Limnol. Oceanogr., 44(6), 1999, 1530–1539

An in situ acetate injection was used to determine the influence of labile dissolved organic carbon (DOC) availability on microbial respiration in the hyporheic zone of a headwater stream. We added bromide as a conservative tracer and acetate as an organic substrate to the hyporheic zone of Rio Calaveras, New Mexico, via an injection well. Tracer was observed in four of eight capture wells. Three of the four wells showed increases in bromide without concurrent increases in acetate concentration, suggesting 100% acetate retention. One well had 38% acetate retention. Pore velocity and acetate retention were negatively correlated, suggesting hydrologic control of acetate retention. Acetate did not significantly sorb to the sandy hyporheic sediments at this site, indicating biological consumption of acetate. Acetate addition stimulated total CO2 production along monitored flowpaths and led to changes in solutes associated with microbial terminal electron-accepting processes (TEAPs). Dissolved oxygen (DO), nitrate, and sulfate significantly decreased, and ferrous iron and methane significantly increased compared to background concentrations in most wells. These results support the hypothesis that microbial respiration in the hyporheic zone is limited by labile DOC availability. Furthermore, we have shown that a suite of metabolic processes, from aerobic respiration to methanogenesis, cooccur and that anaerobic processes dominate heterotrophic metabolism in the hyporheic zone of Rio Calaveras. In the past 10–15 yr, the importance of the hyporheic zone (i.e., subsurface water containing at least 10% surface water, sensu Triska et al. 1989) to lotic biogeochemistry and ecology has been documented for streams and rivers worldwide (e.g., Jones and Holmes 1996; Brunke and Gonser 1997; Gibert et al. 1997). Hyporheic sediments are metabolically active, are an important site for organic matter decomposition, and can have a large impact on respiration (i.e., decrease P : R ratios) in measures of whole-stream ecosystem metabolism (Grimm and Fisher 1984; Mulholland et al. 1997; Nageli and Uehlinger 1997). Microbial metabolism in the hyporheic zone depends on transport of organic substrates and electron acceptors from the surface stream and/ or nearby groundwater (e.g., Findlay 1995; Jones et al. 1995a). Since subsurface organisms are mostly heterotrophic (Jones et al. 1995b) and require allochthonous organic matter sources, it is commonly hypothesized that microbial metabolism in the hyporheic zone is limited by organic matter availability (e.g., Jones 1995). The relationship between hyporheic zone aerobic respiration and organic matter availability has been examined using sediment microcosms (e.g., Pusch and Schwoerbel 1994; Jones 1995; Fuss and Smock 1 Present address: Utah State University, Department of Biology, 5305 Old Main Hill, Logan, Utah 84322-5305. 2 Present address: Virginia Polytechnic Institute and State University, Department of Biology, Blacksburg, Virginia 24061.