Drivers of fish beta diversity vary by habitat and rainfall period in temporary freshwater habitats of the Atlantic Forest

Introduction Temporary freshwater habitats 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. Here, we investigate these processes in temporary freshwater habitats of the Atlantic Forest. Methods We sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. We tested if community composition differs between habitat type and rainfall periods and whether beta diversity components were associated with local environmental and spatial predictors. Results Fish community composition did not differ significantly between habitats or hydrological periods, but temporal variation in spatial beta diversity was consistently high and influenced by turnover. During the dry period, nestedness was positively correlated with differences in volume for temporary pools and roadside ditches. Additionally, during the wet period, turnover in ditches was correlated with differences in pH. In temporary pools during the wet period, turnover increased with differences in distance to the nearest stream. Discussion High turnover for both habitats and periods suggest spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. Nestedness correlations with differences in volume in the dry period indicate that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Meanwhile, turnover correlation with differences in pH for ditches in the wet period is consistent with mass effect dynamics under higher connectivity conditions, with pH acting as a proxy for stream water influx. Turnover correlation with differences in distance to the nearest stream for pools during the wet period suggest colonization by highly tolerant and capable of dispersal species in more isolated sites. Overall, our findings emphasize that beta diversity in temporary aquatic habitats is primarily structured by habitat availability and spatial processes interacting with hydrological dynamics, underscoring that effective conservation must target the entire floodable mosaic rather than isolated sites to maintain this dynamic metacommunity.