Contribution of river-bed interfaces to running water quality by coupling modeling and in situ measurements.

Water quality is strongly related to interactions between the structure and function of ecosystem components at landscape scale of a river. Key interfaces as part of these components, identified in our study, are the epilithic biofilm, the fine sediments in the bottom of the river and the macroporous medium or hyporheic zone located essentially in the active channel where subsurface flow takes place. This article presents three examples of interface sub-models where water and elements fluxes exchanged with the surface water are different, with the focus on organic matter and nitrate dynamics for the Garonne river (France) in its middle course (7th order). Each interface on surface quality (e.g. consumption or production of organic matter) at the river reach scale was examined independently through modeling and in field measurements. We found that (i) influences differ between the interfaces and also within each interface over time as a result of biotic (organisms involved) and abiotic (hydrological and morphological) conditions; and (ii) the interface acts as transient storage zones and contributes to emerging self-purification properties of the river. Analysis of biophysical diversity based on spatial distribution of the different interface, their influences and their interactions can contribute to determining the biogeochemical cycle of surface water.