Assessing Pollution and Diatom-Based Bioindicators in the Arieș River, Romania

The Arieș River, in the Apuseni Mountains of Romania, has been historically impacted by gold mining, resulting in elevated concentrations of metal trace elements. This study assessed the river’s ecological status between 2022 and 2024 by integrating physico-chemical parameters at four sites and diatom-based bioindicators at the same sites in 2024 across monitoring sectors. Key results showed strong mining influence downstream of Baia de Arieș, with episodic acidification (pH down to 5.7), elevated conductivity (>400 μS/cm), and notable exceedances in metal trace elements, particularly Cu (up to 237 μg/L) and Cd (up to 4.18 μg/L). Analysis showed a dominance of α-mesosaprobic taxa (e.g., Amphora ovalis, Navicula cryptocephala, Nitzschia inconspicua), including teratological forms, at polluted sites, while sensitive species persisted upstream. Multivariate analyses indicated that metal trace elements and nutrients are the main drivers of chemical and biological patterns. These findings highlight diatoms as sensitive bioindicators of mining-related impacts. Incorporating diatom-based monitoring into management strategies can support early detection of pollution and more effective protection of freshwater ecosystems.