Identifying the impact of primary production pathways and mixotrophs on energy flow in a tributary backwater of the world’s largest reservoir

Food-web models often simplify producer and microbial pathways while omitting complex feeding strategies, hindering understanding of basal energy flow in ecosystems where such characteristics are important. We developed an Ecopath food-web model for Gaoyang Lake — a tributary backwater of China’s Three Gorges Reservoir (TGR) — incorporating multiple producer and microbial groups, including mixotrophy. The model includes multiple phytoplankton groups (edible taxa, cyanobacteria, and mixotrophic dinoflagellate Ceratium hirundinella), bacterial groups, a detrital pool (allochthonous and autochthonous inputs), and key consumers (zooplankton, zoobenthos and fish). We identified three primary energy pathways: detrital (efficient transfer), grazing, and microbial (channeling energy from phytoplankton and detritus to consumers and linking inedible mixotrophic dinoflagellates to higher trophic levels). The detrital pathway most strongly supported upper trophic levels, while the grazing pathway was limited by low zooplankton abundance. Ecosystem indices indicated early developmental stability, a simplified food chain, and invasive species dominance, consistent with a highly disturbed ecosystem. Simplifying the food web by aggregating lower trophic levels altered ecosystem indices and energy transfer, underscoring the value of including complex producer and microbial pathways for realistic representation of basal energy flow. Modifying the food web to represent bloom conditions increased cycling but reduced lower trophic level transfer efficiency, showing how severe blooms intensify bottlenecks and shift energy among grazing, detrital, and microbial pathways. Including C. hirundinella provided insights, demonstrating that mixotrophy, when coupled with low edibility, may adversely affect the entire food web. Overall, our novel model structure offers a framework to holistically capture complex energy pathways and cycling.