Drivers of stream metabolism in anthropogenically impacted tropical highland streams of Uganda

Headwater streams in tropical highlands provide critical ecosystem services by supporting biodiversity, regulating nutrient and carbon cycles, and maintaining water quality in downstream ecosystems. However, their strong dependence on surrounding landscapes makes them highly vulnerable to anthropogenic disturbances, which can have detrimental effects on their structure and functioning. Stream metabolism is a key indicator of ecosystem functioning, but its drivers in tropical highland streams remain poorly understood. To investigate these drivers, gross primary production (GPP) and ecosystem respiration (ER) were quantified across 11 anthropogenically impacted headwater streams in Uganda over a seven-month period, assessing the influence of proximal (in-stream) and distal (landscape) factors across various spatial scales. GPP was primarily limited by light, with turbidity reducing production in high-elevation streams and canopy cover limiting production in low-elevation sites. ER, on the other hand, increased in response to stream size and nutrient-rich sediments from steep, anthropogenically disturbed slopes. Overall, the combined effects of anthropogenic activities (agriculture, bare land, and built-up areas) and topography (elevation and slope) strongly influenced stream metabolism at distal scales. Landscape drivers affected metabolism indirectly through their impact on proximal factors, with topography often overriding anthropogenic effects. GPP was far lower than ER, as is typical for most tropical headwater streams. However, net heterotrophy in our study was primarily controlled by total suspended solids (TSS) rather than by commonly reported drivers such as terrestrial organic matter inputs and riparian shading. These findings suggest that climate-driven extremes, such as intense precipitation, are likely to amplify net heterotrophy in anthropogenically impacted tropical highland streams, with cascading effects on carbon cycling, water quality, and ecosystem functioning. • GPP declined at high elevations due to light limitation while ER increased. • Nutrients and sediment inputs at high elevations enhanced ecosystem respiration. • Anthropogenic land use and topography jointly shaped metabolic rates.