Global warming affects nutrient upwelling in deep lakes
Measures to reduce lake phosphorus concentrations have been encouragingly successful in many parts of the world. After significant eutrophication in the twentieth century, nutrient concentrations have declined in many natural settings. In addition to these direct anthropogenic impacts, however, climate change is also altering various processes in lakes. Its effects on lacustrine nutrient budgets remain poorly understood. Here we investigate the total phosphorus (TP) concentrations in the epilimnion of the meromictic Lake Zug under present and future climatic conditions. Results are compared with those of other deep lakes. Data showed that TP transported from the hypolimnion by convective winter mixing was the most important source of TP for the epilimnion, reaching values more than ten times higher than the external input from the catchment. We found a logarithmic relationship between winter mixing depth (WMD) and epilimnetic TP content in spring. Warming climate affects WMD mainly due to its dependence on autumn stratification. Model simulations predict a reduction of average WMD from 78 (current) to 65 m in 2085 assuming IPCC scenario A2. Other scenarios show similar but smaller changes in the future. In scenario A2, climate change is predicted to reduce epilimnetic TP concentrations by up to 24% during warm winters and may consequently introduce significant year-to-year variability in primary productivity.
KeywordsLimnology Climate change Winter mixing Phosphorus Lake Zug
We sincerely thank the anonymous reviewers and the editor for their valuable comments. In addition, we are grateful to René Gächter for valuable comments to an earlier version of the manuscript. We also acknowledge the support of the Amt für Umweltschutz Zug, especially Bruno Mathis and Peter Keller, who provided observational data from Lake Zug. Long-term monitoring data for Lake Geneva were provided by the Commission International pour la Protection des Eaux du Léman (CIPEL) and by the SOERE OLA Information System (http://si-ola.inra.fr), INRA Thonon-les-Bains. Data from Lake Zürich were provided by AWEL Zürich. The CH2011 climate predictions were obtained from the Center for Climate Systems Modeling (C2SM; www.ch2011.ch). Meteorological data of this study are archived and distributed by the Swiss Federal Office of Meteorology and Climatology, MeteoSwiss (obtained via their data portal, IDAWEB). Additional information on the data can be provided on request by firstname.lastname@example.org. The source code for SIMSTRAT can be accessed through the archived GIT repository (link: https://github.com/adrien-ga/Simstrat-BSIW/tree/v1.3, https://doi.org/10.5281/zenodo.841084). The first author was supported by Swiss National Science Foundation grants 200021_146652 and 200020_165517.
We dedicate this article to the memory of our wonderful friend and colleague Adrien Gaudard who unexpectedly passed away after a recent avalanche accident.
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