The Shabla–Ezerets complex consists of two adjacent liman-type lakes, formed when Holocene sea-level rise flooded shallow erosion valleys carved into Sarmatian limestone, before longshore sand-barrier growth sealed them off from the sea. Karstic groundwater supplies more than 90 percent of the annual water balance, sustaining a near-freshwater, oligohaline regime. The lakes lie within the Natura 2000 site BG0000621 and serve as a critical stopover along the Via Pontica migratory bird flyway, hosting protected species such as the red-breasted goose and Dalmatian pelican.

To diagnose the system’s condition, the team combined an unusually broad set of methods. Drone-based LiDAR scanning produced a digital terrain model with centimetre-scale accuracy, while hydroacoustic surveys of 66,521 depth points mapped the lake floors. Sedimentologists analysed 136 grab samples for grain size and organic content, and water-column profiles from 2019 to 2024 were compared directly with historical measurements from 1950 to 1963. Fish communities were assessed across seven decades of records, and macrolitter was surveyed on beaches, at the surface and on the lake bed using harmonised European protocols.

The geomorphological picture that emerged is one of a mature, stable, low-energy basin. Shabla Lake reaches a maximum depth of 9 metres and stores about 4.08 million cubic metres of water, acting as the primary sediment and nutrient sink, while Ezerets Lake shows more advanced terrestrialisation with extensive littoral encroachment. Fine, organic-rich sediments dominate the central depressions, with loss-on-ignition values reaching 18 percent, evidence of intense internal nutrient recycling in a system that rarely flushes.

The water-column data reveal the consequences of that stagnation. In summer, warm surface waters of 24 to 26 degrees Celsius overlie cooler bottom layers at 10 to 14 degrees, with a persistent thermocline between 2 and 4 metres depth. Compared with the 1950s and early 1960s, summer surface temperatures have risen by roughly 3 to 4 degrees Celsius. Near-bottom oxygen saturation now frequently drops to just 20 to 40 percent during summer, extending the spatial reach of low-oxygen conditions well beyond what historical records documented. Salinity, meanwhile, has become uniformly oligohaline at 0.1 to 2.1 parts per thousand, replacing the more variable historical range and signalling a weakening of marine influence.

The fish community tells a parallel story of long-term change. Species that once thrived when an artificial sea–lake connection was maintained, including grey mullets, the big-scale sand smelt and European flounder, have disappeared since the outlet ceased to be managed in the late 1960s. Sensitive oligohaline specialists such as the weatherfish and the longtail goby have declined dramatically, while tolerant freshwater and non-native species like Prussian carp and eastern mosquitofish have expanded. Cluster analysis shows the 2001–2009 and 2010–2024 assemblages are the most similar at 85.57 percent, confirming a directional shift toward a simplified, disturbance-tolerant community.

Pollution surveys add a third dimension to the diagnosis. Beach–dune transects recorded 293 to 344 items per 100 metres, with plastics making up 85 to 90 percent of all litter. Floating macrolitter density reached 38.5 items per 10,000 square metres, and benthic litter averaged 3 items per square metre, with hotspots near reedbeds and the artificial canal. Plastic fragments embedded in organic-rich muds point to long residence times and progressive fragmentation into microplastics. The reedbeds themselves act as passive litter traps, capturing lightweight debris such as expanded polystyrene fragments, bottle caps and packaging films that would otherwise be flushed out of a better-connected system.

The study’s central insight is that these three strands, hydrological confinement, climate warming and pollution retention, are not independent problems but reinforcing feedback loops within a single degradation pathway. Restricted water renewal prolongs residence time, which favours stratification, deep-water oxygen depletion, organic accumulation and reed expansion. The expanding vegetation further suppresses circulation and traps more litter, while warming intensifies stratification and oxygen demand. The result is a system that appears geomorphologically stable but is ecologically increasingly vulnerable to hydrological and climatic extremes.

Against this backdrop, the researchers evaluated three management scenarios. Full reconnection to the sea was rejected because both existing outlet channels are topographically perched above lake and sea level, making sustained marine exchange physically impossible without major engineering, and because salinisation would threaten the freshwater habitats protected under Natura 2000. Continued non-intervention would prolong stagnation and deepen oxygen deficits as warming accelerates. The preferred option is controlled seasonal freshwater outflow through an artificial spillway, operational since November 2023, which would enhance water renewal, improve deep-layer oxygenation, moderate reed overgrowth, reduce litter retention and sustain the freshwater character required by European restoration law.

The approach aligns with the EU Nature Restoration Regulation 2024/1991, which prioritises the recovery of natural hydrological regimes in degraded wetlands, and echoes successful hydrological reconnection projects in the Danube Delta. Short-term benefits include the rapid release of stagnant oxygen-poor water, immediate reduction in floating litter within reedbeds and improved water clarity. Long-term gains include slowed terrestrialisation, reduced organic loading and greater resilience against lake heatwaves. The effectiveness of the outlet, the authors caution, depends on the morphodynamic stability of the beach–barrier zone, so regular monitoring of the outlet cross-section will be essential. For now, the Shabla–Ezerets system stands as a model case for how integrated science can guide low-impact, hydrology-centred restoration of Europe’s fragile coastal lakes.

Subject of Research: Hydrological confinement, climate warming and plastic pollution in a Bulgarian Black Sea coastal lake system and nature-based restoration through controlled seasonal freshwater outflow

Article Title: 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

Article References: Bekova, R., Prodanov, B., Lambev, T., & Gussev, C. (2026). 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. One Ecosystem, 11, Article e182524. https://doi.org/10.3897/oneeco.11.e182524

Image Credits: AI Generated

DOI: 10.3897/oneeco.11.e182524

Keywords: Shabla–Ezerets lakes, coastal wetlands, liman lakes, hydrological confinement, climate warming, oxygen depletion, macrolitter, plastic pollution, Natura 2000, nature-based restoration, Black Sea, ichthyofauna