Immobilization and bacterial utilization of dissolved organic carbon entering the riparian zone of the alpine Enns River, Austria

In order to understand the factors controlling the removal of riverine dissolved organic carbon (DOC) in the riparian zone of an alpine river, concentrations of DOC and dissolved oxygen, as well as bacterial abundance and production, were determined in interstitial waters of an experimental bank filtration site of the Enns River, Austria. Four porewater stations exhibiting differing sedimentologic and hydrologic characteristics were sampled over an annual cycle. We found that concentrations of DOG, oxygen, bacterial biomass and production decreased significantly within the first meter from the sediment-water interface. Differences in the grain size distribution among the sampling stations led to spatial heterogeneity in the permeability of the riparian sediments and in the hydraulic residence time of the infiltrating river water, resulting in specific patterns in DOC immobilization and microbial respiration. Porewater bacterial abundance and production and apparent microbial oxygen consumption were positively correlated with the hydraulic residence time of the infiltrating water. DOC occasionally accumulated in the shallow porewater layers during the winter. During the summer, DOC infiltrating from the river surface potentially explained only 36 +/- 25% of the apparent interstitial oxygen consumption. This suggests that particulate organic carbon (POC) contributes substantially to the microbial organic carbon supply in the hyporheic zone. We conclude that: the availability of POC rather than DOC infiltration determines hyporheic microbial metabolism.