Integrated Remote Sensing for Mapping and Monitoring Srebarna Lake Ecosystem, Bulgaria

Srebarna is a shallow freshwater lake situated along the right bank of the Lower Danube, northeastern Bulgaria. In recent times, the natural hydrology of the lake has been modified and managed through a series of embankments and artificial canal which regulate and control the water exchange with the Danube, since their natural connection was interrupted with a dyke built in 1948. Nevertheless, the fundamental importance of the lake’s ecosystem has even increased, considering the degradation of the majority of Lower Danube’s wetlands. Despite its protected status, the lake faces numerous environmental challenges, e.g. eutrophication caused by nutrient enrichment and sediment entrapment, leading to algal blooms, oxygen deficiency, and overall poor water quality, especially during the vegetation season. Desiccation of periphery pools and overgrowth with common reed and grey willow has been observed in the last years, due to the decreased flood potential of the Danube River. Succession in biodiversity like rapid spread of invasive and alien species, followed the degradation of the wetland ecosystem and negatively impact native species diversity. Climate change is a growing threat, as shifts in temperature and precipitation patterns alter the lake's hydrology. In an effort to negate these effects, during 2022 and 2023 several main activities were manged as restoration acitivies: an excavation of second canal in the western part of the lake, floating mats cut off, reed harvest and sapropel excavation. As part of ongoing conservation efforts for monitoring and assessment of the morphological changes after these restoration activities, advanced techniques to collect up-to-date spatial data were utilised. One key aspect is the use of UAS, which employ airborne LiDAR scanning and photogrammetry for high-resolution data collection. The primary objective is to map the variations in the altitude of the lakebed and to assess the dimensions of the artificial channels used to regulate the water flow between the lake and the main river. Scanning is performed during low-water season when environmental conditions and features can be accurately captured. In the areas where LiDAR scanning is ineffective, e.g within the waterbody, sonar CHIRP technology on a boat is employed to gather bathymetric data, providing detailed information about the underwater topography. These tools allow assessment of changes in the lakebed, particularly in the sections that are permanently under water. The lake and its surrounding floodplains are divided into 31 non-equal but similar by area sections, covering overall 981.5 ha. To capture comprehensive data for conservation and monitoring purposes, each section is scanned and photographed individually in a single autonomous flight mission using a battery-powered UAV equipped with both LiDAR and RGB camera sensors. The UAV follows flight paths that consist of two to four parallel trajectories, spanning from shore-to-shore across the lake and its adjacent floodplains. This approach is designed to optimize time efficiency and reduce risks, with each flight starting and ending on the same shore, ensuring the UAV’s safe operation. The flight trajectories maintain an average gap of approximately 70 m, and the UAV operates at an altitude of 130 m asl (approx. 120 m above the target surface), with a forward speed of 6.5 m/s and 70% side overlap, allowing dual-mode scanning. The LiDAR data collected is processed to generate an initial LAS point cloud, which is then converted into a digital elevation model (DEM). Sonar data is integrated into a GIS layer, aligning with the DEM grid cell centres to produce a seamless, improved DEM. This advanced processing technique ensures that the transition between the lakebed and the surrounding dryland surface is smooth and accurate in the final DEM, aiding the evaluation of morphological changes over time. Photogrammetric data is also processed to generate a complete RGB orthomosaic of the entire lake and its surrounding floodplains. The orthomosaic serves as a visual representation for further analysis, enabling the monitoring of water body changes and the assessment of the wetland vegetation dynamics. The high-resolution imagery provides a comprehensive overview of the lake’s current state and facilitates informed decision-making for future conservation activities. Long-term ecological and biological monitoring, the use of advanced technologies, continuous collection and analysis of spatial data, and the application of sustainable management practices, are essential for the preservation of the integrity of this unique wetland, it’s biodiversity and the overall health of the Danube’s floodplain ecosystem.