NOTES Microbiological turnover of amino acids immobilized from groundwater discharged through hyporheic sediments

A pulse of ‘T-labeled dissolved free amino acids (DFAA) was immobilized rapidly from groundwater discharged through cores of hyporheic sediment from an upland stream. Mean turnover distance within the cores was cl.5 cm but, once immobilized, DFAA carbon was retained for longer than would be expected if bacterial cycling was the only retention mechanism. It took 28 weeks for 88% of the original 1.2 pg of DFAA carbon that had been immobilized to be respired and exported from the cores, whereas bacterial respiration alone should have mineralized >99% of this carbon within 7.4-l 1.4 weeks, depending on the bacterial growth efficiency assumed. Abiotic retention processes associated with organic and mineral surfaces appear to enhance the initial immobilization of DFAA and contribute to a delayed recycling of DFAA carbon within the sediment, increasing the effective availability of this labile organic carbon in the hyporheic zone. Abiotic retention of dissolved organic carbon might help sustain the high levels of bacterial production found in hyporheic sediments while reducing the demand for organic carbon imported into the hyporheic zone. Biological activity in the hyporheic zone of running waters depends mostly on the transfer of allochthonous organic carbon, particulate as well as dissolved, from the surrounding environment. Dissolved organic carbon (DOC) is the dominant form of organic carbon turned over in running waters (Fisher and Likens 1973; Mulholland 1981). Apart from leaching of DOC from entrained particulate matter (Moran and Hodson 1989), the availability of DOC within the hyporheic zone depends on rates at which water is exchanged with either surface water (Vervier and Naiman 1992) or shallow groundwater (Hynes 1983). Hynes (1983) suggested that in many running waters shallow groundwater might deliver a substantial amount of DOC to the hyporheic zone, where this DOC can then be immobilized efficiently by attached microorganisms. Due to a lack of direct evidence, there is still some debate over the potential contribution of DOC in groundwater to the carbon budget of running waters (Kaplan and Newbold 1993), although there is circumstantial evidence for the quantitative importance of this input pathway in some rivers and streams (e.g. Naiman et al. 1987; Fiebig 1995). Further, short-term laboratory experiments have shown that up to 27% of the DOC in groundwater can be immobilized during perfusion through shallow cores of hyporheic sediment (Fiebig and Lock 1991). The difficulty of characterizing much of the DOC pool means that our understanding of exactly which compounds can be immobilized from groundwater, and how these are processed within the hyporheic environment, remains rudimentary. It is now clear, though, that a reduction in the concentration of DOC in groundwater after perfusion through hyporheic sediment is not simply due to stripping of microbially labile organic compounds. Fiebig and Lock ( 199 1) showed that concentrations of dissolved free amino acids (DFAA) in groundwater were not reduced significantly after perfusion through hyporheic sediment. In contrast, by tracing a continuous addition of W-DFAA to groundwater over 21 h, Fiebig (1992) showed that 99% of DFAA flowing into the hyporheic sediment was immobilized, mostly in the initial 1.5 cm of sediment encountered. Thus, there must have been a simultaneous release of novel DFAA within the sediment in order to account for the relative consistency of DFAA concentrations in groundwater before and after perfusion through the sediment. These observations suggest a high flux of DFAA within the hyporheic zone, but the fate over time of DFAA carbon after its initial immobilization remains unclear. This is important for assessing the ecological significance of these biologically active compounds in the hyporheic zone and might be indicative for labile DOC in general. For example, longer-term retention of the immobilized carbon in hyporheic biofilms as particulate matter would be trophically more significant at the site of immobilization than rapid mineralization and export of the immobilized carbon as CO,. I address this question here by reporting on the time taken for a radiolabeled pulse of DFAA to be turned over, mineralized, and exported from cores of hyporheic sediment subjected to simulated groundwater discharge. Hyporheic sediments and groundwater were obtained from the Breitenbath, a first-order stream in the uplands of central Germany. Descriptions of the stream and its catchment are available in Marxsen ( 1980) and Cox (1990). Depending on the local flow regime, the streambed sediments vary in size from cobbles to silt. Upwelling and downwelling areas in the hyporheic zone are spatially and temporally variable along the stream, although at several sites the sediments are apparently exposed to groundwater discharge throughout the year (Fiebig 1995). Fifteen sediment cores (7.5 cm deep, 2 cm in diam) were retrieved from a sandy reach of the Breitenbach and installed in a perfusion manifold kept in the dark within a temperature-controlled (10°C) incubator, as described by Fiebig (1992) and Marxsen and Fiebig (1993). Shallow groundwater, from depths of up to 60 cm, was collected as required from 10 sampling wells situated directly next to the Brei-