Role of environmental and spatial processes structuring fish assemblages in streams of the eastern Amazon

Considering the increasing importance of analysing spatial structure in ecological studies, the aims of the present study were to test whether fluvial distances and environmental factors are important drivers of the (beta-diversity of stream fish assemblages, and whether beta-diversity is different in distinct hydrological periods. Specimens were sampled at 33 stream sites in the eastern Amazon. Eight environmental variables were measured at each site and fluvial distances between pairs of stream sites were determined. Environmental variables were the main factors structuring fish assemblages in both periods. However, fluvial distances were important only during the flood period. This can be related to the formation of extensive flood plains in this period, which increases connectivity between streams, breaking habitat isolation and increasing the regional signal for fish species. The higher correlation of beta-diversity with environmental variables during the flood period may be related to decreased dispersal limitations and intermediate dispersal. Finally, beta-diversity was higher during the flood period, highlighting the importance of the heterogeneity of the flood plain to stream biota. The results of the present study indicate that spatial and environmental factors play complementary roles in structuring fish assemblages in Amazon streams, and that (beta-diversity was affected by changes in the habitat connectivity experienced in different hydrological periods.