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Abstract Efforts to reduce nitrogen and carbon loading from developed watersheds typically target specific flows or sources, but across gradients in development intensity there is no consensus on the contribution of different flows to total loading or sources of nitrogen export. This information is vital to optimize management strategies leveraging source reductions, stormwater controls, and restorations. We investigate how solute loading and sources vary across flows and land‐use using high frequency monitoring and stable nitrate isotope analysis from five catchments with different sanitary infrastructure, along a gradient in development intensity. High frequency monitoring allowed estimation of annual loading and attribution to storm versus baseflows. Nitrate loads were 16 kg/km²/yr. from the forested catchment and ranged from 68 to 119 kg/km²/yr., across developed catchments, highest for the septic served site. Across developed catchments, baseflow contributions ranged from 40% of N loading to 75% from the septic served catchment, and the contribution from high stormflows increased with development intensity. Stormflows mobilized and mixed many surface and subsurface nitrate sources while baseflow nitrate was dominated by fewer sources which varied by catchment (soil, wastewater, or fertilizer). To help inform future sampling designs, we demonstrate that grab sampling and targeted storm sampling would likely fail to accurately predict annual loadings within the study period. The dominant baseflow loads and subsurface stormflows are not treated by surface water management practices primarily targeted to surface stormflows. Using a balance of green and gray infrastructure and stream/riparian restoration may target specific flow paths and improve management. 

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 CO₂fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO₂evasion and other biogeochemical processes. The first method, kinematick₆₀₀(k_600‐K), is derived from an empirical relationship between temperature‐adjustedk(k₆₀₀) and the physical characteristics of the stream. The second method, measuredk₆₀₀(k_600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO₂(pCO₂) and CO₂evasion 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 4000 m 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 ink₆₀₀at small spatial scales. Values ofk_600‐Kranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values ofk_600‐Mranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability ink₆₀₀was 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). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates ofk₆₀₀values 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 high solar radiation, high precipitation, and low temperature. They also exhibit some of the highest ecosystem carbon stocks per unit area on Earth. Recent observations have shown that páramos may be a net source of CO₂to the atmosphere as a result of climate change; however, little is known about the source of this excess CO₂in these mountainous environments or which landscape components contribute the most CO₂. We evaluated the spatial and temporal variability of surface CO₂fluxes to the atmosphere from adjacent terrestrial and aquatic environments in a high‐altitude catchment of Ecuador, based on a suite of field measurements performed during the wet season. 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 CO₂fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Our findings revealed that CO₂evasion from stream surfaces was up to an order of magnitude greater than soil CO₂efflux from the adjacent terrestrial environment. Stream carbon flux to the atmosphere appeared to be transport limited (i.e., controlled by flow characteristics, turbulent flow, and water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by CO₂and carbon availability) in the lower reaches of the stream. A 4‐m waterfall along the channel accounted for up to 35% of the total evasion observed along a 250‐m stream reach. These findings represent a first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high‐altitude, tropical, headwater catchments.