Long-Term Monitoring Reveals the Effects of Hydrological Alteration on Abiotic Conditions in Lake Srebarna eLTER Site, Bulgaria

Abstract

Hydrological alteration is among the principal drivers of ecological change in floodplain wetlands, affecting ecosystem functioning, biodiversity, nutrient cycling and the provision of ecosystem services. Distinguishing natural variability from long-term environmental change requires sustained observations based on harmonised monitoring protocols. As part of the Bulgarian Long-Term Ecosystem Research Network (LTER-BG) and the Integrated European Long-Term Ecosystem, Critical Zone and Socio-Ecological Research Infrastructure (eLTER), Lake Srebarna Biosphere Reserve provides a unique natural laboratory for studying ecosystem responses to hydrological modification, ecological restoration and climate variability in one of Europe’s most valuable floodplain wetlands. The hydrology and ecological functioning of Lake Srebarna have been closely linked to its connection with the Danube River. The interruption of this connection following the construction of the protective embankment in 1949 fundamentally changed the lake’s hydrological regime. In 1978, restoration activities were limited to partial dyke removal, while the construction of the Eastern (1994) and Western (2023) Channel—have aimed to improve the water exchange and recover natural ecosystem processes. These successive alterations make Srebarna an exceptional model system for evaluating ecological responses to both hydrological degradation and restoration efforts under increasing climatic variability. The study presents recent observations (2023-2026) generated as part of the long-term environmental monitoring at Lake Srebarna eLTER Site. Approximately 250 monthly samples collected from seven representative monitoring stations were analysed, encompassing open-water areas, channels, groundwater springs, and the Danube River. The monitoring network was designed to capture the principal hydrological gradients and environmental heterogeneity of the ecosystem. Standardised field observations and laboratory analyses were used to characterise the hydrological, physicochemical and hydrochemical conditions, the nutrient dynamics and the indicators of ecosystem functioning, providing one of the most comprehensive up-to-date datasets describing the overall current status of the wetland. The obtained data revealed pronounced seasonal variability together with considerable spatial heterogeneity in the studied abiotic variables. The period was characterised by lack of sufficient flooding events and persistently low water levels, resulting in the desiccation of vast areas of peripheral pools, reduced or absent lateral connectivity within the lake and increased physicochemical differentiation among the sampling sites. These hydrological conditions influence the nutrient dynamics and physicochemical processes, and ultimately affect the overall ecosystem functioning. The spatial differences among monitoring stations further demonstrate the combined effects of natural environmental gradients and anthropogenic hydromorphological modifications, highlighting the importance of maintaining an integrated long-term monitoring network capable of detecting ecological responses to ongoing environmental changes. The observed shifts in the hydrological connectivity and physicochemical conditions indicate changes in the habitat quality and ecosystem processes, revealing the role of the water regime as the principal driver of ecological functioning in the wetland. The generated dataset establishes a baseline for distinguishing natural seasonal variability from long-term changes associated with hydrological alteration and provides a valuable basis for evaluating the effectiveness of recent restoration measures aimed at improving the hydrological connectivity between the wetland and the Danube. Beyond documenting environmental conditions, the data demonstrates the value of long-term ecological observations for translating scientific evidence into informed adaptive management. By contributing harmonised long-term observations to the eLTER research infrastructures, the study supports ecosystem restoration, biodiversity conservation and climate-change adaptation while reinforcing the role of long-term ecosystem research in the sustainable management of protected floodplain wetlands.