Researchers Investigate Cause of Low Oxygen Levels in Lakes

Science has been aware of the lack of oxygen in lake waters for more than 160 years. Increased amounts of waste water in our towns and cities and the use of artificial fertilizers in agriculture have had a growing impact on the ecological balance in lakes all over Europe. A group of international researchers – among them Geography Professor BerndZolitschka from the University of Bremen – recently completed a study on the subject. Their research results have now been published in the journal “Proceedings of the National Academy of Sciences”. They show that the regeneration of our lakes is only progressing slowly.

As long ago as 1850, scientists already knew about the development of low oxygen concentrations in many European lakes. The term hypoxia describes the oxygen deficiency resulting from an excessive accumulation of nutrients in lake waters or, if you want to get really technical, anthropogenic eutrophication. It is brought about by the environmental impact of humans, who have been seriously disturbing the ecological balance of still water lakes since the turn of the twentieth century, long before the large scale use of artificial fertilizers (starting in the 1950s) or global climate change (first proven in the 1970s). An international team of researchers has now shown that change in our lifestyles and the associated growth of population centers (urbanization) are the cause of low oxygen levely in a large number of lakes in Europe.

The results of the study indicate that the effluents contained in waste water since the start of the twentieth century increased the biological productivity in lakes which subsequently led to higher oxygen consumption. Researchers from Germany, Finland, France and Canada – among them Geography Professor BerndZolitschka from the University of Bremen – just published these research results in the renowned journal “Proceedings of the National Academy of Sciences”. The study draws on work carried out by the “Varve Working Group”, which is part of the International Geospheres-Biospheres Program, IGBP-PAGES (Past Global Changes), which BerndZolitschka likewise participated in.

Urban waste water is a main cause of oxygen deficiency

The researchers analyzed everything that could possibly have caused the problem, including climatic parameters, historical land use, and the sedimentation of over 1,500 catchment basins feeding European lakes. For the first time ever on a continental scale, they compared reconstructions of land utilization and its development over time with data relevant for oxygen consumption over the past 300 years. This enabled them to identify urban waste water, especially dissolved phosphorous, as the main causal factors for the alarming increase of hypoxia at the bottom of lakes since the turn of the twentieth century.

The second main cause is fertilizer used in agriculture

The high variation in regional environmental factors as well as their interactions – but also uncertainties in the longitudinal study – presented the authors of the study with quite a challenge. It also had to be taken into account that both point as well as diffuse sources contribute to nutrient accumulation in lake waters. Moreover, their impact varies in space and over time. The presented results document the significance of point sources of urban waste water as the dominant cause of eutrophication in European lakes during the current epoch of the Earth’s history – the Anthropocene. However, over past decades in industrial countries these point sources of urban waste water have been replaced as the main causal factor of eutrophication by diffuse sources as result of the increased use of fertilizer and the removal of point nutrient sources by sewage treatment plants.

Lakes are regenerating only slowly, in spite of improved environmental protection

Despite an overall improvement to the environment in most lakes’ catchment basins since the 1980s, the deepest layers of water remain free of oxygen and the once-established hypoxia continues to exist. The authors therefore conclude: “This extremely slow reaction of the lakes’ systems underlines the importance of historical land utilization studies as well as the need for long-term strategies to maintain and improve the quality of lake waters”.

Publication: “Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes”: Jenny, J.-P., A. Normandeau, P. Francus, Z.E. Taranu, I. Gregory-Eaves, F. Lapointe, J. Jautzy, A.E.K. Ojala, J.-M. Dorioz, A. Schimmelmann, and BerndZolitschka. Proceedings of the National Academy of Sciences (PNAS). 

If you would like to have more information, feel free to contact:
University of Bremen
Faculty of Social Sciences
Institute for Geography
Phone: +49 421-218.67150
email: zoliprotect me ?!uni-bremenprotect me ?!.de

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