Dynamics of Respired Dissolved Organic Carbon in a Stream

Dissolved organic carbon (DOC) represents a large flux of carbon (C) across landscapes and supports respiration (mineralization) in streams. Because DOC comprises many compounds with varying reactivity, and much of what is in transport is the less reactive portion of total inputs, it is difficult to study reactivity and fate of the DOC pool. To address this knowledge gap, we added ¹³C-labeled glucose and plant litter leachate in separate pulse additions to a small, open-canopy stream in Montana, USA, and quantified the timing and amount of respiration of this C tracer. DOC uptake was high for both C sources and matched that from other tracer studies; mean uptake velocities for glucose and leachate were 4.0 and 1.7 m d^-1, and 28–100% was mineralized to the dissolved inorganic carbon (DIC) pool. Mineralization occurred at two timescales, with 10–70% the DOC tracer immediately released as DIC and the remainder retained with a 6–12 h residence time before it was mineralized. Nearly all of the leachate tracer was respired within 24 h, but about half of glucose remained in the stream with an unknown residence time. Scaling tracer removal to the entire DOC pool showed that DOC removal from the water column can support much of the measured respiration in the stream, but respiration was a small amount of total CO2 emissions because of high external DIC inputs.