Environmental gradients shape fish assemblages across estuarine-coastal ecosystems on the eastern Brazilian coast

Estuarine and coastal ecosystems are governed by physicochemical gradients that shape fish community structure, yet system-specific responses to environmental change and disturbance remain incompletely understood. We surveyed fish assemblages across four estuarine-coastal systems in southeastern Brazil (Doce River, Piraquê-Açu, São Mateus, and Caravelas), recording 49,308 individuals representing 193 species. Assemblage composition varied markedly among systems and between habitats with estuarine samples exhibiting higher compositional variability and coastal samples showing comparatively lower variability. Species richness and density differed among systems, with Caravelas showing the highest rarefied richness and the Doce River exhibiting the highest density. Environmental gradients, particularly turbidity, depth, salinity, and dissolved oxygen, were strongly associated with spatial variation in richness, density and species distributions, but the direction and strength of these relationships were system-specific. For example, turbidity and depth emerged as key correlates of richness in several systems, whereas depth was a consistent predictor of density (negative in most systems but positive in the Doce River). Species such as Genidens genidens, Cathorops spixii, and Trinectes microphthalmus were associated with higher dissolved oxygen and lower salinity, while Stellifer rastrifer and Isopisthus parvipinnis showed affinities for more marine-influenced conditions. The Doce River continues to reflect altered physicochemical conditions linked to the 2015 disaster, with implications for species distribution and habitat use. These findings highlight strong spatial heterogeneity in estuarine-coastal fish assemblages and underscore the need for tailored monitoring and management strategies following major anthropogenic disturbances.