Anaerobic microbial metabolism in hyporheic sediment of a gravel bar in a small lowland stream

Distribution of dissolved oxygen, nitrate, sulphate, carbon dioxide and dissolved organic carbon (DOC), acetate and lactate was studied in the stream and interstitial water along the subsurface flowpath in the hyporheic zone of a small lowland stream. Sediments were found to act as a source of nitrous oxide and methane. Interstitial methane concentrations were significantly much higher in comparison to those from surface water, and were significantly lower in the relatively well oxygenated downwelling zone than in the rather anoxic upwelling zone. The interstitial concentrations of O-2, NO3-1 and SO4-2 showed significant decline along the subsurface flowpath, while concentrations Of CO2, N2O, DOC, acetate and lactate remained unchanged. In addition to field measurements, ex situ incubation of sediments was carried out in the laboratory. Maximal methane production was found in the incubation assay using acetate (mean value 380 mu g CH4 kg DW-1 d(-1)). Mean value of the denitrification potential was 1.1 mg N2O kg DW-1 d(-1). Nitrous oxide production potential reached 71-100% of denitrification potential. Our results demonstrate that respiration of oxygen, nitrate, sulphate and methanogenesis may coexist within the hyporheic zone and that anaerobic metabolism is an important pathway in organic carbon cycling in the Sitka stream sediments. Copyright (c) 2005 John Wiley & Sons, Ltd.