Spatial and seasonal patterns in fish assemblages of the Bakırçay river are associated with physicochemical and habitat parameters.

The significance of long-term biodiversity monitoring studies for the protection of natural biodiversity and human well-being is well recognised by the Turkish scientific community. Despite understanding the ecological importance of freshwater ecosystems, spatially or temporally congruent studies using high resolution biodiversity monitoring data from Turkish freshwater resources remain scarce. To determine a biodiversity baseline for future studies, biological and environmental sampling was carried out in 15 different locations from the highly anthropogenically impacted Bakırçay River and its catchment in Western Anatolia between 2017 and 2018. A total of 17 fish species from 10 families were recorded, belonging mainly to the Cyprinidae and Leuciscidae families. These included six non-native, six regionally endemic, and five native species. The endangered endemic Alburnus attalus was the most widespread species, whereas several non-native species were restricted to single sites. Patterns in community composition were primarily associated with pH and stream order. However, community metrics such as species richness, Pielou's evenness, and the Shannon-Wiener diversity index were not significant. Intensifying anthropogenic activity within the Bakırçay basin suggests that sources of pollution and other detrimental stressors like non-native species should be managed to protect riverine biodiversity and maintain the provision of ecosystem services. Our findings therefore not only present a baseline for future studies on fish biodiversity and community composition, but also the possible onset of future monitoring studies in the region. Our findings underline the importance of long-term biomonitoring studies for the conservation of Türkiye's freshwater ecosystems to monitor changes occurring over time.