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Abstract: Changes in land use and intensification of agricultural pressure have greatly accelerated the alteration of the landscape in most developed countries. These changes may greatly disturb the adjacent ecosystems, particularly streams, where the effects of pollution are amplified. In this study, we used the leaf litter breakdown rate to assess the functional integrity of stream ecosystems and river sediments along a gradient of either traditional extensive farming or a gradient of vineyard area. In the benthic layer, the total litter breakdown process integrates the temporal variability of the anthropogenic disturbances and is strongly influenced by land use changes in the catchment even though a low concentration of toxics was measured during the study period. This study also confirmed the essential role played by amphipods in the litter breakdown process. In contrast, microbial processes may have integrated the variations in available nutrients and dissolved oxygen concentrations, but failed to respond to the disturbances induced by vineyard production (the increase in pesticides and metal concentrations) during the study period. The response of microbes may not be sensitive enough for assessing the global effect of seasonal agricultural practices. Finally, the leaf litter breakdown measured in the hyporheic zone seemed mainly driven by microbial activities and was hence more affected by vertical exchanges with surface water than by land use practices. However, the breakdown rate of leaf litter in the hyporheic zone may constitute a relevant way to evaluate the impact on river functioning of any human activities that induce massive soil erosion and sediment clogging. [Copyright &y& Elsevier]

1. The functional feeding group approach has been widely used to describe the community structure of benthic invertebrates in relation to organic matter resources. Based on this functional framework, positive interactions between feeding groups (especially shredders and collector-gatherers) were postulated in the River Continuum Concept. However, relationships with organic matter have been poorly documented for invertebrates living in the hyporheic zone. 2. We hypothesised that the common subterranean amphipod Niphargus rhenorhodanensis would feed on fine particulate organic matter (FPOM), which is more abundant than coarse particulate organic matter (CPOM) in hyporheic habitats, and should be favoured by the occurrence of shredders that produce FPOM from CPOM. 3. We used laboratory experiments to quantify leaf litter processing by N. rhenorhodanensis and a common shredder, the surface amphipod Gammarus roeselii. We estimated rates of feeding and assimilation (using nitrogen stable isotopes) of the two species separately and together to reveal any potential shredder-collector facilitation between them. 4. Measured leaf litter mass loss showed that N. rhenorhodanensis did not act as a shredder, unlike G. roeselii. Organic matter dynamics and N/N ratios in tissues of niphargids indicated that N. rhenorhodanensis was a collector-gatherer feeding preferentially on FPOM. We also found a positive influence of the gammarid shredders on the assimilation rate of N. rhenorhodanensis, which fed on FPOM produced by the shredders, supporting the hypothesis of a positive interaction between surface shredders and hyporheic collector-gatherers. [ABSTRACT FROM AUTHOR]

1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel-based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders. [ABSTRACT FROM AUTHOR]

Stormwater infiltration systems (SIS) have been set up to collect and infiltrate urban stormwater runoff in order to reduce flooding and to artificially recharge aquifers. Such practices produce environmental changes in shallow groundwater ecosystems like an increase in organic matter concentrations that could drive changes in structure and functions of groundwater microbial communities. Previous works suggested that SIS influence groundwater physico-chemistry during either rainy and dry period but no study has examined the impact of SIS on groundwater microorganisms during both periods. This study aimed to fill this gap by assessing SIS impacts on groundwater quality parameters in three SIS with vadose zone thickness < 3 m during two contrasting meteorological conditions (rainy/dry periods). Physicochemical (dissolved organic carbon and nutrient concentrations) and microbial variables (biomass, dehydrogenase and hydrolytic activities, and bacterial community structure) were assessed on SIS-impacted and non-SIS-impacted zones of the aquifers for the three SIS. Using clay beads incubated in the aquifer to collect microbial biofilm, we show that SIS increased microbial activities, bacterial richness and diversity in groundwater biofilms during the rainy period but not during the dry period. In contrast, the significant differences in dissolved organic carbon and nutrient concentrations, biofilm biomass and bacterial community structures (Bray-Curtis distances, relative abundances of main bacterial orders) measured between SIS-impacted and non-SIS-impacted zones of the aquifer were comparable during the two periods. These results suggest that structural indicators of biofilm like biomass were probably controlled by long-term effects of SIS on concentrations of dissolved organic matter and nutrients whereas biofilm activities and bacterial richness were temporally stimulated by stormwater runoff infiltrations during the rainy period. This decoupling between the structural and functional responses of groundwater biofilms to stormwater infiltration practices suggests that biofilms functions were highly reactive to fluxes associated with aquifer recharge events. Unlabelled Image • Stormwater infiltration systems (SIS) produced an increase in biomass of groundwater biofilms during both dry and rainy periods. • Bacterial community structures of groundwater biofilms were impacted similarly by SIS during both rainy and dry periods. • Microbial activities were more stimulated by SIS during the rainy than during the dry period. • Functional response of biofilms to SIS was influenced by stormwater reaching groundwater during rainy periods. • The structural response of biofilms to SIS was persistent whatever the meteorological conditions. [ABSTRACT FROM AUTHOR]

Heterogeneity of hyporheic fauna is associated with geomorphological features and related vertical water exchanges. Constrictions on river floodplain are known to induce groundwater inputs and increase stygobite fauna. Two floodplain constrictions were studied in a large braided river (the Drôme River): one linked to a natural process (valley narrowing), another to an artificial river regulation (early 20th embankment). Spatial distribution of hyporheic organisms were sampled upstream and downstream of the two constrained sections, at 9 stations, 3 positions (left and right sides, centre of the braided strip), 3 replication points and at a depth of 50 cm in the river sediment. The spatial heterogeneity in community composition was higher near the banks than at the centre of the braided strip, no matter the width of the strip. The artificial constriction induced a decrease in spatial heterogeneity of the benthic fraction of the hyporheic fauna, but no changes were detected for the stygofauna. The natural valley narrowing reduced width and thickness of the alluvium and induced an inflow of groundwater resulting in an increase in stygofauna abundance. Natural floodplain narrowing linked to geology thus control the distribution of stygobite species, while artificial constrictions only modify the spatial distribution of the benthic fraction of the hyporheic fauna. Unlabelled Image • Braided channel narrowing can be natural or artificial. • Artificial narrowing induces homogenization of the benthic fraction of the hyporheos. • Natural narrowing induces upwelling of groundwater and increase in stygobite fauna. [ABSTRACT FROM AUTHOR]

Highlights: [•] Evaluation of groundwater quality must include biological and ecological indicators. [•] Crustaceans are used as sentinels organisms caged inside groundwater. [•] Gammarus pulex and Niphargus rhenorhodanensis were chosen. [•] An ecophysiological index combined survival rates to states of the body stores. [•] The ecophysiological index of sentinels was correlated to groundwater quality. [Copyright &y& Elsevier]

Abstract: The European water legislation highlights the necessity of developing ecological criteria for assessing the quality of aquatic ecosystem. While a multitude of ecological indicators has been proposed and validated for streams, rivers and lakes, these indicators are not available for groundwater ecosystems. In the present study, we developed a method based on the measurements of the growth and activity of microorganisms developed on artificial substrates (glass beads) incubated in wells to characterize the trophic status of groundwater. Incubation sites were selected in an urban aquifer where previous works showed contrasting trophic conditions in groundwater due to artificial groundwater recharge practices. Total proteins, total carbohydrates, dehydrogenase and hydrolytic activities were measured on glass beads incubated in wells for two periods of 2 months (October–December 2010 and April–June 2011). Biofilm measured on glass beads was significantly more developed and active in wells where groundwater was enriched with dissolved organic matter due to artificial groundwater recharge. Indeed, most microbial variables (total proteins and dehydrogenase activities for the two incubation periods and hydrolytic activities for the second incubation period) were significantly and positively correlated with the concentrations of dissolved organic carbon (DOC) in groundwater. The availability of phosphorus also tended to influence biofilm growth (assessed by total carbohydrates) when PO4 3− concentrations were lower than 50μg/l. Overall, our study clearly demonstrated that artificial substrates acting as colonisable area for microorganisms could be used to efficiently monitor nutrient enrichment in aquifers. [Copyright &y& Elsevier]