Climate change affects running waters not only by increasing temperatures but also by increasing discharge variability as more frequent and severe floods and more frequent and longer droughts occur, especially in upper reaches. Mediterranean streams are known to experience droughts, but Central European headwaters are also beginning to be affected. The development of bacterial communities (abundance, composition) and the recovery of microbial functions (bacterial production, extracellular enzyme activity) were explored after rewetting desiccated streambed sediments via a sediment core perfusion technique. The bacterial community composition changed only slightly in the sediments from the Central European stream Breitenbach (Germany), but distinctly in the Mediterranean Mulargia River (Sardinia, Italy) during 4 days of experimental rewetting. Breitenbach sediments probably enabled survival of bacterial communities more similar to indigenous streambed communities, because they were less dry. High activity of enzymes involved in polymer degradation at the beginning of rewetting in both sediments indicated the persistence of extracellular enzymes during drought. After 4 days, nearly all microbial activities reached a level similar to unaffected sediments for the Breitenbach, but not for Mulargia. Here, much more intense drying resulted in a more distinct change and reduction of the microbial community, responsible for slower recovery of structure and functions.

We tested whether seasonal changes in the sources of organic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > beta-glucosidase > alpha-glucosidase > exo-1,4-beta-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and beta-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/beta-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). The relative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2-3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.

Fluvial systems transport sediments in suspension and in bed-load together with bacteria adhering to the transported particles. Activity of these bacteria is assumed to be critical for processes of self-purification of river waters. Here we present the results of multidisciplinary research designed to provide insight into how turbulent flow and movable sediments affect bacterial activity. Bacterial density and activity were investigated at 0-2, 9-11 and 18-20 cm sediment depth at three profiles longitudinally spaced along a sandy dune in the lowland River Spree in Germany. Dynamics of riverbed relief were examined via high-resolution spatial and temporal echo-sounding surveys. Three-dimensional turbulent flow structure was measured at three vertical profiles, the locations of which coincided with the positions of bacterial samples. Bacterial activity was high in the upper 10 cm layer of sand which was moving relatively fast in the form of comparatively small sand waves, and significantly lower in the slow-moving deeper layer associated with larger, slowly migrating dunes. The thickness of the upper layer tended to decrease towards the sand dune crests. A transition zone between high and low activity was found in the 9-11 cm sediment layer. The local depth of this transition zone also matched the local values of displacement heights computed from turbulent stresses profiles. We demonstrate that information on characteristic scales of sand wave motion may define regions of high bacterial activity for rivers with non-cohesive, sandy sediment deposits.

Microbial processes within subaqueous dunes of large rivers are important for organic matter retention and decomposition but have rarely been examined. We investigated 3 morphodynamically 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 correlated with protein, carbohydrates, nitrogen and phaeopigments associated with high-quality particulate organic matter. Our findings showed that the morphodynamic differentiation of the subaqueous 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 bedform were focal sites of microbial carbon mineralisation in the large river ecosystem.

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