Aquatic Carbon Export and Dynamics in Mountain Headwater Streams of the Western U.S.

Abstract Mountain headwater streams actively cycle carbon, receiving it from terrestrial landscapes and exporting it through downstream transport and gas exchange with the atmosphere. Although their importance is now widely recognized, aquatic carbon fluxes in headwater streams remain poorly characterized. In this study, aquatic carbon fluxes were measured in 15 mountain headwater streams and were used in a geostatistical analysis to determine how landscape characteristics influence aquatic carbon fluxes. In‐stream sensors were used to measure fluorescent dissolved organic matter (fDOM) (a surrogate for dissolved organic carbon (DOC)) at a subset of sites to characterize dynamic responses to hydroclimatic events. Wetlands have a positive influence on aquatic carbon fluxes, whereas perennial snow/ice has the opposite effect, reflecting differences in soil organic matter content in these landscapes. Mean annual temperature (MAT) has a complex influence on DOC, with peak DOC exports in basins with MAT of 0–2°C. Precipitation has a strong positive influence on aquatic carbon fluxes, and declining snowpacks in the western United States may reduce future aquatic carbon exports. fDOM (and by implication DOC) and showed strong dynamic responses to snowmelt and rain events, with fDOM increasing and decreasing during events. Combining results from this study with those from a companion study on CO 2 exchange yielded total aquatic carbon fluxes of 7.2–15.7 g C m −2 yr −1 (median = 12.22), similar to those from forests and peatlands. Given net ecosystem production (NEP) of similar magnitude, NEP calculations that do not account for losses via the aquatic pathway can substantially overestimate terrestrial carbon sequestration.