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World’s predominant nitrogen fertilizer induces extreme eutrophication of surface waters in central North America

Nature Water (2026)Cite this article

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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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Fig. 1: Changes in phytoplankton abundance measured as biomarker pigments in triplicate fertilized and reference reservoirs across the length of experiment expressed as day of year in 2021.
Fig. 2: Cumulative mass of urea added to each reservoir and the sum of masses of N species in individual dissolved and particulate pools standardized to reservoir surface area during the experiment.
Fig. 3: Ubiquitous application of urea fertilizer has likely degraded surface water quality across the grasslands of central North America.

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.

Author information

Author notes

  1. Cale A. C. Gushulak

    Present address: Department of Biological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada

  2. Deirdre Bateson

    Present address: Saskatchewan Water Security Agency, Saskatoon, Saskatchewan, Canada

  3. Heather A. Sauer

    Present address: Saskatchewan Water Security Agency, Regina, Saskatchewan, Canada

  4. Sydney Jensen

    Present address: Saskatchewan Ministry of Environment, Regina, Saskatchewan, Canada

  5. Jackie R. Webb

    Present address: School of Science, Engineering and Digital Technologies, University of Southern Queensland, Toowoomba, Queensland, Australia

  6. Vanessa J. Swarbrick

    Present address: Office of the Chief Scientist, Government of Alberta, Edmonton, Alberta, Canada

Authors and Affiliations

  1. Institute of Environmental Change and Society, University of Regina, Regina, Saskatchewan, Canada

    Cale A. C. Gushulak, Amir M. Chegoonian, Jess Lerminiaux, Deirdre Bateson, Heather A. Sauer, Sydney Jensen, Jackie R. Webb, Vanessa J. Swarbrick, Kerri Finlay & Peter R. Leavitt

  2. Department of Biology, University of Regina, Regina, Saskatchewan, Canada

    Cale A. C. Gushulak, Jess Lerminiaux, Viraj Shah, Heather A. Sauer, Kerri Finlay & Peter R. Leavitt

  3. Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada

    Mark D. Graham

  4. Department of Biological Sciences, University of Lethbridge, Lethbridge, Alberta, Canada

    Chun Ngai Chan & Matthew J. Bogard

  5. Toxicology Centre, School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

    Markus Hecker

  6. Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China

    Yongqiang Zhou

Authors
  1. Cale A. C. Gushulak
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  2. Amir M. Chegoonian
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  3. Jess Lerminiaux
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  8. Chun Ngai Chan
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  10. Sydney Jensen
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  14. Yongqiang Zhou
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  15. Kerri Finlay
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  16. Peter R. Leavitt
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Contributions

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.

Corresponding author

Correspondence to Cale A. C. Gushulak.

Ethics declarations

Competing interests

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.

Peer review

Peer review information

Nature Water thanks Daniel Conley, Thad Scott and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Supplementary information

Supplementary Information (download PDF )

Supplementary Figs. 1–16, Tables 1–6 and references.

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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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