Diversity and spatio-temporal distribution of benthic macroinvertebrate communities in a transboundary river basin in the Caucasus region (Aras river, NE Türkiye).

This study evaluates the ecological status of the Aras River Basin (Türkiye) by analyzing benthic macroinvertebrate communities in relation to seasonal variations and anthropogenic pressures. During 2014-2015 sampling campaigns, we identified 126 taxa, of which 107 were identified at the species level and 19 at the genus level across 17 stations, with Insecta (87 taxa, 69%) showing the highest richness, followed by Clitellata (23 taxa, 18%). The dominant species Tubifex tubifex (15.87%), Chironomus riparius (15.60%), and Gammarus balcanicus (15.11%) served as key bioindicators, revealing significant organic pollution impacts, particularly in lentic habitats. Canonical Correspondence Analysis (CCA) for the summer period identified dissolved oxygen (DO) and pH as the primary environmental drivers, with pollution-tolerant taxa (e.g., Chironomus riparius) clustering in low-DO areas, while sensitive species (e.g., Baetis rhodani) were predominantly associated with well-oxygenated, alkaline conditions. Seasonal analyses demonstrated autumn as the most productive period (3,765 ind., 91 taxa), with Station 9 maintaining pristine conditions (BMWP score: 66, "Good") year-round. Conversely, spring showed the poorest water quality (BMWP < 25 at most stations), while summer exhibited intermediate conditions. Multivariate analyses (UPGMA, TWINSPAN) confirmed spatial clustering based on pollution gradients, with tolerant taxa (e.g., aquatic leeches, Chironomus spp.) dominating organically enriched sites and sensitive species (e.g., Plecoptera) restricted to high-quality habitats. Our findings highlight: (1) severe degradation at stations receiving agricultural/domestic waste (Stations 1-6), (2) the critical role of seasonal monitoring in detecting climate-driven stressors, and (3) the utility of macroinvertebrate-based indices (BMWP/ASPT) for basin-scale water quality assessment. The study provides a scientific basis for conserving transboundary freshwater ecosystems through targeted pollution control and habitat protection measures.