ALBERT

Notes: 〈list style="custom"〉1Unlike riffles, research has focused rarely on the hyporheic zone of pools. To highlight the functioning of a pool, field investigations were performed in a riffle-pool-riffle sequence by integrating simultaneously physico-chemistry, microbes and invertebrates. The study was conducted in a channel characterised by strong downwelling of surface water.2To include the downstream flux of water within the sediment, a longitudinal profile was studied along six stations situated: at the centre (Station 1) and at the lower end (Station 2) of the first riffle, at the upstream part (Station 3), at the centre (Station 4, at the inflection point) and at the lower end (Station 5) of the pool, and at the centre of the second riffle (Station 6). At each station, three replicate samples were taken and three sample depths were investigated (0.2, 0.5, and 1.0 m below the stream bed) on two dates.3Physico-chemical parameters (vertical hydraulic gradient, oxygen concentration and specific conductance) differed between stations depending on infiltration rates. In contrast, organic matter and microbial parameters presented patchy distributions linked with factors other than the geomorphological pattern. Despite not very pronounced geomorphologic features, the slope variation at the centre of the pool (at the inflection point) affects the distribution of epigean and hypogean invertebrates.4Based upon faunal parameters, the pool could be divided into an upstream and a downstream part, the latter being more strongly influenced by surface water.5The pool should be considered as a heterogeneous area. In that respect, the inflection point of a pool may be as important as the top of a riffle in the functioning of river sediments.

Notes: 1. Hydrological exchange between the surface stream and the hyporheic zone is well documented in the main channel of rivers, especially at the reach scale. Hydrological processes of advection/convection occur at different scales, and in secondary channels of large rivers little is known about these exchanges in the hyporheic zone on a broad scale (i.e. kilometres). This work studied exchanges of water and biota in a secondary channel on a large scale (4 km), using a three-dimensional framework.2. The exchanges of water were described using physicochemical indicators of surface and groundwaters. Samples of water and biota were taken in three dimensions: (i) vertically from benthic (i.e. 0.20 m below the surface of the substratum) to hyporheic (0.50 m) and deep interstitial (1.0 m) zones; (ii) laterally from the right to the left bank (i.e. right, middle and left positions); and (iii) longitudinally from upstream to downstream (seven stations regularly distributed along the channel).3. The physicochemical indicators clearly revealed hydrological heterogeneity in the longitudinal and vertical dimensions, whereas lateral variability was not significant.4. Spatial distribution of biota exhibited strong longitudinal variations that were not gradual as predicted by an upstream/downstream continuum, but patchy and discontinuous. No significant differences were found between the three positions across the channel.5. Analyses of both physicochemical and faunal data sets produced matched ordination of samples and stations, indicating that interstitial–surface flow relationships appear to be an important governing factor in the distribution of interstitial biota at this broad scale.6. Results are discussed in relation to the hypothetical three-dimensional models of the hyporheic zone in rivers. Contrasting with other observations on the main channel (where advection/convection patterns are dominated by morphological changes of the river-bed morphology), it is proposed that water exchanges in backwaters are more likely to be related to local modifications of stream-bed porosity.

Notes: 〈list style="custom"〉1Copepoda, Ostracoda and ‘Cladocera’ are important meiobenthic Crustacea which can be both numerically abundant and species rich in running waters. Harpacticoids and ostracods are well adapted to benthic life because they are typical crawlers, walkers, and burrowers. Many cladocerans are substratum dwellers, but most benthic species among these can also swim. Cyclopoids which are generally good swimmers are nevertheless often bottom frequenters and actively colonise sediment interstices (the hyporheic zone).〈list style="custom"〉2The subclass Copepoda includes 10 orders. With 53 families, the order Harpacticoida dominates the benthos. Only five of these families are represented in fresh waters (ca. 1 000 species and subspecies). The order Cyclopoida includes 12 families of which the Cyclopidae is well represented in freshwater habitats with 900 species and subspecies. Freshwater Ostracods belong to the order Podocopida (5 000 species) with three superfamilies occurring in running fresh waters. The group ‘Cladocera’ contains four orders, 12 families, more than 80 genera, and 450–600 freshwater species. Most of the benthic species are found in the families Chydoridae (39 genera), Macrothricidae, Ilyocryptidae and Sididae.3For each of the three major taxa, morphological characteristics are presented, specimen collection and preparation are described and references to available taxonomical keys are provided.4Biological characteristics are extremely diverse among and within the three taxa, resulting in a great variety of strategies in meiobenthic crustaceans. Characteristics of reproduction, sexual dimorphism, cyclomorphosis and population parameters (i.e. clutch size, lifespan, growth, moulting) are provided for some of the most common species.5Important differences between the three main taxa were found at the species level. Ecological requirements such as hydraulic microhabitats and geomorphologic features of the streambed are the major determinants of species diversity and abundance for benthic microcrustacea of lotic habitats. Many studies on the ecology of these communities are limited by a lack of knowledge of the life history characterisitics of lotic (especially interstitial) crustacean populations.

Notes: 1. Natural experiments, in the form of disturbance from spates, were used to study the resistance and resilience of interstitial communities. Investigations were conducted in a by-passed section of the Rhône River characterized by an artificial hydrology with frequent spates separated by regular minimum discharge of 30 m3 s–1.2. Three areas of a bar were studied, upwellings at the head of the bar (stations 1 and 2), and downwelling at the tail of the bar (station 3). In the head of the bar the substratum was characterized by stable cobbles, while mobile gravels dominated in the tail of the bar. At each station, samples were derived from four depths (0.5, 1.0, 1.5 and 2.0 m below the surface of the substratum). Fifteen spates occurred during the study period whose peak discharge ranged from 50 to 1640 m3 s–1. Temporal variations of the fauna were studied by comparing the spate effect observed 1 day (resistance), 7 days (resilience) and 17 days after the spate. Within-class correspondence analysis was used to compare the temporal variability of the fauna within each class {station/depth}.3. The fauna differed markedly between the three stations, and the relative density of stygobionts (i.e. hypogean fauna) decreased from 55% at station 1 to 4% at station 3. The spatio-temporal variability increased dramatically from station 1 to station 3.4. The results suggest that the hyporheic zone acts as a patchy refugium: the stations were more or less active refugial zones, depending on hydrology (upwelling or downwelling), substratum stability and spate amplitude.5. The downwelling station was the main refugium area for benthic taxa. Important migrations of benthic groups (e.g. Gammarus, Cladocera) or hyporheic taxa (e.g. Cyclopoida and Harpacticoida) were observed deep into the sediment (2 m). Vertical movements of stygobionts (Niphargus, Niphargopsis) were also observed at high amplitude spates. These movements were very important (great numbers of individuals migrated) at low and medium magnitude spates, but were unimportant at high discharge, when the threshold of sediment instability was exceeded. In this case the substratum became mobile and induced drift of benthic organisms.6. Conversely, in the upwelling stable stations, accumulation was less important (lower number of species and lower densities) but more constant with increasing discharge, suggesting that substratum stability is also a key factor.7. Generally recovery was rapid at all stations (within 7 days) but no relationships were found between resilience (rate of recovery) and the amplitude of spates.