Integrated bioassessment of a snowmelt-fed mountain stream: hydromorphology drives longitudinal patterns

Mountain headwater streams are highly sensitive freshwater ecosystems where hydromorphological gradients strongly influence habitat structure and biological communities, highlighting the need for integrated ecological assessment. This study uses Israfil Creek (Denizli, Türkiye) as a model snowmelt-fed mountain headwater to investigate how hydromorphological gradients and metric sensitivity structure ecological patterns. A total of 55 macroinvertebrate taxa were identified, with Chironomidae as the most diverse group. Community composition showed a clear longitudinal gradient, with cold-adapted taxa (e.g., Diamesa spp.) dominating upstream and more tolerant taxa (e.g., Chironomus riparius and Gyraulus albus) increasing downstream. Substrate composition shifted from lithal dominance in headwaters to finer sediments and particulate organic matter downstream. Saprobic indices indicated predominantly xenosaprobic-oligosaprobic conditions upstream, with increasing mesosaprobic influence downstream. Although Biological Monitoring Working Party (BMWP) scores consistently indicated high ecological quality, Ephemeroptera, Plecoptera, and Trichoptera (EPT) percentages declined longitudinally, revealing differences in metric sensitivity. Hydroclimatic analysis highlighted strong elevational control on snow persistence and pronounced precipitation seasonality. Overall, ecological patterns were governed primarily by hydromorphological gradients rather than conventional water quality signals, and reliance on single metrics may obscure subtle ecological transitions. These findings emphasize the importance of integrated, multimetric approaches for assessing climatically sensitive mountain stream ecosystems.