Resource Quality and Quantity Exert Distinct Influences on Trophic Interactions and Food Web Pyramids Under Different Nutrient Conditions

ABSTRACT Resource quality and quantity are critical drivers in shaping trophic interactions and food web structures in aquatic ecosystems. However, the lack of clarity on how these drivers distinctly influence energy flow and trophic pyramids represents a significant gap in understanding the mechanisms governing ecosystem stability and productivity. This study explored how resource quality and quantity influenced trophic interactions and food web pyramids across a spatial gradient of aqueous nutrient levels. Resource quality was assessed by omega‐3 (ω3) long‐chain polyunsaturated fatty acids (LC‐PUFA), while resource quantity was evaluated based on biomass. Food web components were collected, including algae (phytoplankton and periphyton) and their consumers (zooplankton, macroinvertebrates and fish). Our results showed that higher ambient nutrient concentrations significantly boosted phytoplankton biomass, leading to a bottom‐heavy biomass pyramid in the low‐quality food group. However, this increase in quantity was accompanied by a notable reduction in ω3 LC‐PUFA in primary producers, resulting in a distinct FA stock pyramid with a narrowed base and middle. This pattern suggests that despite high phytoplankton biomass, poor food quality created a resource quality bottleneck that constrained the transfer of essential fatty acids. This reduction in resource quality simplified the transfer pathways of ω3 LC‐PUFA to piscivorous fish, limiting their dietary options and weakening trophic connections. Notably, the unusual role of planktivorous fish in accumulating FA at the second trophic level in the study area, in contrast to typical trends observed in other regions, highlights how variations in species composition and resource quality can reshape trophic structure and influence energy flow. Our findings emphasise that declines in resource quality exerted a greater influence on food web dynamics than declines in resource quantity. Our study underscores the importance of considering resource quality, alongside quantity, to maintain ecosystem stability and resilience in nutrient‐enriched aquatic systems.