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Microbial processes within subaqueous dunes of large rivers are important for organic matter retention and decomposition but have rarely been examined. We investigated 3 morpho- dynamically defined zones (stoss side, crestal plateau, and lee side) within a subaqueous dune in the 8th-order River Elbe. Analysis of flow velocity, vertical hydraulic gradient, concentration of mobile fine interstitial particles, and the quantity and biochemical quality of sedimentary organic matter indicated that the stoss and the lee sides of the dune were focal zones of particulate matter retention due to infiltration and sedimentation processes. Bacterial abundance and most measures of microbial activity (sediment community respiration and activities of the extracellular enzymes β-glucosidase, leucine aminopeptidase, β-xylosidase, and exo-1, 4-β-glucanase) were significantly higher in these zones than in the plateau. Increases in bacterial abundance and microbial activity were closely cor- related with protein, carbohydrates, nitrogen and phaeopigments associated with high-quality par- ticulate organic matter. Our findings showed that the morphodynamic differentiation of the sub- aqueous dune resulted in the formation of distinct functional zones in the sediment. The underlying mechanisms can be conceptually summarized by a 2-stage regulatory hierarchy. Microbial activities were controlled directly by the input of dissolved oxygen and easily degradable microbial substrates, and indirectly by hydromorphological processes. We conclude that the subaqueous dune functioned as an efficient filter of particulate organic matter, and that the stoss and the lee sides of this river bed- form were focal sites of microbial carbon mineralisation in the large river ecosystem.

Microbial activity within a subaqueous dune in a large lowland river (River Elbe, Germany)