Sessions

Urbanization is a set of various anthropogenic habitat alterations which can translate into freshwater ecosystems via complex environmental cascades. The confluences of adventitious streams host transitional fish assemblages by the admixture of riverine and stream-dwelling assemblages. These fragile ecosystems are hotspots for local biodiversity and provide key nodes for dispersion. Although the ecological importance of tributary confluences is well recognized, the mechanisms through which urbanization influences fish assemblages in these keystone habitats is poorly understood.

In this study, we examined the estuarine section of 15 adventitious streams in the Upper Danube Freshwater Ecoregion (in Hungary) throughout a year (2024 fall, 2025 spring and summer) to reveal the effects of urbanization on the whole fish assemblage, and the species richness and density of riverine, stream-dwelling, non-native and native groups. We built and tested a hypothetical cascade of environmental variables with Piecewise Structural Equation Modelling (pSEM), which represent different spatial layers, including local land use, riparian vegetation coverage, stream habitat size and habitat gradient (mountain-lowland). In addition, we investigated the effects of these environmental variables on the whole fish assemblage via Partial Redundancy Analysis (pRDA).

We found limited upstream dispersion of riverine species in adventitious streams, since both the species richness and density of this group decreased in the upstream direction, while the opposite was observed in case of the stream-dwelling species. Among the environmental variables, pSEM revealed that urbanization in land use is paired with the increase in artificial coverage in the riparian zone. However, our data did not support the transmission of this effect to the within-stream habitat. Instead, urbanization in land use exerts its negative effect on stream habitat size which was the main factor influencing species richness and density in general. Environmental variables showed high between-stream variation, suggesting heterogenous environment and the importance of local conditions. Variance partitioning from pRDA highlighted the importance of within-stream habitat in fish assemblage composition but with considerable amount of unexplained variance, suggesting strong stochasticity in assemblage organization.

Our findings emphasize that even urbanized stream confluences can still support relatively diverse fish assemblages comprising a mixture of stream-dwelling and riverine species. They also highlight the importance of restoring channel morphology and re-establishing a heterogeneous habitat mosaic, including a diversity of flow conditions, depths, substrates, and refuge habitats. Such measures may enhance habitat availability and connectivity, and thereby strengthen the capacity of stream confluences to support diverse fish assemblages even in urban environment.