Dissolved organic C (DOC) is metabolically important in streams, but its contribution to ecosystem metabolism is not well known because it is a complex mixture of mostly unidentified molecules. The uptake of bioavailable DOC in White Clay Creek (WCC), a 3^rd-order stream in Pennsylvania, was estimated from the results of an experiment using ¹³C-labeled tree-tissue leachate and streambed sediments in recirculating mesocosms. The contribution of DOC in transport to stream metabolism was estimated from measurements of ¹³C-DOC uptake, ¹²C-DOC concentrations, and diel changes in dissolved O₂ in the mesocosms. Eighty percent (±5) of the DOC in the ¹³C-tree-tissue leachate was bioavailable and belonged to 1 of 2 distinct lability classes, readily and intermediately labile. These components made up 88% (±0.6) and 12% (±0.6), respectively, of the biodegradable DOC in the leachate. Uptake mass transfer coefficients for the readily and intermediately labile components were 55 (±24) μm/s and 2.6 (±0.13) μm/s, respectively. Based on our mesocosm measurements, DOC in transport could support 33 to 54% of the bacterial C demand and up to 51% of the community respiration in WCC. Extrapolation of our results to WCC indicates that readily and intermediately labile DOC similar in quality to the ¹³C-DOC would travel 175 and 3692 m downstream in WCC before being taken up by the sediments. These distances represent ∼7% and >150% of the length of the 3^rd-order reach. Our results suggest that readily labile DOC is an important energy source at the reach scale, whereas intermediately labile DOC serves as an energy subsidy from upstream to downstream reaches.