Chemical Stratification and Spatio-Temporal Variability of the Anthroposaline Lake Baia Baciului in an Unstable Salt-Karst Setting (Slănic Prahova, Romania)

Baia Baciului is an anthroposaline lake formed by mining collapse within the Slănic Prahova salt diapir. Its salinity has declined over recent decades, and in January 2024 its level fell by almost 2 m. No vertically and spatially resolved hydrochemical data had been published since these changes, so it was unknown whether the historically reported meromictic structure has persisted. Between July 2025 and September 2026, five campaigns measured electrical conductivity (EC) and salinity at 12 measurement points and three depth levels (149 records); temperature, pH, dissolved oxygen (DO), oxidation–reduction potential (ORP) and turbidity were additionally recorded in December 2025. EC at 1.4 m exceeded surface EC in all 52 matched pairs. Surface EC ranged from 4.3 to 51.0 mS/cm, whereas bottom samples (2.0–4.5 m) ranged from 70.1 to 130.2 mS/cm (EC-derived salinity up to ≈85 g/L). Transect-blocked Friedman tests showed significant differences among campaigns at 0.2 m and 1.4 m (p ≤ 0.002). The water column was nearly isothermal in July and inversely stratified in December, when bottom DO (21–98% saturation) and ORP (down to −332 mV) were low but spatially heterogeneous. Localised surface freshening suggested possible discrete freshwater inputs. The lake was chemically stratified on all sampled dates, consistent with meromixis.