Two perspectives, one river: what macroinvertebrates and fish reveal about mountain river ecosystem

This study provides an integrated ecological assessment of a mountain river by combining responses of fish and macroinvertebrate communities as biological quality elements (BQEs). Macroinvertebrates showed significantly higher taxa richness than fish, with cumulative richness increasing downstream, whereas fish species richness displayed no clear longitudinal pattern. Trichoptera, Ephemeroptera, Coleoptera and Diptera were the most diverse macroinvertebrate groups, with Brachycentrus subnubilis, Cheumatopsyche lepida and Serratella ignita as the most abundant taxa. Eight species typical of mountain rivers were recorded in the fish community. Diversity, evenness and dominance patterns highlighted differences between the two BQEs: macroinvertebrate communities showed high diversity with evenly distributed taxa, while fish communities exhibited moderate diversity and strong dominance of Alburnoides bipunctatus and Barbus balcanicus along the entire reach. Notably, two Balkan regional endemic macroinvertebrates (Dina dinarica and Gammarus balcanicus) and several protected fish species (Cottus gobio, Sabanejewia balcanica, Barbus balcanicus, Alburnoides bipunctatus, Salmo trutta and Squalius cephalus) were recorded. Both BQEs were consistent in indicating favorable ecological conditions. Spearman rank correlations between corresponding fish- and macroinvertebrate community metrics were not statistically significant. However, Wilcoxon signed-rank tests revealed consistent, although non-significant, directional differences, with higher diversity and evenness in macroinvertebrates and stronger dominance in fish. Although fish communities in upper mountain catchments are naturally species-poor and limited in standalone assessment power, their combined use with macroinvertebrates may provide a more comprehensive and ecologically meaningful evaluation. The results contribute to the evidence supporting of multi-BQE approaches in mountain river assessment, highlighting the complementary information provided by fish and macroinvertebrate communities.