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Abstract

Previous studies have indicated that surface inputs of organic carbon were insufficient to sustain high levels of bacterial production measured in hyporheic sediments of the Breitenbach, a first-order stream in central Germany. The aim of this study was to characterize groundwater discharge into the Breitenbach and evaluate the extent to which dissolved organic carbon (DOC) discharged into the stream via groundwater might make up for this apparent deficit in carbon input. Groundwater contributed 18-56% of the water discharged into the lower 2 km of the stream, depending on precipitation regime and season. Groundwater discharge through the hyporheic zone was also spatially variable, both along the stream and over time. Concentrations of DOC in groundwater directly adjacent to the stream (mean = 5.5 mg l^-1) consistently exceeded those in the stream (mean = 1.4 mg l^-1). Mass-balance calculations indicated that an annual mean of 0.92 g C m^-2 d^-1 was discharged to the stream via groundwater, of which 0.66 g C m^-2 d^-1 (71%) was predicted to have been immobilized within the hyporheic sediments before reaching the water column. Both these parameters were predicted to increase with increasing groundwater discharge rates and DOC concentrations, with hydrological variability being the more influential factor. Experimental perfusion of groundwater through cores of hyporheic sediment confirmed that immobilization rates could increase with discharge beyond the range of net groundwater discharge rates measured in the stream. It was concluded that the biofilms of hyporheic sediments are effective at immobilizing ephemeral increases in the DOC load from groundwater, and that this source of organic carbon would make a significant contribution to sustaining high levels of bacterial production in the Breitenbach.