Ontogenetic Carnivory Drives Secondary Production Across a Productivity Gradient

Ontogenetic size shifts and omnivory are central to understanding consumer-resource interactions and energy flow in aquatic ecosystems. However, how these shifts and feeding behavior influence secondary production and organic matter flow remains poorly understood. 2. Here, we quantified the relationship between body size, carnivory (i.e., animal tissue consumption), secondary production, and organic matter flow across a primary productivity gradient in Brazilian Atlantic Forest streams. Using gut content analyses combined with estimates of secondary production for an overabundant net-spinning caddisfly with filter-feeding behavior (Leptonema sp.), we evaluated how body size influences feeding behavior and ecosystem fluxes. 3. Carnivory consistently increased with body size across the primary productivity gradient, with higher average animal tissue consumption at low-productivity sites. Carnivory was positively associated with secondary production but showed no relationship with overall organic matter flow, suggesting a decoupling between consumer-level production and energy throughput. This decoupling may reflect constraints related to resource quality and assimilation efficiency. Additionally, potential prey flux measured with drift nets did not predict carnivory, suggesting that resource quality, as described by the productivity gradient may outweigh prey availability in driving trophic behavior. 4. Our results provide novel evidence that omnivory depicted by size-enhanced carnivory is an adaptive strategy contributing to population-level production, but not necessarily to increased energy throughput at the ecosystem level. Our findings highlight the importance of considering consumer traits, such as body size and trophic flexibility, in understanding energy flow and ecosystem functioning along productivity gradients in riverine networks.