Spatiotemporal patterns in nitrogen concentrations in African rivers

Precipitation and hydrological processes are important drivers of water quality, as they regulate the transport of nitrogen, as well as other substances, through various flow paths, including surface runoff, subsurface and groundwater flow. Climate change, in combination with land use and management changes, will likely affect nitrogen concentrations in rivers and streams, with potential consequences for aquatic ecology and the suitability of water for drinking, agricultural and industrial use. Understanding drivers of nitrogen concentrations can help to improve our ability to predict the impact of future changes on water quality. This study presents an overview of the current knowledge on nitrogen and drivers of spatiotemporal patterns in African rivers and streams and identifies avenues for future research.

Data on nitrogen concentrations were extracted from 243 peer-reviewed studies conducted in sub-Saharan Africa, covering 32 out of 48 countries. Differences were observed between sites characterised by different land use types, with urban sites having highest median total nitrogen and nitrate concentrations (3.9 and 1.2 mg N L⁻¹, respectively), most likely resulting from wastewater discharge. Seasonality influences nitrogen concentrations, showing higher or lower concentrations during the wet season indicating increased inputs or dilution processes, respectively, depending on the nitrogen compound and land use type. These findings highlight the importance of having a thorough understanding of nitrogen transport and transformation processes. Yet, compared to other continents, only few studies investigated these processes in African rivers and stream. Furthermore, only very few long-term (> 8 years) studies are available, impeding the analysis of trends in nitrogen concentrations alongside changes in climate and land cover. To strengthen the knowledge base and improve our ability to predict climate and land use change impacts on water quality, long-term monitoring as well as in-depth research on the underlying processes are required, also covering those parts of Africa, which are currently understudied.