METABOLISM OF STREAMWATER DISSOLVED ORGANIC-CARBON IN THE SHALLOW HYPORHEIC ZONE

Approximately 50% of streamwater dissolved organic C (DOC) disappeared from interstitial water moving along a hyporheic flowpath below a gra vel bar. Concurrent decreases in oxygen and increases in dissolved ino rganic C (DIC) indicate metabolism of streamwater-derived DOC by hypor heic microbes. Loss of approximately 100 muM DOC would account for 24- 39% of the oxygen depletion and DIC accumulation. Sediments were incub ated with DOC collected from the stream channel and two wells along th e flowpath to examine whether streamwater and hyporheic DOC could supp ort growth of sedimentary bacteria. Streamwater and hyporheic DOC coll ected from the upstream end of the flowpath stimulated higher bacteria l growth rates and supported higher biomass than hyporheic DOC collect ed from the downstream end. The supply of streamwater-derived DOC is a significant source of C for hyporheic sediment microorganisms, and th ese organisms are capable of using about half the DOC contained in str eamwater.