Abstract
Urea, the world’s predominant nitrogen fertilizer, has supported human population growth for the past 60 years, yet its effects on freshwater ecosystems are largely unknown. Here urea additions at ecologically relevant rates tripled summer phytoplankton abundance in replicate agricultural reservoirs with significant responses by most eukaryotic algae, but not cyanobacteria or their toxins. Mass budgets reveal that fertilized reservoirs did not become limited by phosphorus due to its continuous release from sediments. Further, most added nitrogen did not accumulate in reservoirs but was lost to the atmosphere, probably as NH3. Sub-continental spatial analysis shows that study reservoirs are characteristic of shallow water bodies within Canada’s largest agricultural region, where >40% of surface waters are vulnerable to degradation by urea. Similar degrees of water quality loss by urea are expected in other global agricultural regions (for example, China, India, North America) where elevated urea use interacts with phosphorus-rich surface waters to induce extreme eutrophication.
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Differential effects of sole and phosphorus based nitrogen fertilizer sources on wheat growth and nutrient use efficiency
Data availability
Data files are available via GitHub at https://github.com/calegushulak/Urea-reservoir-experiment. These include a nutrient dataset from Chinese lakes that were used to produce Supplementary Fig. 16.
Code availability
Code for analyses is available via GitHub at https://github.com/calegushulak/Urea-reservoir-experiment.
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Acknowledgements
We thank J. Kotylak and P. Kotylak for allowing this study to take place on their property. This project was funded by the National Science and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI), the Canada Research Chairs program (CRC), the province of Saskatchewan and the University of Regina. Taxonomic analysis of phytoplankton was conducted at the Forensic Phycology Laboratory (Clear D Results Corp.). We thank Clear D Results Corp. for the laboratory resources used to support algal taxonomy, enumerations and biovolume quantification. This study is a contribution of the Qu’Appelle Valley long-term ecological research programme (QU-LTER). Analyses and conclusions presented in this article do not necessarily reflect the opinions or official position of the Saskatchewan Water Security Agency or the Government of Saskatchewan. This research was conducted on Treaty 2, 4, and 6 lands, territories of the Nêhiyawak (Cree), Anihšināpēk (Saulteaux), Dakota, Lakota and Nakoda First Nations and the homeland of the Métis Nation. We acknowledge the responsibility of sharing these lands in faithful accordance of all treaties and are grateful for First Nations leadership in protection of the natural environment.
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C.A.C.G. designed the study, collected data, performed most statistical analyses, data interpretation, and figure drafting and wrote the paper. A.M.C. conducted landscape analyses and produced maps. D.B. coordinated the ecosystem experiments. J.L. collected cyanobacterial toxin data. H.A.S. conducted the lake survey. V.S. conducted the sediment analyses. M.D.G. conducted enumerations and identification of phytoplankton from water samples. C.N.C and M.J.B. estimated greenhouse gas concentrations, ecosystem C content, and modelled greenhouse gas fluxes. S.J., J.R.W. and K.F. conducted wetland pond and small reservoir surveys. V.J.S. quantified urea biogeochemistry in regional waters. M.H. conducted ecotoxicity experiments. Y.Z. provided and analysed lake data from China. P.R.L. designed and funded all components of the study. All authors edited the paper and approved the final version.
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M.D.G. is an employee of Clear D Results Corp., which provided laboratory facilities for the taxonomic analysis of phytoplankton. This relationship did not influence the scientific integrity or the interpretation of the data presented in this study. The other authors declare no competing interests.
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Gushulak, C.A.C., Chegoonian, A.M., Lerminiaux, J. et al. World’s predominant nitrogen fertilizer induces extreme eutrophication of surface waters in central North America.
Nat Water (2026). https://doi.org/10.1038/s44221-026-00636-7
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DOI: https://doi.org/10.1038/s44221-026-00636-7
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