The role of heterotrophic biofilm of water–sediment interface in detoxification processes was tested in abiotic and biotic conditions under laboratory conditions. Three toxicants, a herbicide (Diuron), a fungicide (Dimethomorph) and an insecticide (Chlorpyrifos-ethyl) have been tested in water percolating into columns reproducing hyporheic sediment. The detoxification processes were tested by comparing the water quality after 18 days of percolation with and without heterotrophic biofilm. Tested concentrations were 30 μg.L−1 of Diuron diluted in 0.1% dimethyl sulfoxide (DMSO), 2 μg.L−1 of Dimethomorph and 0.1 μg.L−1 of Chlorpyrifos-ethyl. To characterise the detoxification efficiency of the system, we performed  genotoxicity bioassays in amphibian larvae and rotifers and measured the respiration and denitrification of sediments. Although the presence of biofilm increased the production of N-(3,4 dichlorophenyl)-N-(methyl)-urea, a metabolite of diuron, the toxicity did not decrease irrespective of the bioassay. In the presence of biofilm, Dimethomorph concentrations decreased compared with abiotic conditions, from 2 μg.L−1 to 0.4 μg.L−1 after 18 days of percolation. For both Dimethomorph and Chlorpyrifos-ethyl additions, assessment of detoxification level by the biofilm depended on the test used: detoxification effect was found with amphibian larvae bioassay and no detoxification was observed with the rotifer test. Heterotrophic biofilm exerts a major influence in the biochemical transformation of contaminants such as pesticides, suggesting that the interface between running water and sediment plays a role in self-purification of stream reaches.

Excessive sedimentation of fine particles on stream beds has been recognized as a major threat to running-water ecosystems. Deposition of fine sediments often affects hyporheic zone (HZ) functioning by (1) reducing hydrological exchanges at the water–sediment interface and by (2) increasing the organic matter (OM) content of surface sediments. These two factors usually occur concurrently to control biogeochemical processes in sediments. In the present study, experimental and modelling approaches were coupled to evaluate the contribution of these factors on the biogeochemical functioning of the HZ. We used a one-dimensional (1D) vertical model taking into account the hydrodynamic properties, the vertical distribution of the OM and the main microbial processes involved in OM processing (aerobic respiration, denitrification, nitrification and sulphate reduction). This Mobile-Immobile Model for Organic Matter (MIM-OM) model was calibrated and validated using experimental data (conservative tracer, dissolved oxygen and nitrate concentrations) obtained in filtration columns filled with a porous sedimentary matrix. Simulations showed that organic carbon content and Darcy velocity acted in concert to shape biogeochemical processes in stream sediments. The use of the MIM-OM model on data obtained in filtration columns impacted by fine sediment deposition indicated that the biodegradability of the OM (modified through the degradation parameter kPOC) also played a key role on biogeochemical processes occurring in sediments. In conclusion, the MIM-OM model appears as an efficient simulation tool to evaluate biogeochemical functioning in river sediments under different conditions (granulometry, quality of surface water and clogging).

Headwater woodland streams are primarily heterotrophic: they receive substantial inputs of organic matter from the riparian vegetation, while autochthonous primary production is generally low. A substantial part of leaf litter entering running waters may be buried in the streambed because of flooding and sediment movement. Although the general significance of the hyporheic zone for stream metabolism has been reported early, organic matter storage within the sediment of streams has received less attention, with most studies only quantifying accumulations at the streambed surface and ignoring other stream compartments. In the present study, the amounts of three fractions of coarse particulate organic matter (CPOM; >16, 4–16 and 1–4 mm) were determined in late autumn and early spring in the interstitial and benthic zones of three headwater streams of the Montagne Noire (South-Western France) differing in their substratum grain size. Our findings demonstrated that the total CPOM content in the interstitial zone can be much (up to one order of magnitude) higher than at the sediment surface. The sandy bottomed stream exhibited a higher amount of CPOM (whatever the size fraction) than the two other streams, suggesting that the sediment particle size may be a major determinant of CPOM storage. Given the large amount of organic matter stored in the interstitial zone, this compartment may play an important role for the carbon turnover and associated trophic dynamics in the stream ecosystem.

Thermal contraction crack polygons modify the generation, transport, and storage of water in Wright Valley gullies. Water generation is contributed to by trapping of windblown snow in polygon troughs. Water transport is modified by changes to the ice-cement table and active layer topography caused by polygon trough formation. Water storage is modified by sediment grain-size distribution within polygons in gully distal hyporheic zones. Patterned ground morphological variation can serve as an indicator of fluvial modification, ranging from nearly unmodified composite-wedge polygons to polygons forming in association with gully channels. Thermal contraction crack polygons may also constrain the gully formation sequence, suggesting the continuous presence of permafrost beneath the Wright Valley gullies during the entire period of gully emplacement. This analysis provides a framework for understanding the relationships between polygons and gullies observed on Mars. If comparable stratigraphic relationships can be documented, the presence of an analogous impermeable ice-cemented layer beneath the gullies can be inferred, suggesting an atmospheric source for Martian gully-carving fluids.

Nitrogen dynamics were investigated in the top centimeters of bottom sediment in three different geomorphologic units(riffles, sand bars, pools) and along two contrasted reaches (nitrogen-rich, nitrogen-poor) of a rural stream. Some predictionsof links between geomorphology and processes were verified: (i) hydrological exchanges and nitrification were lower in poolsthan in riffles and bars, whereas (ii) ammonification and denitrification were higher in pools. Studies of denitrification potentialin six other reaches of three N-rich streams support these conclusions and demonstrate that pools are spots of high microbialactivity in streams.

Le peuplement des Crustacés de la zone hyporhéique d'un ruisseau des Pyrénées, le Nert, a été étudié pendant un an sur une aire de 30 m2 à 0,6 m et 1 m de profondeur. 35 espèces dont 20 stygobies ont été recensées. Ce peuplement est caractérisé par sa persistance due à la stabilité physique de l'environnement interstitiel. Il témoigne cependant d'une très forte hétérogénéité liée aux conditions hydrodynamiques qui règnent au sein des sédiments.