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Environmental changes caused by hydrological shifts between wet and dry seasons in the tropics drive variation in resource availability and feeding interactions for riverine fish assemblages, but studies of trophic structure typically take place over short time scales that do not adequately capture this variation. In this study, we used analysis of stable isotopes (δ¹³C and δ15N) to assess seasonal changes in the diversity and relative importance of production sources supporting fish assemblages in La Venta River, the largest tributary of Netzahualcoyotl Reservoir on the Grijalva River in Chiapas, Mexico. Based on established river ecosystem theories, we expected the relative importance of carbon sources supporting fishes to shift seasonally, with greater use of instream resources during low-water periods and an increasing contribution from terrestrial sources during high-water periods. Abundant, low-trophic-level food resources (i.e., plants and invertebrates) were expected to support most fish species in the wet season, whereas piscivory was predicted to increase as habitat volume declined and fish density increased in the dry season. Somewhat surprisingly, we found that terrestrial energy sources were assimilated more during low-water conditions at some sites, and ranges of δ¹³C were higher in the dry season. Terrestrial carbon supporting fish production in low-water periods likely came from riparian plant detritus that remained in the river after flows receded and/or leaf litter that accumulated in the dry season. Ranges of δ15N were similar across seasons, with no apparent increase in piscivory during the dry season. Higher trophic redundancy was observed in the wet season, likely due to fish taking advantage of the same readily available food items (e.g., pulses of arthropod prey) during high flows. Documenting seasonal use of trophic resources provides critical information as expanding hydroelectric dam construction threatens the natural variability of energy pathways that support fisheries resources in tropical rivers. Inglés