J. N. Am. Benthol. Soc., 1987, 6(3):159-167 © 1987 by The North American Benthological Society Interstitial dissolved organic carbon in sediments of a southern Appalachian headwater stream M. TAD CROCKER AND JUDY L. MEYER 1 Zoology Department and Institute of Ecology, University of Georgia, Athens, Georgia 30602 USA Abstract. This study had two objectives: (1) to compare seasonal and spatial patterns of water- column dissolved organic carbon (DOC) and sediment interstitial DOC (IDOC) concentrations at a headwater spring seep, and (2) to explore interactions between sediment organic matter content, benthic bacterial biomass and production, water column DOC concentration, and sediment IDOC concentration and composition (high versus low molecular weight components). For the first objective, sediment organic matter content, IDOC concentration (sampled from small wells by dialysis or by pipet), water column DOC concentration, and benthic bacterial biomass were measured at a spring seep in Coweeta Hydrologic Laboratory, Macon Co., North Carolina USA, for two years. For the second objective, organic matter content was increased or decreased in experi- mental sediments. The same variables listed above were measured within three weeks of sediment manipulation and again after five and a half months. Benthic bacterial production was measured one week after sediment manipulation. Results indicate benthic organic matter is a source of IDOC, the concentration of which depends on sediment organic matter content, the relative proportion of high and low molecular weight IDOC compounds, and the exchange of interstitial and water column DOC. High molecular weight IDOC (> 50,000) accumulates in sediments in proportion to sediment organic matter content. Low molecular weight IDOC (< 50,000) concentrations are lower than high molecular weight IDOC concentrations and are independent of sediment organic matter content. However, low molecular weight IDOC concentrations are generally greater than water column total DOC concentrations. Low molecular weight IDOC concentrations are greater during the dormant season than during the growing season. Key words: streams, DOC, hyporheos, organic matter, decomposition, bacterial biomass, bacterial production, dialysis, Coweeta Hydrologic Laboratory. Studies of the distribution and dynamics of al. (1981) and Hynes (1983) suggested that dissolved organic carbon (DOC) in streams have groundwater DOC inputs may be trophically considered the following aspects of water col- important through microbial metabolism of umn DOC: its fluvial patterns, mass balance in these compounds. stream reaches and watersheds, and within- Another potentially important source of DOC stream transformations (e.g., Fisher and Likens to this interstitial DOC pool is the gradual de- 1973, Kaplan et al. 1980, McDowell and Fisher composition of particulate organic matter (POM) 1976). Stream sediments have been shown to buried in stream sediments (Meyer et al. 1987b). influence water column DOC by physical ad- The existence of DOC concentration gradients sorption and microbial utilization of DOC ranging from <1 mg/L in the water column (Dahm 1981) and by photosynthetic production (e.g., Moeller et al. 1979), to > 10 mg/L within of DOC (Kuserk et al. 1984). Recent develop- anaerobic leaf accumulations (C. N. Dahm, Uni- ments in the study of hyporheic communities versity of New Mexico, personal communica- have attracted attention to the distribution and tion), to >1000 mg/L within decaying and an- dynamics of DOC in sediment interstitial spaces, aerobic leaf tissues (D. L. Lawson, Michigan State One potentially important source of interstitial University, personal communication) supports DOC to stream sediments is groundwater. Based the idea that long-term particulate organic mat- on DOC concentration differences between ter decomposition is a source of DOC. Linkages groundwater and seep water and the impor- between POM decomposition, DOC produc- tance^of groundwater to streamflow, Wallis et tion, and microbial metabolism have been made in several recent studies in a variety of systems 1 To whom correspondence should be addressed. * (Cole 1982, Cole et al. 1982, Kato 1984, Kepkay 159