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Abstract The hydrological regime leads to fluctuations in the availability of food resources for fish, which may reflect in their diet variation. Our main goal was to analyze the relationship between water-level fluctuations and the diet and the body condition of Trachelyopterus galeatus in the Upper Paraná River floodplain. The fish used in the study were sampled quarterly along nine years, at nine sampling stations. Our results showed that diet of T. galeatus varied in response to oscillations in the hydrometric level, with the consumption of terrestrial invertebrates being positively related to hydrometric level, while the consumption of aquatic invertebrates was negatively related. The trophic niche breadth and feeding activity were not affected by hydrometric level, but the fish body condition was positively affected. The high food plasticity allows T. galeatus to consume the most available resources (e.g., terrestrial invertebrates in high water levels), which probably contributed to the increase in its body condition in higher hydrometric levels. In summary, the hydrological dynamic is fundamental for the diet variation of omnivorous fish, which can take advantage of the resource availability according to the hydrometric level, increasing its body condition when allochthonous resources as terrestrial invertebrates are supposed to be more available.

An intensively debated issue in ecology is whether the variations in the biodiversity patterns of different biological groups are congruent in space and time. In addition, ecologists have recognized the necessity of accounting for both taxonomic and functional facets when analysing spatial and temporal congruence patterns. This study aimed to determine how the cross-taxon congruence of taxonomic and functional beta diversity varies across space and time, using data from four floodplains at a continental scale. Our general hypothesis was that the congruence between aquatic biological groups, either taxonomic or functional, would decrease with the “between-group” functional distance. Also, we examined how congruence patterns varied across spatial and temporal scales by focusing on how the cross-taxon relationships differ among Brazilian floodplains and between dry/wet periods. Our study comprised information on eight biological groups from the four largest Brazilian river-floodplain systems, and cross-taxon congruence was assessed using Procrustes analysis. Our results show how detailed analyses can reveal different patterns of cross-taxon congruence, and partially support the hypothesis that the strength of cross-taxon congruence is negatively related to between-group functional distance.

Morphological similarity, resource sharing, and differences in habitat use by species are factors that favor their coexistence. The objective of this study was to test possible differences in ecomorphology and diet composition of two Heptapterids (Imparfinis mirini and Cetopsorhamdia iherengi) to identify patterns related to resource use. Samplings were carried out in ten streams in Southern Brazil and 123 individuals were caught. A total of 21 ecomorphological indices were calculated for each individual and the volumetric and occurrence methods were used to quantify stomach contents. Both species presented significant differences in some ecomorphological traits, mainly related to foraging behavior. Even though both species were considered insectivorous, the consumed feeding resources differed between them. Cetopsorhamdia iheringi diet was dominated by Simuliidae and terrestrial Coleoptera larvae, while Imparfinis mirini, presented a diet dominated mainly by Trichoptera larvae, Ephemeroptera nymph, and Annelida. Although ecomorphological patterns cannot be used as an absolute factor to explain diet variations, they provide relevant information about how species share resources. These mechanisms allow us to obtain important subsidies for the conservation and management of freshwater ecosystems since they provide an effective understanding on the interactions that occur between the species.