Anthropogenic activities affecting Arreo Lake (N Spain) during the last 2500 years

Arreo Lake is a small (288 ha surface area) karstic lake 25 m deep located at the northwestern edge of the Ebro Basin (NE Spain). The integration of sedimentary facies, element geochemistry, mineralogy, and biological proxies (pollen and diatoms), together with a robust chronological model provided by 15 AMS radiocarbon dating, 137Cs analyses, and varve counting, permitted a reconstruction of the main phases of anthropogenic activity affecting the Arreo Lake dynamic in the context of the climate variability in the Iberian Peninsula during the last two millennia. A high-resolution study of the lacustrine facies and diatoms, combined with their detailed comparison with recent regional instrumental climatic data (1952-2007), limnological monitoring of the lake (1992-2008), and recent land-use changes affecting the lake watershed show the strong influence of human activities in lake dynamics during the last 60 years. The main impacts are a large increase in sediment delivery to the lake after the 1980s, fluctuations in lake level caused by water extraction for irrigation, and changes in the mixing status of the lake. Littoral and distal sediment cores record the long history of the use of natural resources (salt, water, forest and farming) and their significant impacts in the lake during the last 2500 years. Periods of higher anthropogenic activities linked to increased salt production in the nearby Salinas de Añana during the Roman Period and the Early Middle Ages were coincident with deforestation and increased sediment delivery to the lake. The Modern period was characterized by an abrupt increase in the sedimentation rate. Forest expansion and reduced clastic input to the lake were synchronous with documented depopulation of the area during the Late Middle Ages and the 20th century. The synergy between climate and human activities is shown by the correspondence of increased human pressure and more favourable climate conditions, such as it is recorded during the Roman Humid Period and the Medieval Climate Anomaly. Paleolimnological reconstructions provide the framework for management policies to integrate the use of lake and watershed resources and the preservation of unique lacustrine ecosystems like Arreo Lake.