Eutrophication-Driven Changes in Plankton Trophic Interactions: Insights from Trade-Offs in Functional Traits

Understanding how plankton trophic interactions, particularly phytoplankton nutrient uptake and zooplankton grazing, respond to eutrophication is important for maintaining aquatic ecosystem functions and developing effective mitigation strategies. Phytoplankton exhibit trade-offs in functional traits between growth rate and antipredation defense, thereby regulating these trophic interactions. However, the combined effects of eutrophication and such trait-based regulation on plankton communities and interactions remain poorly understood. In the present study, we investigated these effects by integrating trait-based mechanistic modeling and field observations in China's eutrophic Pearl River Estuary. Our model predicted that the species with the weakest defensive capacities dominated under nutrient-poor conditions. As eutrophication increased, a concave growth-defense trade-off favored species with high growth rates and strong defense capacities, whereas a convex trade-off curve favored species that were either the least or the most well-defended. High grazing pressure accelerated these shifts. In the estuary, similar patterns emerged in the relative abundance of different phytoplankton species along a gradient of the nitrogen to phosphorus ratio (N:P), indicating changes from high nutrient uptake and low grazing under oligotrophic conditions to eutrophic conditions, in which some phytoplankton face considerable grazing pressure despite high nutrient uptake, whereas others grow slowly with less grazing pressure. These results enhance our understanding of trait-based plankton interactions in eutrophic bodies of water and provide support for more effective conservation and management strategies.