Relationships between fish assemblages, macrophytes and environmental gradients in the Amazon River floodplain

During the flood season of 1992-1993, 139 species of fishes were collected from a floodplain Like system in the central Amazon Basin. Fish species distribution was examined relative to abiotic variables in seven vegetation strata on Marchantaria Island, Solimoes River. Both environmental variables and species distributions were influenced by a river channel to floodplain-interior gradient. Species diversity was significantly higher ill vegetated areas than in unvegetated areas, with deeper water Paspalum repens stands harbouring the highest diversity. As a result, species richness and catches were positively related to habitat complexity, while catch was also negatively related to dissolved oxygen (DO) and water depth. Low DO and shallow waters appeared to act as a refuge from predation. Fish assemblages were related to water chemistry, but species richness was not. Canonical correspondence analysis provided evidence that floodplain fish assemblages formed by the 76 most common species were influenced by physical variables, macrophyte coverage and habitat complexity, which jointly accounted for 67% of the variance of fish species assemblages. Omnivores showed no pattern relative to the river channel to floodplain-interior gradient while detritivores were more likely to be found at interior floodplain sites and piscivores closer to the river. Piscivores could he further separated into three groups, one with seven species associated with tree-floating macrophytes in deep water, a second with five species found in shallow waters with rooted grasses and a third with six open water orientated species. The results suggest that fish assemblages in the Amazon floodplain are not random associations of species. (C) 2003 The Fisheries Society of the British Isles.