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.     

Effects of spates on interstitial assemblages of the Rhône River. Importance of spatial heterogeneity

The dynamics of interstitial assemblages, after a spate and during low discharge, was studied in a regulated channel (Miribel Canal) of the Upper Rhône River, France. Using a Bou-Rouch pump, three stations were samples: 1) Station IIA, a site fed by superficial water infiltrations, 2) Station IIC, a site fed by riparian phreatic water, and 3) Station IA, a site fed by both surface and phreatic waters. The spate greatly influenced the interstitial assemblages, their dynamics were different according to the hydrology of the given site. At Station IIA, the spate had a wash-out effect on the assemblages (reduction in abundance and diversity), whereas during low discharge the interstitial layer received a continuous influx of epigean organisms (benthic and limnophilous). At Stations IIC and IA, the spate introduced numerous limnophilous and benthic invertebrates into the interstices, which function as an organismic trap. However, these sites appear to be more isolated from surface waters during low discharge. Stygobites decreased or disappeared after the spate. They appear highly sensitive to hydrologic perturbations in the surface waters.     

Oligochaete drift in a large river (French Upper Rhône): the effect of life cycle and discharge

In the framework of a macroinvertebrate drift study, the oligochaete community was analysed over one year (1989) at six sites located in a partly impounded reach of the Upper Rhône River (France). Drift composition between sites was somewhat affected by the environmental conditions. These differences were caused by significant deviations in drift rates (P < 0.05) of endobenthic oligochaetes and non-swimming Naididae. There was no significant difference (P > 0.05) among the swimming Naididae. As the latter strongly dominated, global spatial differences in the drift were weak. Temporal changes constituted the main source of variability in drifting oligochaetes. Life cycles and reproductive ecology of four major swimming Naidid species, Nais elinguis, Stylaria lacustris, Nais christinae, and Nais barbata, explained most of the drift structure over the year. Hydrological fluctuations also affected drift structure, both for swimming and non-swimming species. Taxon richness and relative abundance in the drift were very similar to those obtained in the river benthos from a reference reach previously studied. There was a lack of stygobiont and stygophilous species that are frequent in area of groundwater seepage. Biological and environmental implications of oligochaete drift are stressed. The drift phenomenon is appropriated to provide ecological informations on oligochaete communities in fast flowing and deep, large rivers.     

Impacts of global changes and extreme hydroclimatic events on macroinvertebrate community structures in the French Rhône River

We assessed the temporal changes in and the relationships between the structures of the macroinvertebrate communities and the environmental conditions of the French Rhône River (the river from Lake Geneva to the Mediterranean Sea) over the last 20 years (1985–2004). Multisite environmental and biological datasets were analysed using multiple CO-inertia analysis (MCOA) and Procrustean analysis. Changes in environmental conditions were mainly marked by an improvement in water quality between 1985 and 1991 and by an increase in water temperature from 1985 onwards due to climate change. Improvement in water quality seemed to delay changes in community structures under global warming. We then observed trends in community structures coupled with high temperatures and a decrease in oxygen content. Interestingly, we observed both gradual changes and rapid switches in community states. These shifts seemed coupled to extreme hydroclimatic events (i.e. pulse disturbances). Floods and the 2003 heatwave enhanced the development of eurytolerant and invasive taxa which were probably able to take advantage of gradual warming environmental conditions. Despite various site-specific “press” constraints (e.g. hydropower schemes, nuclear power plants), similar changes in community structures were observed along the French Rhône River. Such consistency in temporal processes at large geographical scales underlined the strength of hydroclimatic constraints on community dynamics compared to specific local disturbances. Finally, community structures did not show any sign of recovery, and their relative sensitivities to extreme hydroclimatic events seemed to increase with time. Thus, our results suggest that global changes may reduce the resilience of current community states.     

Effects of a plume front on the distribution of inorganic and organic matter off the Rhône River

Hydrological and chemical structures off the Rhône River estuary resulting from the introduction of the river flow into the Mediterranean Sea are described. The effect of the fresh-water/sea-water interface on the distribution of inorganic and organic matter off the Rhône river is investigated. Strong vertical gradients of inorganic and dissolved organic matter such as lipids characterized the first few meters in this area (from 83.7 to 0.6 N-NO₃ µgat l^–1, from 6.39 to 0.92 N-NH₄ µgat l^–1 and from 299 to 73 µg l^–1 of total dissolved lipids). At the interface, substantial increases of particulate organic (PON: from 45 µg l^–1 at surface to 118 µg l^–1 at the interface, POC: from 462 to 876 µg l^–1, total particulate lipids: from 33 to 648 µg l^–1) and suspended matter in general (from 18 to 22.2 mg l^–1) were observed. High phytoplanktonic production may account for some of this enrichment, although passive accumulation might also be involved.     

Influence of wastewater treatment on the microbial ecology of a large, temperate river system — the Danube River

In an attempt to assess the influence of tertiary-treated sewage influx on bacterial metabolism in the Danube River, bacterial abundance, ectoenzymatic activity, faecal coliforms, faecal streptococci, chlorophyll a (chl a), concentrations of dissolved (DOC) and total organic carbon (TOC) and inorganic nutrients were measured upstream of the sewage influx and compared with sampling sites downstream. Additional samples were taken near the outlet of the sewage treatment plant. Bacterial abundance as determined by epifluorescence microscopy was compared with plate counts of total heterotrophic bacteria. Significantly higher values were obtained at the stations downstream from the sewage influx only for faecal coliforms and faecal streptococci, for glucuronidase activity and bacterial biovolume. All the other parameters were not significantly different from values obtained at the upstream sampling site. Strong seasonal dependence was detectable for nitrate with high concentrations during the winter ( 250 µM) and low concentrations during summer ( 100 µM). A distinct spring phytoplankton bloom was noticeable in the river reaching chl a concentrations of 70 µg 1^–1; during the remaining seasons chl a concentrations were <20 µg 1^–1. Highly significant correlations were found between faecal coliform counts and glucuronidase activity. C:N:P ratios of 13.9:10.7:1 (for the upstream station) and 11.7:9.2:1 (for the downstream stations) of dissolved nutrients are rather low indicating severe carbon limitation of bacterioplankton. Based on our results we conclude that the heterotrophic bacterial community is not significantly effected by the input of treated sewage but faecal contamination is readily detectable over a comparatively long reach of 30 km.     

Influence of the Włocławek Reservoir on hydrochemistry and plankton of a large,lowland river (the Lower Vistula River,Poland)

The research on the W?oc?awek Reservoir (WR), situated on the Lower Vistula River (Poland), was carried out during two periods: between 1986 and 2002, hydrochemistry was studied, and between 1994 and 2000 – plankton. WR is 70 km² in area, its capacity is 370 × 10⁶ m³ and the retention time with the average flow rate (Q) amounts to 930 m³ s^?1 – only 5 days (!). Until 2002, due to cyclic work of a hydroelectric power plant, considerable variability in the daily flow rate was recorded (from 600 to 1800 m³ s^?1). Its specific hydrology and fluvial character mean that WR functions in a completely different way compared to dam reservoirs described in textbooks, with significantly longer retention time.WR significantly improves most of the water quality variables in the river. It reduces the concentration of suspended matter (on average by more than 50%), P_tot (by 16%), BOD₅ (by 40%) and chlorophyll “a” (by 50–60%) (unpublished data).Phytoplankton is rich in species – altogether 441 taxa, with the count of 7.4 × 10⁶ individuals per dm^?3 and biomass of 16 mg dm^?3. Throughout the research period, a significant reduction in the abundance of phytoplankton was recorded in WR, sometimes more than 60%. The total number of zooplankton taxa at the site upstream from WR and in the reservoir was almost identical (68 and 67), however in the reservoir itself, a small decrease in the number of Rotifera species and an increase in the number of Crustacea species was observed, as well as a significant, fourfold increase in the total zooplankton biomass. The aforementioned reduction of phytoplankton and enrichment of the river with Crustacea are one of the major biocenotic consequences of the functioning of the W?oc?awek Reservoir.     

Demographic structure and life history traits of the common goby Pomatoschistus microps (Teleostei,Gobiidae) in a Mediterranean coastal lagoon (Rhône River delta,France)

Demographic structure and life history traits of the common goby Pomatoschistus microps were investigated in a brackish water lagoon of the Rhône River delta (Mediterranean Sea, southern France). The size frequency distribution and gonadosomatic index indicated that three different age groups occurred and reproduced successively in the lagoon, resulting in a long spawning period from March to September and a high investment in reproduction. This high investment in reproduction, which contrasts with that found in other mostly northern European populations, probably relates to the unpredictability of the gobyˈs environmental conditions.     

Rates of nitrification and carbon uptake in the Rhône River plume (northwestern Mediterranean Sea)

Nitrification rates were measured along a salinity gradient in the Rhône River estuary, using specific inhibitors (allylthiourea and chlorate) coupled with the measurement of change in nitrite concentration and inorganic carbon uptake by nitrifiers. Rates of ammonium and nitrite oxidation were similar up to 15 practical salinity units (from 1 to 2 mol N oxidized liter^-1 day^-1). For higher salinities, nitrite and ammonium oxidation rates were 0.14 and 0.23 mol N oxidized liter^-1 day^-1, respectively. Ammonium oxidizers assimilated 19–150 × 10–3 mol C liter^-1 day^-1, while nitrite oxidizers fixed 4.8–72.6 × 10–3 mol C liter^-1 day^-1. The amounts of nitrogen oxidized and C incorporated demonstrated a linear correlation (r ² > 0.99). The ratio of N oxidized to C incorporated ranged between 14.3 to 12.3 for ammonium oxidizers, and between 31.6 and 29 for nitrite oxidizers, the lower values being measured in seawater. Offprint requests to: M. Bianchi.     

Succession and fluctuation in the aquatic vegetation of two former Rhône River channels

The vegetation dynamics in two former braided channels of the Rhône River was studied at two time scales in order to test the following hypothesis: fluctuations would occur within seasons (flood disturbances, hydrological fluctuations, phenology) while successions would occur between years. The vegetation was surveyed in 1983, 1988 and 1989 during summer for the interannual investigation, and in spring 1989, summer 1989, winter 1989 and spring 1990 for the seasonal investigation. Terrestrialization, which was observed within the same period in other braided former channels of that river, did not happen here despite the 1989 drought. However, a vegetation zone situated in the upstream part one channel seems to represent some successional trend, resulting in the establishment of Nasturtium officinale and the increasing abundance of Chara vulgaris. In disagreement with the tested hypothesis, only fluctuations are observed at the two temporal scales in the other vegetation zones. The amplitude of cyclic trajectories observed in the seasonal study depends of the degree of hydraulic disturbances (floods, drought) that affects each vegetation zone. The channel that is closer to the river is maintained at a steady state by the periodical inputs of kinetic energy during river overflows and fast floods; the disturbances wash away fine deposits and rejuvenate the vegetation mosaic. In the other former channel that is less disturbed by floods and is characterized by a thick layer of fine sediments, the groundwater inputs from numerous limnocrene springs carry away organic matter and slow down ecological successions.Abbreviations C.A. Correspondence Analysis     

Transfers between a eutrophic ecosystem,the river Rhône,and an oligotrophic ecosystem,the north-western Mediterranean Sea

Transfer processes from a eutrophic ecosystem, the river Rhône, to an oligotrophic environment, the north-west Mediterranean Sea, were studied during the winter period, when inputs of dissolved and particulate matter are at their highest.During a cruise carried out in February 1987, salinity, temperature, turbidity, nutrients, pigments, suspended particulate matter and particulate carbon and nitrogen were measured in surface and subsurface waters near the Rhône outlet.For most of the parameters the results show that during this period a simple dilution process occurs in the river plume of the Rhône without adsorption, desorption, precipitation or biological uptake, contrary to recordings for some other rivers. All the nutrients discharged into the sea by the Rhône are available for utilization by marine phytoplankters.     

Cross frontal variability in hydrological and biological structures observed in a river plume area (Rhône mouth, NW Mediterranean Sea)

The lake herring (Coregonus artedi) is an important coldwater planktivore in the Laurentian Great Lakes and in smaller inland lakes in portions of Canada and the northern United States. Lake herring cruise the pelagia and feed selectively in both gulping and particulate modes. They are visual predators in environments with adequate illumination. Visual predation by fish consists of a series of discrete steps. We studied the first step in the predation sequence, reaction to prey, at light intensities of 2–1500 Lx in a simulated pelagic environment at 10–13°C. We measured lake herring reactive distances, the distance at which a prey item will be detected and attacked, to live Limnocalanus macrurus, a natural prey of lake herring in Lake Superior. We used the reactive distances and associated angles of bearing and elevation, which described the location of the prey relative to the lake herring, to calculate reactive volume. This reactive volume can be envisioned as an irregular sphere surrounding the fish, within which prey are detected and attacked. All of the attacks on prey occurred in the anterior portions of the sagittal and lateral planes of the lake herring, as would be expected for a pelagic, cruising fish. The reactive volume surrounding the lake herring was generally spherical, but was more irregular than the simple spheres, hemispheres, cylinders, cones or other geometries assumed in previous studies. The reactive distances and the reactive volume changed with light intensity and were significantly smaller at 2–10 Lx than at 40–1500 Lx. At 40–1500 Lx, the reactive volume was expanded over that observed at 2–10 Lx laterally and caudally. Collectively our results indicate that lake herring can visually forage most effectively in environments with light levels >10 Lx.     

Genetic structure of fragmented populations of a threatened endemic percid of the Rhône river: Zingel asper

Zingel asper is an endemic percid of the Rh?ne basin considered to be critically endangered. This species was continuously distributed throughout the Rh?ne in 1900, but today only occupies 17% of its initial area. In the present study, five microsatellite loci were used to assess the level of genetic variability within and among populations localized in different sub-basins. Contrasting results were obtained for the three main populations from the Rh?ne. A reduced allelic diversity was observed for the two populations displaying the lowest patch sizes (length of the river system occupied); of these, a recent genetic bottleneck was detected for the population showing a particularly low density. However, the third population was characterized by a relatively large spatial extent, high local fish concentrations and an allelic diversity that was twice as high and associated with an equilibrium between mutation and drift. Thus, this population shows an apparently better evolutionary potential for long-term survival. Since 1930, a marked fragmentation of the whole Rh?ne system has appeared, related to the development of dams, and we assume that the significant genetic differentiation detected between the populations could mainly reflect the impact of this fragmentation. The high turnover of the Z. asper populations, and the major role of dispersal in population persistence (highlighted in a recent population dynamics study), indeed suggest that the differentiation observed could mainly have arisen from habitat fragmentation in recent history.     

Influence of ionic strength and conductivity on benthic diatom communities in a tropical river (Monjolinho), S?o Carlos-SP, Brazil

Benthic diatoms are important indicators of ecological conditions in lotic systems. The objective of this study was to elucidate the confounding effects of eutrophication, organic pollution and ionic strength and conductivity on benthic diatom communities. Benthic diatoms and water quality sampling was done at 10 sites during summer base flow period (2008 and 2009). Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to determine environmental gradients along which species vary with respect to ionic strength and conductivity and other environmental variables. Using variance partitioning, we assessed the individual importance of a set of environmental variables (eutrophication and organic pollution) versus ionic strength and conductivity on diatom community structure. The effects of ionic strength and conductivity and organic pollution, eutrophication and other environmental variables were integrated into overall resultant benthic diatom communities. Through partial CCA, we partitioned the variance in diatom data between two sets of exploratory variables, i.e. ionic strength and conductivity (26.9%); other variables, particularly eutrophication and organic pollution (23.0%); shared variance (11.3%) and unexplained variance (38.8%). Due to the interaction of the effects of ionic strength and conductivity and other variables in this study, laboratory experiments must be performed to confirm the observed effects of ionic strength and conductivity.     

Do parafluvial zones have an impact in regulating river pollution? Spatial and temporal dynamics of nutrients,carbon, and bacteria in a large gravel bar of the Doubs River (France)

The changes in both physical and chemical properties of interstitial water were investigated within a large gravel bar to determine if the parafluvial zone was a hotspot for nutrient transformations in a lowland eutrophic river, the Doubs (Eastern France). Interstitial water was sampled in 30 piezometers along five transects across the gravel bar, surface water was sampled in main and chute channels. Five campaigns of sampling were performed among summer and winter. In both interstitial and surface samples, water chemistry (electrical conductivity, pH, chloride, dissolved oxygen concentrations—DO) and nutrient concentrations (nitrate—NO₃ ⁻, ammonium—NH₄ ⁺, soluble reactive phosphorus—SRP, dissolved organic carbon—DOC) were measured. Moreover, temperature, water level, sediment grain size distribution and total bacterial abundance were assessed along a flowpath through a lateral gravel bar of the Doubs River. Measurements of water table elevation in the bar, main and chute channels indicated that the parafluvial flowpath was perpendicular to the main channel. Very low changes in chloride concentration and electrical conductivity showed minor groundwater input along the flowpath. The parafluvial zone was 0.9 m thick under the mean piezometric level, hydraulic gradients along the flowpath were 0.3%, and the discharge of interstitial water through the bar calculated at low flow was 40.6 m³ day⁻¹. Most changes in interstitial nutrients occurred during the warm season, suggesting that biotic mechanisms occurred. Along the flowpath, DO, DOC, and bacterial abundance declined, while phosphate increased. Temporal trends of nitrate were less clear, exhibiting a slight increase during spring (organic matter biodegradation) and a decrease during summer (denitrification). Using the parafluvial discharge and nutrient concentrations of water infiltrating into and seeping from the gravel bar, the retention capacity of the bar was 171 gC day⁻¹ for DOC and 48.3 gN day⁻¹ for nitrate. Phosphate production ranged from 0.65 to 2.3 gP day⁻¹. These values were low compared to the nutrient fluxes in the river Doubs, suggesting a minimal local impact of the parafluvial zone in regulating of river pollution. Handling editor: J. Padisak     

Are the banks a source of recolonization after disturbance: an experiment on aquatic vegetation in a former channel of the Rhône River

Hydrolysis of natural dissolved organic phosphorus (DOP) in three hardwater lakes of different trophic level was calculated from kinetic data of phosphatase activity (PA) in different size fractions. DOP as well as kinetics of PA were determined every fortnight in depth profiles during the year 1990. 60% of DOP was assumed to be suitable substrate for phosphatases. The rate of hydrolysis increased markedly with higher trophic level. Average hydrolysis rate of DOP in polytrophic lake Thaler See was 3.26 nM P min^–1 (6 µg P-PO₄ l^–1 h^–1). In oligotrophic Lake Herrensee, dissolved phosphatases were responsible for more than half of the total hydrolysis. In the other two lakes, bacterial and algal surface PA dominated hydrolysis in changing parts depending on kinetics and DOP concentration. The regeneration rate of phosphate by PA was compared to phosphorus (P) excretion rate of zooplankton. Excretion was calculated from zooplankton data and excretion equations from the literature. In oligotrophic Lake Herrensee, excretion by zooplankton recycled in average 18% of the phosphate amount which was hydrolysed from DOP by PA. With higher trophic level, relevance of P excretion from zooplankton decreased drastically.     

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.     

Using γ-emitting artificial radionuclides,released by nuclear plants,as markers of restricted movements by chub,Leuciscus cephalus,in a large river,the Lower Rhône

Synopsis On the basis that -emitting artificial radionuclides, present in nuclear plant radioactive wastes, can be considered as natural collective markers of fish living downstream from a radioactive discharge, we studied the restricted movements of chub in a reservoir on the Lower Rhône river where the Marcoule nuclear plant is located. A qualitative determination, based on the detection of specific radioelements in our samples originating from radioactive waste, and a quantitative determination of the cesium-137 concentration in the samples, were used to identify fish radioactively marked by the effluent. Individual measurements of -radioactivity in 49 adult chub captured at two stations, each 2 km long and 6 km apart, divided the fish into two distinct sub-units: one living downstream of the discharge pipe, in which 73% were marked, and the other upstream, in which 79% were not marked. Similar results were found in two neighbouring stations when we analysed previous radioecological measurements of -radioactivity in groups of chub. After combining all the data concerning chub, detailed information was obtained on the spatial stability of the chub population in the reservoir. Two spatially different stocks were found and each stock can be divided in two components: a sedentary component that remains in a restricted zone (its home range), and a mobile component that undertakes movements between the two zones.