Seasonal dynamics of fish diversity and assemblage composition in temporary pools of a protected Atlantic Forest area.

The Atlantic Forest is one of the top five biodiversity hotspots on the planet and is considered a global priority for conservation, largely due to extensive human-driven deforestation. Temporary pools in the Atlantic Forest harbour unique and vulnerable fish assemblages, yet remain poorly studied, particularly within protected areas. In this study, we assessed fish diversity, composition and environmental drivers in 34 temporary pools sampled across dry and wet hydrological periods in the Serra do Mar State Park, southeastern Brazil. A total of 574 individuals representing eight species were recorded, indicating low regional richness, with estimates not exceeding 10 species. Fish abundance, richness, effective number of species, Margalef's K and species composition did not differ between periods. Habitat availability and isolation influenced fish abundance only during the dry period, reflecting contrasting ecological strategies of dominant species. In the wet period, the assemblage structure showed no association with environmental predictors, suggesting a stronger influence of stochastic processes driven by unpredictable flood dynamics. Shrimp abundance had no detectable effect on fish assemblage metrics. Our results indicate that temporary pools are shaped by a combination of historical environmental filtering acting on the regional species pool and contemporary stochastic processes. These findings highlight the ecological dynamics of temporary aquatic habitats, often overlooked in conservation planning, and also reinforce the role of protected Atlantic Forest areas as microclimatic refugia in a rapidly changing world.