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This study evaluated the effect of seasonality on diet composition and overlap in fish species inhabiting the littoral zone of a small lake. We analyzed whether the way the species exploit food resources may help explain their coexistence. Fish samples were collected in a lake in the Upper Paraná River floodplain in September (dry period) and December (rainy period) of 2010. The diet of six species was evaluated through stomach content analysis, which showed that the most consumed resources were microcrustaceans, aquatic insects, aquatic higher plant and algae. Diet composition was significantly different between periods for all species studied. Diet overlap values were low in both periods, but trophic segregation was even higher in the rainy period, indicating that with greater abundance of food resources, species could exploit preferential resources. Trophic segregation, evidenced by the low overlap values in both periods, indicates the absence of interspecific competition, which may explain the coexistence of these species in the littoral zone of this small lake.

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Hydrological connectivity interferes directly in dispersal rates of organisms and in similarity of environmental conditions among floodplain environments. Consequently, connectivity promotes changes in food resources availability to fish. Here we tested the predictions that (a) isolated floodplain lakes have greater environmental heterogeneity than connected lakes, (b) fish diet differs more among isolated than among connected lakes and (c) trophic niche breadth of the species is smaller in isolated than in connected lakes. We used one invertivorous and one algivorous species of fish to test these two last predictions. The environmental heterogeneity (evaluated in relation to abiotic variables and the macrophyte composition) tended to be greater in isolated lakes. Diet of both species differed significantly in all isolated lakes populations, and in the majority of the populations in connected lakes; however, the main items consumed for invertivorous species were common in all connected lakes. Trophic niche breadth of the invertivorous fish was significantly greater in the connected lakes; however, the algivorous species showed no significant difference between connected and isolated lakes. Our findings suggest that connectivity can facilitate the dispersal of organisms, allowing the increase in food item richness and consequently, trophic niche breadth, as observed for invertivorous species. However, connectivity likely plays a minor role on the algivorous species, whose diet may be more affected by local conditions, such as habitat structure. Our results support the view that regional (dispersion) and local (habitat structure) factors interact with feeding habit to determine fish food composition in the floodplain lakes we studied.

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