Microbial metabolism of dissolved organic carbon in stream hyporheic zone

Stream hyporheic zones (saturated sediments found below and lateral to the open-stream channel in active exchange with surface water) can alter stream biogeochemistry and may influence organic carbon transport. I examined the fate of stream-derived dissolved organic carbon (DOC) during transport through hyporheic zones. Specifically, I examined DOC concentrations along hyporheic flowpaths in five streams known to represent the range in surface-water DOC in the Hudson River Valley and Catskill Mountain region of eastern NY. Transects of wells were installed along two hyporheic flowpaths in each of the five streams and sampled for subsurface DOC and dissolved oxygen concentrations. In addition, I examined bacterial productivity and abundance on ceramic tiles incubated in separate wells along these flowpaths.;Hyporheic DOC concentrations ranged from ∼100% of surface water to ∼50% of surface water. Hyporheic DOC significantly decreased along hyporheic flowpaths in four of five streams, however hyporheic DOC did not decline in the stream with the lowest surface-water DOC. Hyporheic dissolved oxygen significantly decreased along hyporheic flowpaths in four of five streams (range: 100%--0% surface water concentration) and decreases were concomitant with DOC loss. In addition, bacterial productivity and abundance declined in concert with hyporheic DOC.;Differences in the hydrology among flowpaths can confound interpretation of biogeochemical patterns since variation in residence time can influence gradients in water chemistry. Mesocosms that simulate hyporheic flowpaths ("hyporheotron") were constructed and used to examine DOC removal under normalized hydrologic regimes. DOC losses along hyporheotron flowpaths were similar to those documented along natural hyporheic flowpaths and were reliable predictors of changes in bacterial productivity, abundance, and extracellular enzyme activities.;Overall, my findings suggest removal of DOC in hyporheic zones is ultimately mediated by microbial metabolism, hence DOC losses along hyporheic flowpaths can be used to evaluate the bioavailability of surface- water DOC entering these environments in situ. Bioavailable DOC varies markedly among streams regionally and is a reliable predictor of sediment-bound bacterial productivity. DOC exiting hyporheic flowpaths is generally unavailable, hence significant hyporheic exchange of surface water may influence the amount and composition of DOC transported to downstream ecosystems.