We found higher gamma and beta fish diversity in the free-flowing section, where Brillouin diversity increased in a downstream direction however, in the impounded section, we found decreasing diversity downstream.

We used electrified benthic trawls and stable isotope analysis of δ13C and δ15N to determine the complex relationships between taxonomic diversity and food web structure of fish assemblages among sites in the free-flowing and impounded reaches of the Allegheny River, Pennsylvania, USA. 'Abstract Although dams are a common feature on rivers throughout the world, their effects on diversity, composition, and structure of fish assemblages are often unclear. 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. The type of isolation of lakes from the river channel had no apparent effect on community structure. Visually oriented fishes had highest abundance in clear lakes, whereas fishes with adaptations to low visibility were most abundant in turbid lakes.

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). Canonical correspondence analysis revealed that among the environmental descriptors of lakes, only Secchi transparency and maximum depth were significantly related to fish community structure. 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. We examined the relationship between structure of fish communities and 11 environmental variables in floodplain lakes from the Middle Araguaia River, Brazil.

Riffle and associates