VARIATION IN BIOAVAILABILITY OF DISSOLVED ORGANIC CARBON AMONG STREAM HYPORHEIC FLOWPATHS

Abstract

Dissolved organic carbon (DOC) dominates the flux of organic matter in most stream ecosystems, but the proportion susceptible to microbial degradation is often presumed to be low. The fraction of bulk DOC contributing to microbial metabolism was assessed in five streams representing the regional range in surface-water DOC concentration in eastern New York State, USA (range 0.5–7.7 mg/L; n = 82). Transects of shallow wells along two hyporheic flowpaths (i.e., saturated sediments found below and lateral to the open-stream channel in active exchange with surface waters) in each of five streams were sampled monthly at baseflow to determine changes in subsurface DOC and dissolved oxygen concentrations. Hyporheic DOC concentrations ranged from 50% to 100% of surface-water concentrations and decreased along hyporheic flowpaths in four of five streams. Dissolved oxygen losses along hyporheic flowpaths paralleled DOC loss, and bacterial activity on tiles incubated at points along the flowpaths generally declined as hyporheic DOC was depleted. DOC losses along natural flowpaths exceeded the quantity of DOC lost during laboratory bottle incubations, even when samples were amended with inorganic nutrients. Hyporheic mesocosms were used to examine the fate of stream-derived DOC along replicated flowpaths under controlled hydrologic conditions. The overall patterns of DOC losses along mesocosm flowpaths supplied with water from previously studied streams were similar to DOC losses along natural flowpaths. DOC declines were paralleled by declines in bacterial activity and dissolved oxygen. Mesocosm results indicated that variation in percentage of DOC loss along natural flowpaths was not a function of dilution, residence time, or initial DOC concentration and that subsurface DOC dynamics were linked to variation in microbial metabolism. The fraction of total DOC available for metabolism varied markedly among regional streams and was independent of initial DOC concentration. DOC near the end of hyporheic flowpaths was not subject to further degradation, regardless of the bioavailability of surface-water DOC entering these flowpaths. Hence, in streams with significant hyporheic exchange, the amount and bioavailability of DOC transported to downstream ecosystems may be affected by subsurface metabolism. DOC depletion during hyporheic transport may provide a general in situ measure of bioavailable DOC in surface water and be a powerful predictor of rates of heterotrophic activity in sediments at the reach level.