Trophic States Regulate Assembly Processes and Network Structures of Picophytoplankton Communities in Subtropical Coastal Ecosystems

Abstract Background: Long-term coastal eutrophication especially in semi-enclosed marine areas is driven by increased amounts of nutrients derived from anthropogenic activities. Given that accelerating nutrients may constitute a strong environmental filter, understanding the diversity, assembly process and co-occurrence pattern of picophytoplankton communities in response to increasing coastal eutrophication is clearly of great importance. Results: We investigated picophytoplankton community changes using rbcL gene amplicon sequencing. The results exhibited that the alpha diversity (ANOVA, p < 0.001) and beta diversity (ANOSIM, p < 0.001) were significantly different among eutrophic states. Further, phylogenetic based β-nearest taxon distance analyses revealed that stochastic processes mainly provided 69.26% contribution to picophytoplankton community assembly, whereas deterministic processes dominated community assembly in a heavy eutrophic state. Integrated co-occurrence networks modularly responded to eutrophic states and revealed that keystone taxa mainly belonged to the oligo eutrophic group, which may play fundamental roles in network persistence. Importantly, increased environmental disturbances, such as nitrogen and phosphorus nutrients, could alter picophytoplankton community structure and disrupt ecological processes. Conclusion: Stochastic and deterministic processes simultaneously influenced the assembly of picophytoplankton communities in the subtropical coastal ecosystems. Eutrophic disturbances alert the assembly processes and network structures of picophytoplankton community. Our findings promote the understanding of fundamental ecological processes along eutrophic gradients in subtropical coastal ecosystems.