River Flow Regulation and Land Use Changes Shape Food Web Structure and Trophic Diversity of Tropical Fish Communities in Southern Mexico

ABSTRACT Tropical rivers support a substantial share of global biodiversity but are increasingly affected by anthropogenic disturbances such as flow regulation and deforestation. We investigated how such threats shape trophic resource use and food web structure in fish communities across four rivers in the Grijalva‐Usumacinta Basin, southern Mexico, spanning a gradient of land use and hydrological alteration. Using carbon, nitrogen, and hydrogen stable isotopes, we assessed the relative contributions of autochthonous and allochthonous resources to fish diets. Isotopic mixing models revealed that forested, unregulated rivers sustained more diverse and seasonally dynamic food webs, with fish relying on both in‐stream and terrestrial inputs. In contrast, disturbed systems, particularly the regulated river, showed reduced trophic diversity, narrower niche breadth, and a predominant reliance on in‐stream production. These shifts were linked to diminished resource diversity, loss of trophic specialization, and simplified food web structures. Our findings underscore how land use changes and flow regulation compromise ecosystem resilience, and highlight the value of multi‐isotope approaches for assessing functional changes in riverine ecosystems. Conservation efforts should prioritize maintaining natural flow regimes and riparian vegetation to sustain trophic complexity and ecosystem integrity in tropical rivers.