Contribution of dissolved organic C to stream metabolism: a mesocosm study using 13C-enriched tree-tissue leachate

Abstract. 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 3rd-order stream in Penn-sylvania, 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 (65) 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 % (60.6) and 12 % (60.6), respectively, of the biodegradable DOC in the leachate. Uptake mass transfer coefficients for the readily and intermediately labile com-ponents were 55 (624) mm/s and 2.6 (60.13) mm/s, respectively. Based on our mesocosm measure-ments, 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 down-stream in WCC before being taken up by the sediments. These distances represent;7 % and.150