Future nutrient pollution increases risk to Africa’s freshwater fish biodiversity

Excessive levels of nutrients, particularly nitrogen and phosphorus, can trigger eutrophication leading to harmful algal blooms and oxygen depletion thus endangering freshwater fish species. Despite the widespread awareness of these risks, efforts to protect freshwater fish species remain largely ineffective, especially under climate change and other anthropogenic pressures. This study therefore identifies potential future hotspots for nutrient pollution in African river systems, specifically assessing the risk that elevated total nitrogen (TN), and total phosphorus (TP) levels pose to freshwater fish species. We identified areas where nutrient concentrations are likely to exceed critical thresholds, that could increase the occurrence of eutrophication and thus threatening freshwater fish biodiversity. Using two large-scale water quality models; SWAT+ and IMAGE-GNM, we analysed annual concentrations of TN and TP for 2010 and 2050 under the combined shared socio-economic pathways (SSPs) and representative concentration pathways (RCPs) combined scenario, SSP5-RCP8.5. Basing on the United Nations Environment Programme (UNEP) target thresholds used for the assessment of SDG indicator 6.3.2 that designates a waterbody as having “good ambient water quality”, both models predicted that from 2010 to 2050, the percentage of African rivers exceeding the critical thresholds of 0.7 mg/L for TN and 0.02 mg/L for TP will increase by 15% under the SSP5-RCP8.5 scenario. High nutrient levels in river basins such as the Niger, Nile, and Limpopo overlap with areas of high fish species richness, posing a significant threat of exposure to nutrient pollution. At continental scale, from 2010 to 2050, the proportion of freshwater fish species at high risk from TN pollution significantly increased by 23%. In contrast, >90 % of fish species remained highly vulnerable to TP pollution throughout the 2010 and 2050 periods. Our findings highlighted regions where proactive management and policy interventions should be prioritised to mitigate the potential adverse effects of nutrient pollution on freshwater fish biodiversity.