Reconstructing the environmental impact of mining on mountain lakes

Mountain lakes are particularly fragile ecosystems undergoing important ecological and depositional transformations associated with ongoing global change. However, the history of anthropogenic impacts on mountain lakes and their catchments is much longer, in many cases featuring millennia of summer pastoral farming. More recently, the growing demand for raw materials and energy linked to industrialization, particularly accelerated since the 19th century CE, meant a further increase in human impact on mountain areas. The Cantabrian Range (northern Spain) constitutes a paradigmatic case of southern European mountain range experiencing intense human impact during the past millennia and particularly the past two centuries. Here, we have reconstructed the environmental dynamics of this area during the last millennium, with a particular focus on the impact of mining, based on the multidisciplinary analysis (sedimentology, biogeochemistry, magnetic susceptibility, diatoms, pollen, charcoal and dung fungal spores) of sediment cores from Lago de La Cueva (43°03'N, 6°06'W, 1550 m a.s.l.). Fire induced deforestation during the 15th century CE increased erosion during the Little Ice Age. The onset of iron mining in the catchment 200 years ago significantly impacted the lake, increasing sedimentation rates and mining waste containing hematite and potentially toxic elements. Diatoms showed that lake regulation since the early 20th century CE severely altered the natural hydrological regime introducing rapid seasonal lake-level oscillations and increasing lakeshore erosion, water turbidity and nutrient loads. The recent environmental restoration, finished in 2006, involved the re-deposition of mine tailings. Although mining wastewater still reaches the lake, restoration effectively reduced erosion and nutrient loads. This study illustrates the complex interactions between human activities (grazing, mining, hydropower) and climate change in shaping mountain landscapes through time. Our findings highlight the usefulness of Paleolimnology to quantitatively assess the effectiveness of lake restoration programs.