Reorganization of Subtropical Phytoplankton Communities in the Warming Ocean

Abstract Global warming profoundly influences marine phytoplankton, yet the reorganization of phytoplankton communities in the future warming ocean remains uncertain. Here, we investigated phytoplankton diversity and community structure along a temperature gradient in the thermal discharge zone of a nuclear power plant. 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°C. In the typical temperature region of 23°–25°C and the strongly heated region of 28°–30°C, the phytoplankton communities were characterized by relatively high diversity and complexity of co‐occurrence networks. However, chlorophyll a (Chl a ) concentration was lower in the intensely heated region. In contrast, the phytoplankton community in the moderately heated region of 25°–28°C displayed the lowest diversity and complexity of co‐occurrence networks but had the highest Chl a concentration. Integrative analysis of species richness, dominant groups, co‐occurrence networks, and on‐site and global ocean Chl a concentrations confirmed a thermal tipping point at approximately 25.6°C for subtropical phytoplankton communities. This study demonstrates that phytoplankton responses to ocean warming are nonlinear and complex and that phytoplankton could assemble new functional communities under various warming scenarios.