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

• Microbial processes within subaqueous dunes of large rivers are important for organicmatter retention and decomposition but have rarely been examined. We investigated 3 morphodynamicallydefined zones (stoss side, crestal plateau, and lee side) within a subaqueous dune in the8th-order River Elbe. Analysis of flow velocity, vertical hydraulic gradient, concentration of mobilefine interstitial particles, and the quantity and biochemical quality of sedimentary organic matterindicated that the stoss and the lee sides of the dune were focal zones of particulate matter retentiondue to infiltration and sedimentation processes. Bacterial abundance and most measures of microbialactivity (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 thesezones than in the plateau. Increases in bacterial abundance and microbial activity were closely correlatedwith protein, carbohydrates, nitrogen and phaeopigments associated with high-quality particulateorganic matter. Our findings showed that the morphodynamic differentiation of the subaqueousdune resulted in the formation of distinct functional zones in the sediment. The underlyingmechanisms can be conceptually summarized by a 2-stage regulatory hierarchy. Microbial activitieswere controlled directly by the input of dissolved oxygen and easily degradable microbial substrates,and indirectly by hydromorphological processes. We conclude that the subaqueous dune functionedas an efficient filter of particulate organic matter, and that the stoss and the lee sides of this river bedformwere focal sites of microbial carbon mineralisation in the large river ecosystem.

Nyckelord

Extracellular enzymes · Particulate organic matter · Subaqueous dune · Sediments ·River · Bacteria · Hydrodynamics

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