Drivers of stream metabolism in anthropogenically disturbed mountainous streams of Uganda.

Stream metabolism is a fundamental ecosystem function that includes gross primary productivity (GPP) and ecosystem respiration (ER). These processes measure the energy supply and consumption in the aquatic system and are the basis for the green and brown food webs. Because of their sensitivity to environmental stressors, they are important in aquatic ecosystem management as functional indicators of the ecosystem’s health. They also have the benefit of being an integrative indicator of ecosystem change since they are influenced by multiple factors at different scales. Although the drivers of metabolism in river systems are known, there is great variation in the factors controlling stream metabolism within and between individual river systems due to natural and anthropogenic drivers. Additionally, limiting factors can vary from one system to another, leading to distinct metabolic regimes.
In Uganda's mountainous regions, the interaction between natural factors such as elevation and human-induced disturbances, including deforestation, agriculture and urbanization can cause metabolic patterns to deviate from those predicted for headwater streams. Understanding the drivers of stream metabolism in these anthropogenically impacted ecosystems is therefore crucial for their sustainable management. Considering the increased impact of anthropogenic activities on headwater streams and the general lack of understanding of the drivers of metabolism in these systems, this study examined the drivers of metabolism in anthropogenically disturbed headwater montane streams in western Uganda.
Over 7 months, metabolism and its hypothesised drivers were measured in 11 tropical stream reaches at high elevation. Stepwise regression was used to build models to understand the factors influencing GPP and ER at catchment and local scales. At large scales, stream order, catchment area, and percentage of agriculture and forest cover influenced GPP, while stream order, elevation and the percentage of urban land use influenced ER. Structural equation modelling showed that catchment factors influenced GPP through effects on local drivers such as stream width, ammonia and phosphorous concentrations in sediments, turbidity and canopy cover. On the other hand, the catchment drivers controlled ER through influence on discharge, temperature, phosphorus, and ammonia. Our results suggest that metabolism in mountainous streams is not only affected by anthropogenic activities, but elevation also plays an important role for the observed patterns. The high elevation and steep slopes initiate further sediment-related processes, erosion and sedimentation, influencing metabolism.