Spatiotemporal dynamics of CO2 gas exchange from headwater mountain streams

Mountain streams play an important role in the global carbon cycle by transporting, metabolizing, and exchanging carbon they receive from the terrestrial environment. The rates at which these processes occur remain highly uncertain because of a paucity of observations and the difficulty of measuring gas exchange rates in steep, turbulent mountain streams. This uncertainty is compounded by large temporal and spatial variability in stream carbon dioxide (CO₂) concentrations in mountain environments. In this study, we measured diel, seasonal, and annual variations in CO₂ partial pressure (pCO₂) in seven headwater streams and a groundwater spring in the Colorado Rocky Mountains to determine how CO₂ exchange fluxes () vary with time, annual precipitation, and landscape characteristics. Our results show that temporal variability in pCO₂ and  in mountain streams is large and is strongly influenced by solar radiation, the accumulation and melting of seasonal snowpacks, and interannual variations in precipitation. Spatial variations in pCO₂ and  were related to landscape characteristics, with soil organic matter, wetlands, and likely groundwater discharge zones having a positive influence. Periglacial features, such as ice and rock glaciers, had a negative influence on stream pCO₂ and . Estimated  from streams in an alpine/subalpine region of Colorado was 3.4 kg C m⁻² yr⁻¹ normalized to stream surface area (95% CI: 2.1–5.0 kg C m⁻² yr⁻¹), consistent with recent work on CO₂ exchange from mountain streams in the Swiss Alps. Our results highlight the importance of mountain streams as substantial contributors in the global carbon cycle.