Environmental heterogeneity restructures trophic organization across agricultural streams: evidence from a freshwater decapod

Agricultural streams are exposed to multiple interacting environmental drivers that may reorganize food-web structure, yet trophic responses of dominant benthic consumers remain poorly understood. We evaluated spatial variation in the trophic ecology of Aegla uruguayana populations inhabiting five Pampean streams previously characterized by contrasting environmental conditions. Using stable isotopes of carbon and nitrogen (δ¹³C and δ¹⁵N), isotopic niche metrics, and Bayesian mixing models, we assessed differences in isotopic composition, niche breadth, and estimated resource contributions among populations. Populations occupied distinct isotopic spaces among streams, indicating marked spatial variation in carbon and nitrogen pathways and trophic organization. δ¹⁵N showed partial correspondence with environmental quality, with populations from Very good and Good sites exhibiting higher or similar values than those from sites of lower environmental quality. Isotopic niche breadth showed a different pattern, with the broadest niche occurring under intermediate environmental conditions. Mixing models revealed pronounced among-site differences in estimated resource contributions, indicating substantial spatial variation in trophic pathways. These contributions did not consistently correspond to the relative dry biomass of potential macroinvertebrate resources, revealing that trophic organization was not simply proportional to resource representation within streams. Overall, trophic organization did not follow a single environmental-quality gradient. Instead, the combined isotopic patterns suggest that spatial trophic differentiation may arise from variation in basal isotopic pathways and population-level trophic plasticity.These findings demonstrate the value of combining multiple stable-isotope metrics to characterize trophic variation across environmentally contrasting agricultural streams.