Using modern coexistence theory to understand the distinct states of phytoplankton communities in a subtropical eutrophic river network

Plain river networks are increasingly threatened by eutrophication, bringing out various types of algal blooms. However, limited information is available on the coexistence mechanisms of interactive phytoplankton communities and how they vary under distinct environmental conditions, hampering the deep understanding of the ecological mechanism of algal blooms. By coupling the modern coexistence theory and the alternative stable states theory, this study for the first time analyzed coexistence patterns of phytoplankton communities under two distinct states in a subtropical river network. The observed bimodal distribution based on phytoplankton biomass was aligned with the water trophic level index (TLI), segregating the community into the high-trophic state (A) and the low-trophic state (B). In state A, the phytoplankton community exhibited high abundance but low diversity and evenness. Conversely, state B displayed the opposite pattern. In terms of coexistence patterns, the coexistence probability of selected high-abundance genera in state A was 60.7 %, attributed to reduced fitness differences (72.92); whereas in state B, it was 47.2 %, attributed to higher niche differences (0.98). Thus, the fitness difference was recognized as the primary catalyst for the discrepancies between the two states. Variations in fitness differences of the core genus Cyclotella significantly impacted the dynamics of the phytoplankton community. Flow velocity and nutrient availability, including TN, NO₃^--N, and NH₄⁺-N, were identified as significant factors influencing both biomass and coexistence parameters. The structural equation model indicated that environmental conditions exerted a more direct influence on the community in state B, while their impacts on state A were strongly mediated through interactions among genera. This study sheds light on the role of internal interactions in shaping the phytoplankton community, offering novel insights for aquatic ecological restoration through the lens of species coexistence.