Drivers of fish beta diversity vary by habitat and rainfall period in ephemeral aquatic habitats of the Atlantic Forest
Ephemeral aquatic habitats, such as temporary pools and roadside ditches, are characterized by cyclical flooding and drying that create highly dynamic conditions for fish community assembly. These environments can exhibit high spatial and temporal variability in community structure driven by local topography, distance to source habitats and interannual variation in rainfall, which affects connectivity dynamics. As such, they provide unique opportunities to test hypotheses about the processes shaping metacommunities, particularly through the partitioning of beta diversity into turnover and nestedness components. Here, we investigate these processes in ephemeral aquatic habitats of the Atlantic Forest, where we sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. Fish communities composition did not differ significantly between habitats or hydrological periods, but beta diversity was consistently high and predominantly influenced by turnover. This suggests spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. We tested whether beta diversity components were associated with local environmental (temperature, dissolved oxygen, pH, volume) and spatial (distance to the nearest stream) predictors. During the dry period, nestedness was positively correlated with differences in volume for temporary pools, indicating that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Additionally, turnover was associated with differences in distance to the nearest stream in roadside ditches during the dry period, likely reflecting differential colonization by stream-associated species. During the wet period, differences in pH were correlated with turnover in roadside ditches, consistent with Mass Effect dynamics under higher connectivity conditions. Meanwhile, in temporary pools during the wet period, turnover increased with differences in distance to the nearest stream, suggesting colonization by highly tolerant and capable of dispersal species in more isolated sites. These contrasting relationships may also reflect the spatial organization of habitats, with linear ditches facilitating directional dispersal along roads, whereas spatially dispersed pools depend on more stochastic colonization. Thus, pools act as seasonal refugia for tolerant or amphibious species during low connectivity, and for stream-dwelling species during high connectivity, whereas ditches function as more persistent nodes that facilitate dispersal during wetter periods. This seasonal complementarity helps explain the observed temporal and habitat-related variation in beta diversity, highlighting that effective conservation must target the entire floodable mosaic, rather than isolated sites, to maintain this dynamic metacommunity.