Heterogeneity of macrophyte banks affects the structure of fish communities in flooded habitats of the Amazon Basin

In freshwater ecosystems, macrophyte contribute to habitat heterogeneity with their varying structural forms, ranging from free submerged to rooted species. Macrophytes provide substrates that support fish populations, providing food and refuge from predators, especially in semi-lentic environments, like river floodplains, which tend to provide little shelter for small fish. We investigated how the species richness, abundance, and morphological traits of the fish communities of river floodplains in the Amazon region are affected by the heterogeneity of macrophyte beds, based on the relative contribution of each plant species present in these beds. Simple linear regressions were used to investigate the relationship between macrophyte bed heterogeneity and fish species richness, abundance, and morphological traits, evaluated using a community-weighted mean index analysis (CWM). We recorded 16 aquatic macrophyte species and 21 fish species at 34 study sites. Eichhornia azurea and Eichhornia crassipes dominated the macrophyte beds, while the most abundant fish species were Hemigrammus ocellifer and Laimosemion strigatus . The results of the linear regressions of the heterogeneity of the macrophyte beds were only significant for two morphological traits, both linked to fish locomotion. Our results indicate that the heterogeneity of the macrophyte beds acts an environmental filter for the fish species with a high degree of maneuverability, given that these species are able to swim within the macrophyte structures. Although this does not affect the richness and abundance of the fish, the macrophyte beds favor certain species that use this vegetation during the different stages of their life cycle, which present distinct adaptations. The heterogeneity provided by the aquatic macrophytes is thus important to a range of different fish species, by providing shelter and protection for the smaller species with morphological traits that permit their survival in this environment.