Réjean Fortin

Fish community structure in relation to environmental variation in floodplain lakes of the Araguaia River, Amazon Basin

We examined the relationship between structure of fish communities and 11 environmental variables in floodplain lakes from the Middle Araguaia River, Brazil. Samples were collected at the beginning and at the end of the dry season in standardized surveys of six seasonally isolated lakes and six lakes permanently connected to the river mainstem. Canonical correspondence analysis revealed that among the environmental descriptors of lakes, only Secchi transparency and maximum depth were significantly related to fish community structure. Most of the patterned variation in community structure was associated with marked seasonal changes in lake transparency (means: early dry season=90.8 cm; late dry season = 33.2 cm) and depth (means: early dry season = 4.6 m; late dry season = 2.0 m). Visually oriented fishes had highest abundance in clear lakes, whereas fishes with adaptations to low visibility were most abundant in turbid lakes. The type of isolation of lakes from the river channel had no apparent effect on community structure. The results suggest that sensory adaptations for detection of prey under prevailing optical conditions play a major role in the organization of these communities, as predicted by a model of fish community organization originally developed for floodplain lakes of the Orinoco River, Venezuela.