Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach LOUIS A. KAPLAN*, TRACY N. WIEGNER* ,1 , J. D. NEWBOLD*, PEGGY H. OSTROM † AND HASAND GANDHI † *Stroud Water Research Center, Avondale, PA, U.S.A. † Department of Geological Sciences, Michigan State University, East Lansing, MI, U.S.A. SUMMARY 1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole- stream addition of a 13 C-DOC tracer coupled with laboratory measurements of bioavail- ability of the tracer and stream water DOC. 2. The tracer, a leachate of 13 C-labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2-h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer. 3. From the longitudinal 13 C uptake curve, we resolved labile and semi-labile DOC classes within the 13 C-DOC tracer comprising 82% and 18% of the tracer respectively. 4. Plug-flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi-labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients. 5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi-labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi-labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans. Keywords: bioavailability, dissolved organic carbon, stable isotopes, streams, tracer addition Introduction In streams and rivers, dissolved organic carbon (DOC) supplies energy and carbon (C) to heterotro- phic bacteria, and DOC incorporation into the microbial loop or aggregation into particles affects its transfer to higher trophic levels. These organic molecules constitute the largest pool of organic matter in rivers, and while they were long considered biologically refractory (Wetzel, 1995), laboratory measurements have shown that humic substances (Volk, Volk & Kaplan, 1997) and lignin (Frazier, Kaplan & Hatcher, 2005) from headwater streams are bioavailable. Additionally, in situ gas flux measurements (Cole & Caraco, 2001) and C-isotope analyses (Mayorga et al., 2005) provide evidence that terrestrial C is assimilated and respired in large rivers. Yet, we do not know the length of the river the DOC travels before it is taken up because no study to date has directly measured uptake lengths for total stream water DOC or its constituent lability classes. Correspondence: Louis A. Kaplan, Stroud Water Research Center, 970 Spencer Road, Avondale, PA 19311, U.S.A. E-mail: lakaplan@stroudcenter.org 1 Present address: Tracy N. Wiegner, 200 W. Kawili Street, Hilo, Hawaii 96720, USA. Freshwater Biology (2008) 53, 855–864 doi:10.1111/j.1365-2427.2007.01941.x Ó 2007 The Authors, Journal compilation Ó 2007 Blackwell Publishing Ltd 855