48 J. N. Am. Benthol. Soc., 2005, 24(1):48–67  2005 by The North American Benthological Society Contribution of dissolved organic C to stream metabolism: a mesocosm study using 13 C-enriched tree-tissue leachate TRACY N. WIEGNER 1 ,LOUIS A. KAPLAN 2 , AND J. DENIS NEWBOLD 3 Stroud Water Research Center, 970 Spencer Road, Avondale, Pennsylvania 19311 USA PEGGY H. OSTROM 4 206 Natural Science Building, Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824-1115 USA 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 3 rd -order stream in Penn- sylvania, was estimated from the results of an experiment using 13 C-labeled tree-tissue leachate and streambed sediments in recirculating mesocosms. The contribution of DOC in transport to stream metabolism was estimated from measurements of 13 C-DOC uptake, 12 C-DOC concentrations, and diel changes in dissolved O 2 in the mesocosms. Eighty percent (5) of the DOC in the 13 C-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 com- ponents were 55 (24) m/s and 2.6 (0.13) m/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 13 C-DOC would travel 175 and 3692 m down- stream 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. Key words: carbon-13, dissolved organic carbon, ecosystem metabolism, leachate, stable isotopes, tracers, streams. Dissolved organic matter (DOM) plays an im- portant metabolic role in streams and rivers by supplying energy and C to heterotrophic bac- teria (Meyer et al. 1988). The incorporation of dissolved organic C (DOC) into the microbial food web affects the amount and rate at which DOC is supplied to higher trophic levels. DOC stimulates both pelagic and benthic bacteria in running waters (e.g., Bott et al. 1984, Servais et al. 1987, Volk et al. 1997). The primary site for DOM use and bacterial activity in low-order streams is in the benthic sediments (Lock and Hynes 1976, Dahm 1981, Fischer and Pusch 2001, Fischer et al. 2002b). However, the relative contribution of DOC from the water column to 1 Present address: Marine Science Department, Uni- versity of Hawaii at Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091 USA. E-mail: wiegner@hawaii.edu 2 E-mail addresses: lakaplan@stroudcenter.org 3 newbold@stroudcenter.org 4 ostrom@pilot.msu.edu stream and riverine metabolism is not well known, in part because DOC is a complex mix- ture of mostly unidentified molecules from var- ious sources with varying labilities (Thurman 1985, Fischer et al. 2002a). In addition, in situ measurements of DOC use in stream water are complicated by processes that continually pro- duce, transform, and consume DOC molecules in transport. Thus, a tracer with characteristics similar to bioavailable stream/riverine DOC would be helpful in determining the contribu- tion of DOC in transport to ecosystem metab- olism. Stable and radioactive C isotopes are useful tracers for elucidating flow paths and transfor- mations of dissolved and particulate organic C in lentic and lotic ecosystems (e.g., Bott et al. 1977, Dahm 1981, Hall 1995, Hall and Meyer 1998, Cole et al. 2002). Complex mixtures of 14 C- labeled compounds from leaf leachates have been used to examine stream DOC dynamics in