Spatio-temporal organization patterns in the fish assemblages of a Neotropical floodplain

Communities can be structured over time by deterministic, stochastic, or both mechanisms. This study evaluated whether the fish assemblages of the Upper Parana River floodplain were spatio-temporally structured and examined the mechanisms driving these organizational patterns. The floodplain ichthyofauna was sampled quarterly with seines and gillnets. The structure of the assemblage was assessed using the C-Score co-occurrence index and null models. The influence of both the environment (physical and chemical variables and hydrometric levels) and time on the species' organization patterns was assessed via principal component analysis, multiple and simple regressions, and analyses of variance. Assemblages with larger species showed patterns of species segregation at both spatial and temporal scales, while those composed of smaller species often exhibited random patterns. The physical and chemical variables did not predict co-occurrence among species. For assemblages of large species, the patterns of co-occurrence tended to be random when the river level increased but only during high-water months. Therefore, the life history traits of the species, such as body size, may predict the structure of the assemblages in floodplains, but this effect depends on variations in the hydrometric level.