Cécile Claret

Seasonal dynamics of nutrient and biofilm in interstitial habitats of two contrasting riffles in a regulated large river

Abstract: To describe nutrient and biofilm dynamics in interstitial habitats of riffles (in a downwelling - upwelling sequence), oxygen, nitrate, dissolved organic carbon (DOC) contents, bacterial abundances, hydrolytic and deshydrogenasic activities of biofilms were studied for 18 months in two geomorphologically contrasting riffles of a regulated channel of the Rhône river (a 7th-order stream). The first one was located in an section of the channel affected by river bed incision, whereas the other section has only been slightly impacted by river bed incision. During the transit of surface water through river bed sediments, oxygen consumption, DOC immobilization, and nitrate production were observed especially during the warm seasons when biofilm activity was high. Organic matter decomposition by interstitial biofilms may induce nitrate release by mineralization and nitrification. When the oxygen content decreased strongly during summer, exceptional nitrate depletion occurred in the sediments due to denitrification. A comparison with the Maple River, a 3rd-order stream (Hendricks, 1993) highlights that differences were mostly linked to the origin of groundwater, to differences in stream order (high quantities of nutrients), and to local geomorphological characteristics (heterogeneity in fine sediments and total organic matter - TOM - deposition). River bed incision might induce (1) weak biofilm development and activities in the sediment, (2) changes in vertical distribution of bacteria and biofilm activities, (3) marked temporal variations of biofilm activities, and (4) decrease of sediment efficiency in oxygen consumption, DOC immobilization, and nitrate production.

Effects of management works on the interstitial fauna of floodplain aquatic systems (River Rhône, France)

Long-term changes in composition, structure and biodiversity (i.e. taxonomic richness, diversity index, species traits and habitat-affinity) of interstitial assemblages were studied in two floodplain systems: a restored backwater and an artificial drainage canal. Before restoration, the backwater, affected by both terrestrialisation and eutrophication, was weakly populated by a low diversified fauna dominated by walkers, macrofauna, detritivores, and stygoxenes (i.e. taxa that occur incidentally in ground waters) that reproduce biparentally and lack parental care. This backwater displayed an upstream–downstream gradient in response to restoration works. Upstream, the dredging of fine organic sediments favoured inputs of nutrient-poor groundwater and exchanges between groundwater and surface water that induced an increase in taxonomic richness (in both herbivores and stygoxenes). Downstream the deposition of fine sediment that was suspended in the water column by restoration work enhanced colmation that induced a decrease in herbivore and swimming taxa, and an increase in mesofaunal taxa, whilst phreatobites (i.e. taxa specialized to interstitial life) remained absent from the system. The drainage canal that was artificially hollowed-out to lower the surrounding water table, harbors mixed assemblages of epigean (i.e. taxa of surface-water habitats) and hypogean (i.e. taxa of groundwater habitats) taxa. The upstream part, which is weakly influenced by surface waters, was colonized by phreatobites as oligotrophic conditions increased. The intermediate part, which is fed by surface water and where mesotrophic conditions occurred as habitats progressively matured and diversified, showed diversification of its fauna. The downstream part of the drainage canal displayed the reverse dynamic – this suggests a reduction in groundwater supply due to the clogging of sediment interstices fine sediments, the deposition of which is linked to the Rossillon backwater restoration works.

Influence of benthic and interstitial processes on nutrient changes along a regulated reach of a large river (Rhône River, France)

Benthic and hyporheic (i.e. the water-saturated interstitial zone beneath river bed-sediments) processes together modify particulate and dissolved nutrient fluxes in streams, but the relative importance of these processes remains unstudied in large rivers. Changes in material and nutrient fluxes (total suspended matter, biodegradable and refractory dissolved organic carbon, and nitrate) were monitored along a reach with a regulated discharge (a by-passed section downstream of a dam). Mass balance diagrams highlight a contrasting functioning between upstream and downstream parts of this sector of the Rhône River (Pierre-Bénite sector, France). At the upstream location close to the water input, characterized by partially clogged sediments and large lentic zones, decrease in total suspended matter (TSM) and biodegradable dissolved organic carbon (BDOC) fluxes resulted from high rates of benthic processes (i.e. sedimentation, respiration and mineralisation). At the downstream location, characterized by strong vertical water exchanges in a large riffle, nutrient changes resulted from the combination of processes that took place at the surface of the stream bed and during water percolation through riffle sediments. Physical benthic processes (scouring of periphyton, erosion and resuspension of sediment due to higher surface velocity) lead to an increase of TSM and DOC fluxes. Within the riffle, decomposition and mineralisation of organic matter in the first meter of the infiltration zone, and physical entrapment of RDOC, make the riffle as a sink for DOC and a source of nitrate. The significant contribution of the riffle to the self-purification capacities of this large river reach shows the relevance of including interstitial compartment in self-purification studies.

Comparison of solutes, nutrients, and bacteria inputs from two types of groundwater to the Rhône river during an artificial drought

Solute, nutrient and bacterial inputs to the River Rhône from the interstitial habitat of a gravel bar and the floodplain aquifer were investigated during an artificial drought. Eight springs were investigated: four groundwater-fed springs in the floodplain, located at the bottom of the bank; and four interstitial-fed springs located at the downstream end of a gravel bar. During this period, the inflows of groundwater to the river represented an average input of 0.77 mg l−1 of nitrogen (of which 93.3% were nitrates), 0.0187 mg l−1 of total phosphorus (of which 42.2% was orthophosphate), 3.56 mg l−1 of silica, 2.315 ± 0.703 mg l−1 of dissolved organic carbon (DOC, of which 47% was biodegradable) and 7.3 × 104 ± 3.7 × 104 bacteria per ml (of which 8.8% were active). Silica, DOC, biodegradable DOC, and bacteria concentrations displayed temporal variations during the study, which seem to be linked to the biological activity of the groundwater biofilm. There was a strong heterogeneity between the two types of groundwater that flow to the river: concentrations of calcium and alkalinity were higher in bank springs than in gravel bars springs. In these latters, sulfate, sodium, nitrogen, phosphorus were significantly higher.

Hyporheic flowpaths and interstitial invertebrates associated with stable and eroded river sections: interactions between micro- and meso-scales.

Erosion of the river beds linked to human activities is currently a crucial issue. By inducing strong changes in river dynamics, this erosion alters many of the related ecological processes. The modification of the relationships between the river and its aquifer due to erosion is not really understood. The large scale consequences of erosion for the interactions between river and the underlying aquifer must be clarified, together with the small scale effects of river erosion on hyporheic flowpaths. The role of water exchange within the hyporheic zone on the distribution of interstitial invertebrates is universally recognized, but nothing is known on the potential links between riverbed erosion, the subsequent modification of water exchange at a local scale and their consequences on the distribution and biomass of hyporheic invertebrates. Two sections, stable versus eroded, were studied on the Herbasse, a third-order stream, by sampling hyporheic water and invertebrates at two depths within the substrate (−15 and −50 cm deep), upstream and downstream of ten riffles. Seven physical and chemical parameters were used to identify the hydrological flowpaths at both the riffle (up and downwelling areas) and the section scales (eroded vs stable) and to specify the level of nutrients in the hyporheic habitat. Significant differences were observed between the stable and eroded sections both in the vertical and longitudinal dimensions, with for example available oxygen decreasing with depth and along hyporheic flowpaths, and revealing more obvious differences in the eroded section gaining groundwater than in the stable section. Most of the hypogean organisms (Niphargus sp., Salentinella juberthiei, and Proasellus sp.) were most abundant at the deepest level, in the upwelling zones and the eroded section, while typical epigean taxa (Gammarus sp., Plecoptera and Ephemeroptera) were mostly present near the surface and in the stable section, suggesting that erosion processes may influence the development of hyporheic assemblages at riffle and reach scales.

Interstitial fauna in newly-created floodplain canals of a large regulated river.

Artificial drainage canals are often dug in large river floodplains to prevent winter inundation when groundwater level increases. Nothing is known about the biodiversity of the interstitial fauna of these artificial aquatic systems. The water chemistry and interstitial fauna of four drainage canals along the River Rhone (dug 11–15 years ago) were sampled in July during 3 years (1995–1997). A total of 53 taxa were found, with both epigean and hypogean organisms, and some rare phreatobites previously considered as absent from this sector of the Rhone. The faunal assemblage is characterized by limited temporal variations between the 3 successive years. Differences in interstitial fauna composition between the four drainage canals were mostly linked to oxygen availability and to heterogeneity in water origin (true ground water or surface water infiltration through embankment). Low oxygen content results in poorly diversified assemblages, which are always dominated by the same small set of species. In contrast, heterogeneity in water origin resulted in elevated faunal diversity. Copyright

Tributaries under Mediterranean climate: their role in macrobenthos diversity maintenance.

The taxonomic richness erosion and the role of tributaries in the maintenance of the taxonomic richness were considered in a Mediterranean catchment in southeastern France. Nine stations were chosen along the Arc stream (three stations downstream from an organic effluent and one station upstream from the pollution source) and on two groups of tributaries (three intermittent and two perennial). High biodiversity erosion was noticed in the main stem, revealing diffuse sources of pollution added to the expected effect of the localized organic pollution. Jackknife richness estimator and beta diversity indicated that the intermittent tributaries had the highest richness values and harboured 70% of the taxa recorded at the catchment scale. The intermittent flow tributaries seem to play a major role in maintaining the taxonomic richness in such catchments, highly impacted by anthropogenic activities. The detailed examination and the preservation of these ecosystems should be an important step in catchment management, and support the need for catchment-scale conservation of freshwater invertebrates.

Impact of incision on invertebrate assemblages of the hyporheic zone: assessment from taxonomic and functional classifications

The hyporheie zone, the dynamie interfaee between surfaee and subsurfaee waters, is of major signifieanee for stream funetioning (BouLTON et al. 1998), but many anthropogenie aetivities are likely to alter exehanges through this interfaee (BRUNKE & ÜONSER 1997, HANCOCK 2002). For instanee, dams and regulations have indueed ineision of bed-sediments in many European rivers (BRAVARD et al. 1997) that eauses ehanges in sediment grain size and distribution (e.g., surfaee pavement and reduetion offine sediment eontents) and the hydraulie eonneetion between surfaee and subsurfaee eompartments (e. g., inerease in surfaee water infiltration). These physieal ehanges may have eonsequenees on the eomposition (taxonomie and funetional) and vertieal distribution of invertebrate assemblages inhabiting this zone. Few attempts have been made to use funetional classifications to assess the integrity of the hyporheie faunal assemblages (CLARET et al. 1999, BouLTON 2000), although speeies traits have been widely used as monitoring and management tools in surfaee eeosystems ( e.g., DoLÉDEc et al. 1999, 2006, UssEGLIO-POLATERA et al. 2000, STATZNER et al. 2001). In this study, we foeused on the vertiea1 distributions of invertebrates in 2 riftles, a referenee and an eroded, eonsidering abundanee, riehness, and funetional traits (groundwater habitat affinity and trophie groups; CLARET et al. 1999) to assess potential impaet of ineision on surfaee-subsurfaee interaetions.