Restoration of high-mountain lakes: effects on nutrient concentrations and biological communities

Lakes worldwide are under severe pressure from increasing anthropogenic impacts and global warming, creating a growing demand for restoration efforts and sustainable lake management strategies. However, there has been limited practice in restoring high-altitude lakes, particularly those under protected status. This paper focuses on passed lake restoration with special emphasis on the effects of restoration on lake ecosystems. Restoration efforts in four lakes within Rila National Park, Bulgaria, were carried out in two steps: macrophyte harvesting followed by sediment removal. These were immediately followed by a monitoring programme to evaluate the effects of human activities. The most significant effects on nutrient concentrations (PO 4 -P, NO 3 -N, TP and TN) and chlorophyll-a levels were observed one week after the restoration, with these effects diminishing one year later. Changes in bacterial metabolism were characterised by a reduction in metabolic activity, a shift from preferential utilisation of carbohydrates and polymers to an increased use of carboxylic acids and a decrease in functional richness and diversity, followed by partial recovery. Some of the changes observed in lake environments and biotic components were attributed to seasonal shifts, while others were linked to the implementation of human activities. We conclude that the initial steps of lake restoration were effective, with minimal to negligible environmental impact. However, the long-term effectiveness of restoration remains uncertain, so the monitoring programme will continue to address still unresolved questions, particularly to distinguish between seasonal and anthropogenic effects on ecosystems.