ABSTRACT:

Streams in high-elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce and estimates of CO2 fluxed are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, ‘Kinematic k600’ (k600-K), is derived from an empirical relationship between temperature-adjusted k (k600) and the physical characteristics of the stream. The second method, ‘Measured k600’ (k600-M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5-week period during the wet season of a peatland-stream transition within a páramo ecosystem located above 4,000m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250-m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600-K ranged from 7.42 to 330 m d-1 (mean =116 ± 95.1 m d-1), whereas values of k600-M ranged from 23.5 to 444 m d-1 (mean = 121 ± 127 m d-1). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L s-1). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L s-1). Our study provides the first estimates of k600 values in a high elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co-regulating gas transfer velocities in páramo streams.

ABSTRACT:

High-altitude tropical grasslands, known as “páramos", are characterized by their high solar radiation, high precipitation, and low temperature. They exhibit some of the highest rates of ecosystem carbon storage per unit area on Earth. Recent observations have shown that paramos may be a net source of CO2 to the atmosphere as a result of climate change; however, little is known about the source of this excess CO2 in these mountainous environments or whether specific landscape positions may be disproportionally contributing more CO2 than others. We evaluated the spatial and temporal variability of surface CO2 fluxes from adjacent terrestrial and aquatic environments based on a suite of field measurements performed over seven weeks. Our findings revealed the importance of hydrologic dynamics in regulating the magnitude and likely fate of dissolved carbon in the stream. While headwater catchments are known to contribute disproportionately larger amounts of carbon to the atmosphere than their downstream counterparts, our study highlights the spatial heterogeneity of CO2 fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Stream carbon flux to the atmosphere appeared to be transport-limited (i.e., controlled by flow characteristics, turbulent flow, water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by carbon availability) in the lower reaches of the stream. These findings represent first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high-altitude, tropical, headwater catchments.

ABSTRACT:

Inland waters emit large amounts of carbon and are key players in the global carbon budget. Particularly high rates of carbon emissions have been reported in streams draining mountains, tropical regions, and peatlands. However, few studies have examined the spatial variability of CO2 concentrations and fluxes occurring within these systems, particularly as a function of catchment morphology. Here we evaluated spatial patterns of CO2 in three tropical, headwater catchments in relation to the river network and stream geomorphology. We measured dissolved carbon dioxide (pCO2), aquatic CO2 emissions, discharge, and stream depth and width at high spatial resolutions along multiple stream reaches. Confirming previous studies, we found that tropical headwater streams are an important source of CO2 to the atmosphere. More notably, we found marked, predictable spatial organization in aquatic carbon fluxes as a function of landscape position. For example, pCO2 was consistently high (>10,000 ppm) at locations close to groundwater sources and just downstream of hydrologically connected wetlands, but consistently low (<1,000 ppm) in high gradient locations or river segments with larger drainage areas. Taken together, our findings suggest that catchment area and stream slope are important drivers of pCO2 and gas transfer velocity (k) in mountainous streams, and as such they should be considered in catchment-scale assessments of CO2 emissions. Furthermore, our work suggests that accurate estimation of CO2 emissions requires understanding of dynamics across the entire stream network, from the smallest seeps to larger streams.