Phytoplankton dominance alters basal energy pathways and food web structure in eutrophic shallow lakes.

Eutrophication is widely recognized for increasing primary production, yet how changes in basal energy allocation reorganize food web architecture and stability remains insufficiently resolved. Across three eutrophic shallow lakes spanning a productivity gradient, we combined stable isotope analysis with a Bayesian mixing model (EcoDiet) to quantify the relative contributions of five basal resources to consumers. We then linked phytoplankton dominance to energy-channel diversity, trophic redundancy, and interaction-weighted modularity. Results showed that increasing phytoplankton dominance progressively concentrated basal carbon allocation into a single pelagic pathway. This shift reduced energy-channel diversity and trophic redundancy, suggesting erosion of functional buffering capacity. Conversely, phytoplankton dominance simultaneously strengthened network modularity. These results revealed a structural-functional decoupling that shifted food webs toward more pelagic-based trophic pathways and structurally modular configurations. Thus, nutrient enrichment influences ecosystem stability in multidimensional ways, altering not only productivity but also the distribution and pathways of energy through trophic networks.