Drivers of functional diversity in the hyporheic zone of a large river.

The effects of regional (hydrogeology and geomorphology) and local (sediment and hydrology) characteristics on hyporheic assemblages were studied along a 40-km reach of a large gravel-bed river. Hyporheic water and fauna were sampled at the upstream and downstream positions of 15 large gravel bars. The resulting 30 stations varied in their sediment grain size, stability and direction of river-aquifer exchanges. The study concludes that at the 40-km (sector) scale, the longitudinal distribution of hyporheic fauna was controlled by 1) the hydrogeology of the valley (i.e. gaining vs loosing sectors) that modifies abundance and taxonomic richness of stygobites 2) current channel morphometry of the river (i.e. shape and location of meanders), and 3) historical changes (i.e. river incision) which modify abundance and richness of assemblages. At the local scale, we found that surface grain size and stability of the sediment evaluated by visual observation were poor predictors of hyporheos composition. In contrast, the local hydrology (i.e. downwellings, upwellings, low vertical exchanges) explained a large part of the abundance, taxonomic richness and composition of the hyporheic assemblages. Stations with low vertical exchanges were found poorly colonized, while the upwelling zones were rich in stygobites and downwelling areas harbor abundant and species-rich temporary hyporheos. It was also observed that functional diversity was controlled by the same parameters, with high relative abundances of stygobites in upwelling zones and POM feeders in downwelling zones. The heterogeneity of hydrological patterns, with alternation of upwellings and downwellings may represent the optimal spatial structure for hyporheic biodiversity conservation and resilience in rivers.

Does spatial heterogeneity of hyporheic fauna vary similarly with natural and artificial changes in braided river width?

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.

Na+ /K+ -ATPase gene duplications in clitellate annelids are associated with freshwater colonization.

Major habitat transitions, such as those from marine to freshwater habitats or from aquatic to terrestrial habitats, have occurred infrequently in animal evolution and may represent a barrier to diversification. Identifying genomic events associated with these transitions can help us better understand mechanisms that allow animals to cross these barriers and diversify in new habitats. Study of the Capitella telata and Helobdella robusta genomes allows examination of one such habitat transition (marine to freshwater) in Annelida. Initial examination of these genomes indicated that the freshwater leech H. robusta contains many more copies (12) of the sodium-potassium pump alpha-subunit (Na+ /K+ -ATPase) gene than does the marine polychaete C. telata (2). The sodium-potassium pump plays a key role in maintenance of cellular ionic balance and osmoregulation, and Na+ /K+ -ATPase duplications may have helped annelids invade and diversify in freshwater habitats. To assess whether the timing of Na+ /K+ -ATPase duplications coincided with the marine-to-freshwater transition in Clitellata, we used transcriptomic data from 18 annelid taxa, along with the two genomes, to infer a species phylogeny and identified Na+ /K+ -ATPase gene transcripts in order to infer the timing of gene duplication events using tree-based methods. The inferred timing of Na+ /K+ -ATPase duplication events is consistent with the timing of the initial marine-to-freshwater transition early in the history of clitellate annelids, supporting the hypothesis that gene duplications may have played a role in the annelid diversification into freshwater habitats.

Do thermal infrared (TIR) remote sensing and direct hyporheic measurements (DHM) similarly detect river-groundwater exchanges? Study along a 40 km-section of the Ain River (France).

Water exchanges through the hyporheic zone are crucial to many ecological processes in streams. One major challenge for river managers is to find a practical method for localizing these exchanges using rapid data acquisition techniques. This work compares spatially continuous data, acquired by Thermal Infrared (TIR) techniques, with discrete data collected in surface water and in the hyporheic zone (DHM), at sites of expected water exchanges (gravel bars). Forty gravel bars, distributed along a 40 km-sector of the Ain River were sampled at upstream- and downstream-bar positions (80 sites) in order to reveal hyporheic exchanges. At each site, 4 physico-chemical parameters were measured at 0, -20 and -50 cm beneath the sediment surface. The field collections of TIR high-resolution images were conducted concomitantly, at low flow and high surface-water temperatures. Among the 80 sites selected for field measurements, 14 were identified as upwellings (groundwater inputs) and 66 as downwelling sites. From those 14 upwellings, 13 were also identified with TIR. The 44 additional sites identified with TIR corresponded to small-sized cold-water patches situated along the gravel bars or to groundwater discharge sites located between the bars (19 lateral seeps). Nevertheless, the DHM method documented on downwelling exchanges (infiltration of surface water), which were not captured by TIR images, and may represent hyporheic hotspots especially for benthic invertebrates. Along the studied sector of the Ain River, these downwelling zones were much more numerous than upwelling ones. Both methods in combination provide a rather complete picture of water exchange along rivers and are needed to evaluate the potential as refuges zones during critical dry periods.