencing marine phytoplankton. However, a debate remains about structural and functional organization of phytoplankton communities in the future warming ocean, owing to the lack of research using real ocean warming scenarios. Here, we investigated diversity and community structure of phytoplankton along a temperature gradient in the thermal discharge zone of the Daya Bay Nuclear Power Plant located in the subtropical coastal area of China. Our results revealed that temperature was the primary regulator of phytoplankton diversity and community structure. Phytoplankton formed distinct communities at various temperature intervals within the range of 23–30℃. In the normal temperature region of 23–25℃ and the strongly heated region of 28–30℃, the phytoplankton community was characterized by relatively high diversity and complexity of co-occurrence networks. However, chlorophyll a (Chl a) concentration was lower in the strongly heated region. In contrast, the phytoplankton community in the moderately heated region of 25–28℃ displayed the lowest diversity and complexity of co-occurrence networks but the highest Chl a concentration. Integrative analysis of species richness, dominant groups, co-occurrence networks, and Chl a concentration of the investigation sea area as well as other multi-source phytoplankton data confirmed a temperature tipping point for subtropical phytoplankton community structure and function at approximately 25.6℃. This study demonstrates that the responses of phytoplankton to ocean warming are nonlinear and complicated, and that phytoplankton could assemble new functional communities under various warming scenarios.

phytoplankton community,diversity,ocean warming,tipping point,reorganization