The researchers analyzed information such as climate, land use, and lake sediment data from more than 1,500 European watersheds. For the first time, they compared reconstructions of land occupancy and land use dynamics on a continental scale to their own data of oxygen depletion over the past 300 years. This allowed them to identify urban waste, primarily phosphorus, as the factor responsible for triggering hypoxia at the bottom of lakes starting at the beginning of the 20th century.

"Accurately identifying the source of the nutrient responsible for oxygen depletion was a real challenge because of the variations in environmental stress factors throughout the region and their interactions, as well as the reliability of long term data," explains Professor Francus of the INRS Centre Eau Terre Environnement.

"Point and diffuse sources have always contributed to nutrient supplies in lakes, but at intensities that vary in time and space," adds Jean-Philippe Jenny, now affiliated with the Max Planck Institute for Biogeochemistry in Germany. "Our results show that urban point sources of phosphorus are the dominant cause of eutrophication of European lakes during the Anthropocene."

The researchers recognize, however, that during recent decades, diffuse sources have gradually become the major cause of fresh water eutrophication in developed countries with the increase in the use of chemical fertilizers and the elimination of point sources due to the installation of waste treatment plants.

"Despite the many cleanup initiatives in the 1980s, the deepest layers in the lakes we studied still are not being reoxygenated and the hypoxia persists. This illustrates the importance of studying historical land use and the need to put long-term strategies in place to maintain and restore water quality in lakes," say the study's authors.

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Journal Reference:

1. Jean-Philippe Jenny, Alexandre Normandeau, Pierre Francus, Zofia Ecaterina Taranu, Irene Gregory-Eaves, François Lapointe, Josue Jautzy, Antti E. K. Ojala, Jean-Marcel Dorioz, Arndt Schimmelmann, Bernd Zolitschka. Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes.Proceedings of the National Academy of Sciences, 2016; 201605480 DOI: 10.1073/pnas.1605480113