Sources and driving factors of greenhouse gas emissions from tropical high-mountain Andean lakes: marks of human impact.

Tropical high-mountain lakes represent a biogeochemically distinct but poorly characterized subset of global lake research, with their greenhouse gas (GHG) dynamics remaining largely unexplored. Here, we present the first comprehensive assessment of CO2, CH4 and N2O fluxes, dissolved concentrations, and isotopic signatures in high-mountain Andean lakes (3,100-4,450 m elevation). We combined diel measurements, depth profiling, and stable isotope analyses (δ13C-CO2, δ13C-DIC, δ13C-CH4, and δ15N- and δ18O-N2O) to characterize GHG sources, production pathways, and environmental drivers across gradients of morphometry, lithology, and human impact. Shallow wetland-connected lakes functioned as strong CO2 (up to 1,639 mg m-2 d-1) and CH4 (up to 604 mg m-2 d-1) sources but persistent N2O sinks (up to -45.4 µg m-2 d-1), while deeper, less connected lakes exhibited lower CO2 and CH4 emissions and slight N2O effluxes. Diel patterns revealed metabolic control over CO2 dynamics, with nighttime fluxes 3.2 times higher than daytime values, and temperature-driven episodic CH4 flux peaks in the shallowest systems. Deep lakes maintained well-oxygenated water columns with respiratory CO2 accumulation at depth, while elevated surface CH4 suggested the presence of lateral littoral inputs and oxic methane paradox production. Isotope signatures indicated that CO2 originated primarily from C3 plant organic matter respiration and bedrock composition (δ13C-CO2: -40 ‰ to -16 ‰), CH4 from biogenic methanogenesis with partial oxidation during upward transport, and minimal N2O cycling under nitrogen-limited conditions. Anthropogenic disturbance amplified CH4 emissions by an order of magnitude while enhancing N2O consumption through complete denitrification. Our findings demonstrate that Andean remote oligotrophic lakes are sensitive to even minor catchment disturbance and exhibit distinct GHG dynamics given their unique characteristics, with implications for incorporating high-mountain lakes into global emission inventories.