Ecosystem Functioning under Climate Changes, Hydrological Confinement and Pollution Risks in the Shabla–Ezerets Lake System (Northern Bulgarian Black Sea Coast): Controlled Seasonal Outlet as a Pathway to Nature-based Restoration

This study provides an integrated geomorphological, hydrological, physico-chemical, ecological and macrolitter assessment of the Shabla–Ezerets coastal lake system. High-resolution LiDAR and bathymetric data reveal a shallow, low-energy basin with strong hydrological confinement, extensive littoral development and fine-sediment accumulation. Contemporary water-column profiles (2019–2024), compared with historical measurements (1950–1963), show a stable summer thermocline at 2–4 m depth and an increase of approximately 3–4 °C in summer surface temperatures. Near-bottom oxygen saturation frequently declines to 20–40% during summer, extending the spatial expression of historically observed low-oxygen conditions. Salinity remains uniformly oligohaline (0.1–2.1 ‰), contrasting with the more variable historical range. Ecological data indicate long-term shifts towards freshwater-tolerant ichthyofaunal assemblages and expanded reedbed dominance. Macrolitter surveys document moderate plastic-dominated accumulation across shoreline, surface-water and benthic compartments, reflecting limited flushing and efficient retention. The combined evidence shows that hydrological confinement, gradual climate-related warming and restricted water renewal jointly structure present-day ecosystem functioning. Controlled seasonal freshwater outflow is identified as an appropriate restoration measure for improving water-column renewal, reducing stagnation and decreasing litter retention while preserving the system’s freshwater character.