Seasonal variation in basal resources supporting fish biomass in longitudinal zones of the Usumacinta River Basin, southern Mexico

Aquatic food webs in tropical rivers are affected by spatial and temporal variations in basal resources and the composition of consumer assemblages. We used stable isotope analysis to estimate seasonal variation in basal resources supporting fish biomass in zones along the longitudinal gradient of the Usumacinta River Basin, the largest of Mesoamerica. A Bayesian isotope mixing model was used to estimate the proportional contributions of six basal resources: seston, filamentous algae, periphyton, aquatic macrophytes, riparian C3 plants and C4 plants. Models estimated that fish in the upper zone were largely supported by periphyton and riparian C3 plants during the dry season and by filamentous algae during the wet season. In the dry season, fishes in the middle and lower zones assimilated material derived from seston and, to a lesser extent, periphyton, whereas aquatic macrophytes were more important during the wet season. Chlorophyll-a measurements from the middle and lower zones suggested that seston contained large fractions of suspended algae. These patterns are consistent with the river wave concept, which predicts flow-associated changes in contributions of basal resources to the aquatic food web. The findings of this study reinforce the major role of flow regime in the ecological dynamics of rivers and how these dynamics vary depending on location and local conditions within fluvial networks. eng