Cascading effects of land use on ecosystem function in Afrotropical headwater streams

Abstract Anthropogenic activities in catchments, such as urban and agricultural land use, negatively impact the biogeochemical cycles of carbon, nitrogen and phosphorus in streams by increasing concentrations of these nutrients and altering the composition of dissolved organic matter (DOM). In tropical climates with high temperatures and intense precipitation, streams are particularly vulnerable to high loading from the catchment. The combination of high nutrient loading from the catchment and high processing rates at high temperatures can lead to even higher concentrations and more severe impacts on biogeochemical cycles. However, studies linking human activity to changes in nutrient and DOM composition, and the resulting impacts on stream functions, are still scarce in tropical streams. This study addressed this gap by examining the relationships between land use and water column chromophoric DOM (CDOM), nitrogen and phosphorus across seasons in an Afrotropical watershed. In addition, the effects of nutrient enrichment and changes in DOM composition on stream metabolism were investigated. The results showed that urban land use had the most substantial influence on nutrient concentrations and DOM composition in the studied streams. Streams with a high proportion of urban land use in their riparian zone had high nutrient concentrations and a pronounced autochthonous DOM signal. In contrast, streams with more forest cover in their riparian zone had lower nutrient concentrations and a more allochthonous DOM signal in their water column. Stream metabolism increased with nutrient concentrations and autochthonous organic matter contribution, and these changes were more pronounced in the dry season, pointing to the combined effects of high nutrient loading and processing rates on carbon biogeochemistry. These results confirm that changes in nutrient loading and organic matter composition caused by human activities and seasonal changes will likely impact river ecosystem processes, with implications for food webs and tropical biogeochemical cycles.