Biodegradation of labile dissolved organic carbon under losing and gaining streamflow conditions simulated in a laboratory flume

Carbon in surface waters is widely recognized as a key element that influences nutrient cycling, metal availability, and water quality. Its degradation in streams occurs primarily by benthic microbial communities that colonize the underlying sediment, which is commonly termed the hyporheic zone (HZ). The biodegradation of a labile dissolved organic carbon (DOC_L), exemplified by sodium benzoate, was studied in a novel laboratory flume system under a combination of different overlying water velocities, losing or gaining fluxes, and biofilm distribution (“surficial” or “homogeneous distribution”). The overall objective of this study was to evaluate the effect of different flow conditions on DOC_L biodegradation in the HZ. The results showed that overlying velocity was the dominant factor affecting DOC_L biodegradation, regardless of biofilm distribution. Gaining flow conditions also induced a slight increase in the biodegradation rates as compared to losing or neutral flow conditions, due to additional oxygen input from the upwelling water. The aerobic reactive zone under all flow conditions was limited to the upper section of the benthic biofilm (several millimeters), where the surficial biofilm showed the highest activity. Our results demonstrate the processes affecting DOC_L biodegradation in the hyporheic zone and will help to implement future modeling of DOC transport in streams.