Making Sense of Sensors: Stream Carbon Flux Determination at the Five USGS WEBB Watersheds

Abstract:

At the five diverse forested and/or alpine headwater sites of the U.S. Geological Survey (USGS) Water, Energy, and Biogeochemical Budgets (WEBB) program, we measure stream concentrations of all forms of carbon – DOC, PC (assumed to be POC), CO₂, CH₄, and bicarbonate alkalinity. One goal of this work is to estimate DIC, DOC, and POC fluxes from headwater landscapes. For DOC, POC, and CO₂, we utilize high-frequency measurements with in situ sensors. We also take discrete samples over a range of hydrologic conditions for independent flux calculations, and to verify the sensor measurements and test proxy relations. In situ fluorescing dissolved organic matter (FDOM) serves as a proxy for DOC. Turbidity is needed to adjust FDOM, and also serves as a proxy for POC. DIC flux is computed from the discrete alkalinity analyses.

Sensor measurements at the five sites – in Colorado, Wisconsin, Vermont, Georgia, and Puerto Rico -- began either in 2011 or 2012. The sensors generally functioned well, but occasional down time required gap filling, typically using strong relations with stream discharge. We computed at least one full year of carbon flux at each site. In this paper we compare sensor-based and sampling-based carbon flux estimates. We interpret the differences among sites in total carbon flux and its relative apportionment among DIC, DOC, and POC, with respect to climate and ecosystem type. Finally, we evaluate the significance of stream carbon flux as a component of the ecosystem carbon balance.