High-altitude freshwater ecosystems are particularly vulnerable to anthropogenic impacts, yet spatial and seasonal variations in biological responses remain poorly understood. This study assessed the health status of narrow-clawed crayfish (Pontastacus leptodactylus) and habitat condition in two hydrologically and environmentally distinct bays of Lake Sevan, Armenia, using an integrated hydrochemical and multibiomarker approach. Water quality and crayfish populations were measured in Lichk and Lchashen Bays in May, July, and October 2024 and 2025. In total, 120 crayfish specimens were examined, and oxidative stress responses were assessed in the hepatopancreas, gills, and muscle using MDA, GSH, GST, KAT, and SOD. Lchashen Bay experienced more adverse environmental conditions, characterized by pronounced hypoxia in the summer (3.4–3.9 mg O₂ L⁻¹), higher biochemical oxygen demand and permanganate oxidation, and elevated K, Na, and Mg concentrations. Biomarker responses showed significant seasonal variation, typically with highest values in July. Crayfish from Lchashen Bay consistently demonstrated higher biomarker responses than crayfish from Lichk Bay across all tissues and sampling periods. Integrated biomarker response (IBR) values confirmed this spatial pattern, indicating greater physiological stress in Lchashen Bay. The combined hydrochemical and biomarker approach demonstrates the suitability of P. leptodactylus as an indicator species for assessing sublethal environmental stress in Lake Sevan.

River fragmentation is a major anthropogenic disturbance that modifies hydrological conditions, influences abiotic parameters, and thus also affects phytoplankton composition, diversity, and abundance. Mountain river ecosystems are particularly vulnerable. In the local context of Caucasus rivers, disruptions of ecological continuity are also important because the flux of untreated wastewater or the discharge of drinking-water-quality effluents affects water quality and thus biota substantially. However, the response of phytoplankton communities to river fragmentation is not well studied yet. Therefore, the present study addresses such a gap in the knowledge by investigating the phytoplankton community of the heavily fragmented Hrazdan River in the central part of Armenia and evaluating the effects on its taxonomic composition, diversity, and community structure. Comprehensive analysis involving hydrochemical, hydrophysical, and phytoplankton metrics revealed that both river fragmentation and disruption of ecological continuity have a strong influence on phytoplankton communities. Factors such as severe organic enrichment, elevated nutrient loads, and critical oxygen depletion create chemical barriers that induce additional ecological fragmentation. PCA and cluster analysis showed major patterns of impacts of the interaction between physical barriers and localized anthropogenic stressors, which drive distinct taxonomic regrouping within the phytoplankton community. Overall, this study demonstrates that phytoplankton dynamics in regulated mountain rivers are primarily governed by the coupling between chemical stressors and fragmentation-induced habitat alterations, highlighting the need to conduct river restoration at various parts.