Drivers of Greenhouse Gas Emissions in Tropical Riverine Systems Across a Gradient of Human Impact

ABSTRACT

Tropical lotic ecosystems are important sources of greenhouse gas emissions, yet the magnitude and variability of such emissions remain poorly constrained. Here, we quantified spatial and short‐term temporal variability of carbon dioxide (CO ₂ ) and methane (CH ₄ ) fluxes across tropical riverine systems spanning a gradient of human impact, while identifying potential biogeochemical and physical drivers of these fluxes. All systems acted as net sources of both gases. Fluxes varied widely among sites [49–1,701 (CO ₂ ) and 0.6–85.9 mmol m ⁻² d ⁻¹ (CH ₄ )], while short‐term temporal variability was limited, likely due to relatively uniform climatic conditions during the study period. In particular, treated wastewater inputs, not properly captured by conventional land use metrics, contributed to elevated emissions. CO ₂ emissions were driven by ecosystem respiration, highlighting the role of net heterotrophy, with additional contributions from sediment inorganic carbon and methane fluxes. By contrast, CH ₄ emissions were more closely linked to pH and gas exchange velocity, reflecting the combined influence of sediment production and physical transport processes. These findings advance understanding of the controls on carbon emissions in tropical riverine systems, emphasizing the role of wastewater inputs and metabolic processes in shaping and scaling emissions. They also provide a framework for improving the representation of tropical river networks in regional and global budgets and for guiding strategies to mitigate emissions in impacted freshwater systems.