Integrating Perspectives on Dissolved Organic Carbon Removal and Whole‐Stream Metabolism

Abstract

AbstractQuantifying organic carbon (OC) removal in streams is needed to integrate the functional role of inland waters into landscape carbon budgets. To illustrate how in‐stream OC removal measurements can be used to characterize ecosystem and landscape carbon fluxes, we compared two common methods: (a) Bioassays measuring water column dissolved organic carbon (DOC) uptake and (b) daily rates of whole‐stream metabolism and OC spiraling calculated from fluorescent dissolved organic matter, oxygen, and discharge measurements. We then assessed how OC removal rates from these two methods, measured in two low‐productivity heterotrophic streams, affected estimates of terrestrial OC loading and export using a mass balance model. OC mineralization velocities calculated from whole‐stream metabolism (0.06 ± 0.03 m d−1 (mean ± SD)) were greater than water column bioassay DOC uptake velocities (0.01 ± 0.01 m d−1), which resulted in higher in‐stream OC removal estimates (0.5%–15.2% and 0.02%–4.2% removal for whole‐stream metabolism and bioassays, respectively). Furthermore, the terrestrial OC inputs needed to sustain in‐stream OC concentrations differ among methods, with simulated inputs ranging from 79 to 1,300 or 3–350 g OC d−1 for whole‐stream metabolism or bioassays, respectively. We show how in‐stream OC removal can be used to quantify terrestrial‐aquatic linkages by estimating OC inputs needed to fuel whole‐stream metabolism in low‐productivity streams, and offer future directions to better link OC removal with whole‐ecosystem OC budgets. Without appropriate conversions to whole‐stream processes, bioassays systematically underestimate whole‐stream carbon cycling. By integrating whole‐stream metabolism with OC transport, we can better elucidate the role of running waters in landscape carbon budgets and the global carbon cycle.