Recent expansion of anoxia has become a global issue and there is potential for worsening under global warming. At the same time, obtaining proper long-term instrumental oxygen records is difficult, thus reducing the possibility of recording long-term changes in oxygen shifts that can be related with climate or human influence. Varved lake sediments provide the better time frame to study this phenomenon at high resolution. We tracked the oxic/anoxic shifts of the varved Lake Montcortès since 1500 CE, and tried to recognise anthropogenic and climatic influences combining biological and geochemical proxies. Four main scenarios emerged: 1) years with abrupt sediment inputs (A); 2) years with outstanding mixing and oxygenation of the water column (B); 3) years with strong stratification, anoxia, intense sulfur bacterial activity and increased biomass production (C); 4) years with stratification and anoxia, but relatively less biomass production (D). In line with current limnologic trends, high supra-annual variability in the occurrence of oxygenation events was observed. Interestingly, at least 45.3% of the years were mixing years and, like the meromictic ones, were mostly clustered into groups of consecutive years, thus alternating years of monomixis with years of meromixis. Most years of D belong to the period 1500–1820 CE, when human activities were the most intense. Most years of A belonged to the climatic unstable period of 1850–1899 CE. Years of B were irregularly distributed but were best represented in the period 1820–1849 CE. Most years of C belonged to the 20th century. More than 90% of the years with climatic instrumental records belonged to B and C. Current climate warming seems to be taking control over the oxygenation capacity of the lake, especially since the second half of the 20th century. Our results support recent findings related to hypoxia spreading at the global scale.
Bibliographical noteFunding Information:
Elisabet Safont, Núria Cañellas, Joan Gomà, Santiago Giralt, Sandra Garcès, Arantzazu Lara and Eric Puche helped performing the fieldwork. Important demographic and paleoclimate information was provided by Jesús Sánchez (Arxiu Comarcal de Sort) and Myriam Khodri, (IRD/CNRS Université P. et M. Curie), respectively. We acknowledge the Council of Baix Pallars, the cultural association Lo Vent de Port and the Busseing Pallars Company for their direct involvement in the project and their continuous support. Funding was granted by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (projects MONT-500, ref. CGL2012-33665 ; GLOBALKARST, ref. CGL2009-08145 ), the Institute of Catalan Studies (PIRIMOD and POLMONT), and the Catalan University and Research Management Agency ( AGAUR ) (project 2014 SGR 1207 ). Training research grants were awarded to M. Carmen Trapote and Núria Pérez-Zanón and funded by MINECO (BES-2013-065846) and the University Rovira i Virgili (Martí Franquès program, 2104PMF-PIPF-22), respectively. Fieldwork permits were provided by the Territorial Service of the department of Agriculture, Livestock, Fishing and Natural Environment of Catalonia.
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