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International audience; Water resources and aquatic ecosystems are an essential concern of any policy for sustainable development. In the context of the European Water Framework Directive, which aims to achieve good chemical and ecological status of waters, the herbicide diuron was banned in France in December 2008. In lotic ecosystems, benthic microbial assemblages are considered as useful potential indicators of ecological status because they integrate the effects of multiple anthropogenic disturbances and have strong capacities to adapt to novel environmental conditions. Field studies have thus revealed that in situ diuron exposure can induce microbial adaptation leading to an increase in diuron biodegradation potential of sediment microbial communities. It suggests that microbial biodegradation capacities can represent a powerful ecological indicator for monitoring spatio-temporal variations in pesticide contamination and evaluating associated ecological effects. The aim of this study was thus to evaluate the use of freshwater sediment biodegradation potential as an ecological indicator for monitoring microbial recovery following a decrease in chronic pesticide exposure. For this purpose, a four-year case study (2008–2011) was conducted in a small stream (Morcille river) long exposed to high diuron concentrations, increasing from upstream to downstream. Our results show that the ban on diuron resulted in a progressive decrease in its concentrations in the Morcille river over the survey period. The decrease in the level of chronic diuron exposure in the river caused a fall in sediment diuron-mineralizing capacities, assessed by radiorespirometry using [ring-U-14C] diuron. It thus revealed a corresponding recovery of microbial communities, showing that the use of freshwater sediment biodegradation potential may be useful for assessing microbial recovery after a decrease in chronic exposure to pollutants, opening prospects for developing a new class of ecological indicator to monitor the recovery of biological quality of water resources.

Intermittent rivers and ephemeral streams (IRES) encompass fluvial ecosystems that eventually stop flowing and run dry at some point in space and time. During the dry phase, channels of IRES consist mainly of dry riverbeds (DRBs), prevalent yet widely unexplored ecotones between dry and wet phases that can strongly influence the biogeochemistry of fluvial networks. DRBs are often overlooked because they do not strictly belong to either domain of soil or freshwater science. Due to this dual character of DRBs, we suggest that concepts and knowledge from soil science can be used to expand the understanding of IRES biogeochemistry. Based on this idea, we propose that DRBs can be conceptually understood as early stage soils exhibiting many similarities with soils through two main forces: i) time since last sediment transport event, and ii) the development status of stabilizing structures (e.g. soil crusts and/or vascular plants). Our analysis suggests that while DRBs and soils may differ in master physical attributes (e.g. soil horizons vs fluvial sedimentary facies), they become rapidly comparable in terms of microbial communities and biogeochemical processes. We further propose that drivers of DRBs biogeochemistry are similar to those of soils and, hence, concepts and methods used in soil science are transferable to DRBs research. Finally, our paper presents future research directions to advance the knowledge of DRBs and to understand their role in the biogeochemistry of intermittent fluvial networks., Earth-Science Reviews, 188, ISSN:0012-8252, ISSN:1872-6828