Trophic responses of macrophyte-associated ichthyofauna to seasonality in a Neotropical river

Abstract

In tropical rivers, aquatic macrophytes constitute complex habitats that mediate predator–prey interactions and support diverse fish assemblages. In the present study, we examined how seasonality influences prey use and the trophic network structure of macrophyte-associated ichthyofauna in the lower São Francisco River, Brazil. Fishes were sampled bimonthly in the wet and dry periods at four independent sites. Stomach contents of adult individuals were analyzed to quantify prey richness, composition, and origin (autochthonous vs. allochthonous). Bipartite networks were constructed to investigate nestedness and modularity. Empirical patterns were compared to null models using Z-scores. No differences were found in prey richness between hydrological periods. Prey composition showed a significant but weak shift between hydrological periods (R² = 0.02), reflecting the dominance of autochthonous resources (algae, detritus, microcrustaceans, and immature insects) supported by aquatic macrophytes. The predator–prey network was predominantly nested in the wet period, indicating hierarchical interactions mediated by generalist species, whereas no clear pattern of nestedness or modularity was found in the dry period compared to null expectations. These results highlight the central role of macrophytes in ensuring prey availability and stabilizing trophic networks, even under hydrological fluctuations. Despite this apparent stability, low allochthonous input and increasing human impact make these networks functionally vulnerable to change. Therefore, protecting riparian vegetation and mitigating anthropogenic stressors are critical for maintaining trophic complexity and resilience in tropical river ecosystems.