LAUSR

Metacommunity structure depends on environmental and spatial factors. Stream fishes are constrained to disperse within dendritic networks and waterfalls and other barriers add resistance to species dispersal. We evaluated the importance of environmental and spatial effects on fish metacommunity composition, adopting different models of spatial distance: overland, watercourse, resistance (physical barriers), cost distance (watercourse resistance), and asymmetrical dispersal (considering flow direction). Fishes were sampled in 31 riffle sites of a small subtropical basin of southern Brazil (~ 250 km 2 effective study area). Using redundancy analysis (RDA) and variance partitioning, we found the best model was the one including flow direction weighted by watercourse distance plus environmental variables (overall adj- R 2  = 0.42). However, the explanation of fish composition was greatly improved by weighting spatial distance by stream channel sinuosity or height of barriers. Reach slope and width were the two significant environmental variables. Our study supports that dispersal, in addition to environmental factors, affects stream fish metacommunity even in a watershed of small spatial extent. Detecting spatial effects in a small watershed depends on considering realistic watershed features that affect fish dispersal in drainage networks such as waterfalls and other physical barriers, and distance along the watercourse and flow direction.