Approximately 50% of streamwater dissolved organic C (DOC) disappeared from interstitial water moving along a hyporheic flowpath below a gravel bar. Concurrent decreases in oxygen and increases in dissolved inorganic C (DIC) indicate metabolism of streamwater-derived DOC by hyporheic microbes. Loss of $\thicksim 100 \mu M DOC$ would account for 24-39% of the oxygen depletion and DIC accumulation. Sediments were incubated with DOC collected from the stream channel and two wells along the flowpath to examine whether streamwater and hyporheic DOC could support growth of sedimentary bacteria. Streamwater and hyporheic DOC collected from the upstream end of the flowpath stimulated higher bacterial growth rates and supported higher biomass than hyporheic DOC collected from the downstream end. The supply of streamwater-derived DOC is a significant source of C for hyporheic sediment microorganisms, and these organisms are capable of using about half the DOC contained in streamwater.
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