Future nutrient pollution poses a threat to Africa’s freshwater fishes

Nakkazi, M.T., Nkwasa, A. ORCID: https://orcid.org/0000-0002-8685-8854, La Fuente, S., Lekarkar, K.L., Suresh, K., Peyton, J. ORCID: https://orcid.org/0000-0002-8313-6194, Beusen, A., & van Griensven, A. (2026). Future nutrient pollution poses a threat to Africa’s freshwater fishes. Environmental Research: Water 10.1088/3033-4942/ae892f. (In Press)

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Abstract

Excessive levels of nutrients, particularly nitrogen and phosphorus, trigger eutrophication leading to harmful algal blooms and oxygen depletion that endangers freshwater fish species. Despite widespread awareness of these risks, efforts to protect freshwater fish species remain largely ineffective, especially under climate change and other anthropogenic pressures. Here, we assess present and future nutrient pollution risks across African river systems using two water quality models, IMAGE-GNM and SWAT+, under the SSP5–RCP8.5 scenario for 2010 and 2050. We evaluate total nitrogen (TN) and total phosphorus (TP) concentrations against ecological thresholds derived from fish metrics for freshwater fish species (0.7 mg N/L and 0.07 mg P/L). Both models project increasing nutrient pollution risks between 2010 and 2050 across major river basins and ecoregions, although SWAT+ consistently estimates higher impacts than IMAGE-GNM. By 2050, IMAGE-GNM projects that 42% of freshwater fish species will have at least half of their geographical range threatened by nutrient pollution, while SWAT+ projects similar high threats for more than 70% of species. Across all ecoregions, the potentially affected fraction of species is projected to increase between 2010 and 2050 with major river basins such as Niger, Volta and Turkana having over 80% of their fish species potentially affected by 2050. High risk conditions are particularly evident in habitat types such as temperate upland rivers, coastal rivers, and large lake basins. Our findings highlight regions where proactive management and policy interventions should be prioritised to mitigate the adverse effects of nutrient pollution on freshwater fish biodiversity in Africa.

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