Geomorphology of large braided rivers as driver of biodiversity : how it can shape patterns of aquatic invertebrate communities and populations structure

Les rivières en tresses sont des grandes rivières alluviales de piémont montagneux à forte dynamique spatio-temporelle et à géomorphologie particulière. Leur cours traverse alternativement de vastes plaines et des rétrécissements de vallées ou canyons. Cette géomorphologie influence fortement les échanges d’eau entre la rivière avec sa nappe souterraine et sur l’épaisseur sédimentaire. Les échanges d’eau entre la rivière et sa nappe se produisent à différentes échelles allant de la vallée jusqu’à des bancs de graviers et peuvent créer des filtres biotiques et abiotiques qui influencent les communautés d’invertébrés. Les canyons créent des zones de moindre épaisseur sédimentaire voire d’absence de sédiments et peuvent représenter de fortes barrières à la dispersion pour des organismes inféodés au milieu sédimentaire souterrain. Ce travail de thèse a cherché à évaluer dans quelle mesure la géomorphologie pouvait donc structurer les communautés d’invertébrés de surface et souterraines et pouvait jouer sur la dispersion d’un organisme souterrain Proasellus walteri. Les différents résultats obtenus ont permis de montrer que la géomorphologie structurait les communautés d’invertébrés en mettant en évidence une forte réponse des communautés souterraines mais pas de surface et créait des zones de forte biodiversité à l’aval des plaines. Les résultats de cette étude ont également permis de conclure sur un effet positif de la géomorphologie des rivières en tresses sur la structuration génétique de P. walteri et de mettre en évidence de grandes tailles de populations ainsi que de fortes capacités de dispersion, permettant d’écarter certaines idées reçues sur le milieu souterrainBraided rivers are large alluvial rivers found in piedmont mountainous areas. These rivers are very dynamic systems in space and time and exhibit particular geomorphology. The river flows alternatively into large alluvial plains or narrowing parts (also defined as canyons). This geomorphology impacts groundwater-surface water exchanges and sedimentary thickness. Groundwater-surface water exchanges occur at different scales, then interacting to shape biotic and abiotic filters for invertebrate communities. Canyons can reduce sedimentary continuity or interrupt it and may represent strong barriers to dispersal for sedimentary-dwelling organisms. This present work aimed at evaluating the effects of geomorphology in invertebrate community structure and as a potential barrier to dispersal in the subterranean organism Proasellus walteri. The different results obtained have shown that geomorphology structured invertebrate communities, highlighting a strong response in groundwater communities but not in surface communities and have shown that downstream parts of alluvial plains were hotspots of biodiversity. The results of this study also concluded on a positive effect of geomorphology in braided rivers on the genetic structure of P. walteri and underlined large effective population size and high dispersal ability, then removing some misconceptions about subterranean environmen

Effets de la géomorphologie des rivières en tresses sur les communautés d’invertébrés aquatiques et sur la structuration génétique des populations du crustacé isopode souterrain Proasellus walteri

Braided rivers are large alluvial rivers found in piedmont mountainous areas. These rivers are very dynamic systems in space and time and exhibit particular geomorphology. The river flows alternatively into large alluvial plains or narrowing parts (also defined as canyons). This geomorphology impacts groundwater-surface water exchanges and sedimentary thickness. Groundwater-surface water exchanges occur at different scales, then interacting to shape biotic and abiotic filters for invertebrate communities. Canyons can reduce sedimentary continuity or interrupt it and may represent strong barriers to dispersal for sedimentary-dwelling organisms. This present work aimed at evaluating the effects of geomorphology in invertebrate community structure and as a potential barrier to dispersal in the subterranean organism Proasellus walteri. The different results obtained have shown that geomorphology structured invertebrate communities, highlighting a strong response in groundwater communities but not in surface communities and have shown that downstream parts of alluvial plains were hotspots of biodiversity. The results of this study also concluded on a positive effect of geomorphology in braided rivers on the genetic structure of P. walteri and underlined large effective population size and high dispersal ability, then removing some misconceptions about subterranean environmentLes rivières en tresses sont des grandes rivières alluviales de piémont montagneux à forte dynamique spatio-temporelle et à géomorphologie particulière. Leur cours traverse alternativement de vastes plaines et des rétrécissements de vallées ou canyons. Cette géomorphologie influence fortement les échanges d’eau entre la rivière avec sa nappe souterraine et sur l’épaisseur sédimentaire. Les échanges d’eau entre la rivière et sa nappe se produisent à différentes échelles allant de la vallée jusqu’à des bancs de graviers et peuvent créer des filtres biotiques et abiotiques qui influencent les communautés d’invertébrés. Les canyons créent des zones de moindre épaisseur sédimentaire voire d’absence de sédiments et peuvent représenter de fortes barrières à la dispersion pour des organismes inféodés au milieu sédimentaire souterrain. Ce travail de thèse a cherché à évaluer dans quelle mesure la géomorphologie pouvait donc structurer les communautés d’invertébrés de surface et souterraines et pouvait jouer sur la dispersion d’un organisme souterrain Proasellus walteri. Les différents résultats obtenus ont permis de montrer que la géomorphologie structurait les communautés d’invertébrés en mettant en évidence une forte réponse des communautés souterraines mais pas de surface et créait des zones de forte biodiversité à l’aval des plaines. Les résultats de cette étude ont également permis de conclure sur un effet positif de la géomorphologie des rivières en tresses sur la structuration génétique de P. walteri et de mettre en évidence de grandes tailles de populations ainsi que de fortes capacités de dispersion, permettant d’écarter certaines idées reçues sur le milieu souterrai

Evaluation de l’intégrité des fonds marins dans le cadre de la DCSMM

International audienceLa directive cadre stratégie pour le milieu marin (DCSMM) vise, par une approche écosystémique, à atteindre ou restaurer un bon fonctionnement des écosystèmes marins. Sa mise en œuvre est organisée selon des cycles de 6 ans, permettant tout d’abord d’évaluer l’état environnemental du milieu marin, complété d’une évaluation socio-économique, puis de se fixer des objectifs environnementaux et enfin de prendre les mesures nécessaires à sa préservation. La mise en œuvre d’un programme de surveillance multi-thématiques permet de recueillir les données nécessaires aux évaluations.L’état de l’environnement marins est qualifié par onze descripteurs (thématiques) qui traitent des compartiments biologiques, physiques et des pressions exercées sur le milieu. Le descripteur 6, piloté par le BRGM au niveau national, concerne l’intégrité des fonds marins et vise à garantir que la structure et les fonctions des écosystèmes sont préservées et que les écosystèmes benthiques, en particulier, ne sont pas perturbés.Le descripteur 6 évalue les modifications sédimentaires ou morphologiques des fonds marins induites par les activités anthropiques, en différenciant celles qui sont permanentes (pertes physiques) de celles qui sont temporaires ou réversibles (perturbations physiques). De plus, l’intensité des perturbations physiques doit être évaluée afin de déterminer si elles produisent des effets néfastes sur les habitats benthiques. Les activités anthropiques pouvant engendrer ces pressions physiques (pertes ou perturbations) sur le milieu sont nombreuses et variées : extraction de granulats, dragage et immersion de matériaux, rechargement de plage, ouvrages côtiers et au large, aquaculture, mouillages, pêche professionnelle au fond, pêche à pied de loisirs. La quantification des pressions physiques induites par ces activités (et des effets sur les habitats benthiques) nécessite de nombreuses données qui ne sont pas disponible à l’échelle de toutes les façades métropolitaines. Néanmoins, pour la plupart des activités, on dispose d’informations sur leur localisation et leur étendue possible (en terme de zones autorisées pour certaines), voire d’information plus quantitatives telles que des volumes de sédiments immergés annuellement ou des volumes de granulats autorisés à l’extraction. Ces informations rendent possible un premier niveau d’évaluation nationale et une cartographie à faible résolution des fonds marins subissant des pressions physiques. Mais elles sont largement insuffisantes pour évaluer de façon précise et fiable les impacts morpho-sédimentaires des activités traitées dans le cadre de la DCSMM.Pour certaines activités, des études d’impacts et des suivis environnementaux réglementaires sont demandés par les services de l’état aux maitres d’ouvrages afin d’anticiper les effets possibles des activités, puis de suivre les impacts éventuels sur le terrain. Ces dossiers réglementaires représentant des sources d’informations potentiellement très utiles et quantifiée pour les évaluations de la DCSMM.Cette contribution vise à présenter les données disponibles à l’échelle nationale pour différentes activités anthropiques et les évaluations de pressions physiques qu’elles engendrent sur les fonds marins. Les perspectives et possibilités d’améliorations qu’offrent la collecte et l’analyse des suivis environnementaux seront détaillées

Invertebrate distribution across nested geomorphic features in braided-river landscapes

International audienceRiver landscapes are increasingly viewed as a collection of nested geomorphic features, the hydrological effects of which juxtapose to create a mosaic of aquatic habitat conditions. We examined how the combined hydrological influences of stream reaches and gravel bars affect the composition of hyporheic invertebrate assemblages along the longitudinal and vertical dimensions of river landscapes. We worked in 12 braided-river valley segments of tributaries to the Rhone River, France. Valley segments were bounded downstream by geological knickpoints so that bedform-induced exchange flows beneath gravel bars and riffles were predictably embedded in exchange flows occurring at the valley-segment scale. In upstream reaches (UR) and downstream reaches (DR) of each valley segment, we collected invertebrates at the heads and tails of gravel bars in the hyporheic zone and at the upstream and downstream ends of riffles in surface channels adjacent to the bars. Patterns of vertical hydraulic gradient and specific conductance and the resulting spatial heterogeneity of temperature, dissolved O-2, and particulate organic C suggested that DR were points of flow convergence and water mixing. Specific conductance increased and temperature decreased more steeply along gravel bars in DR, indicating that water from long hyporheic flow paths or lateral aquifers was discharging at the tails of bars in DR. Density hotspots for insect larvae corresponded to hyporheic patches at the heads of bars in DR, which received the highest organic matter inputs from localized downwelling of surface water and were not affected by inputs of cold water from hyporheic flow paths or lateral aquifers. Density of epigean taxa at the tails of bars decreased more steeply with depth in DR than in UR, and density of hypogean taxa increased more steeply. Viewing local hyporheic assemblages as the outcome of a series of environmental filters operating over multiple exchange flows induced by nested geomorphic features provides a general framework that may foster our understanding of biodiversity patterns in river landscapes

Effect of host plant species and insecticides on the evolution of genetic diversity of a crop pest

For agricultural pests capable of attacking different plant species, crop distribution and selection pressure imposed by insecticide use are two key constraints imposed on their evolutionary trajectory. Here, we assessed the contribution of host-based genetic differentiation to the dynamics of resistance alleles to three insecticide modes of action in Myzus persicae. This major aphid pest is infamous for its ability to resist to multiple insecticides. Two distinct samplings were conducted: reference sampling on identified crops and continuous random sampling for 7 years using a suction trap. All aphids were genotyped at 14 microsatellite markers and four insecticide-resistant loci. We analyzed the genetic structure of these populations using an individual-centered approach. Four well-defined genetic clusters were found in the aerial samples, three of which could be linked to specific crops. We found a sharp differentiation between peach and herbaceous individuals. Within the individuals sampled on herbaceous hosts, two distinct genetic clusters were identified, one of which seems to be more strongly associated with tobacco. The fourth group was only found in the aerial samples and display strong genetic difference with other groups. The 4-loci resistance genotypes showed a strong association with the four genetic clusters, indicative of barriers to the spread of insecticide resistances. The 7-year continuous random sampling revealed a rapid turnover in aphid genotypes and associated insecticide resistance patterns. This study highlights the importance of considering landscape-scale population structure to identify the risk of emergence and spread of insecticide resistance for a particular crop

Host plants and insecticides shape the evolution of genetic and clonal diversity in a major aphid crop pest

International audienceUnderstanding the spatiotemporal dynamics of pesticide resistance at the landscape scale is essential to anticipate the evolution and spread of new resistance phenotypes. In crop mosaics, host plant specialization in pest populations is likely to dampen the spread of pesticide resistance between different crops even in mobile pests such as aphids. Here, we assessed the contribution of host-based genetic differentiation to the dynamics of resistance alleles in Myzus persicae, a major aphid pest which displays several insecticide resistance mechanisms. We obtained a representative sample of aphids from a crop mosaic through a suction trap for 7 years and from various crops as a reference collection. We genotyped these aphids at 14 microsatellite markers and four insecticide-resistant loci, analyzed the genetic structure, and assigned host-based genetic groups from field-collected aphids. Four well-defined genetic clusters were found in aerial samples, three of which with strong association with host-plants. The fourth group was exclusive to aerial samples and highly divergent from the others, suggesting mixture with a closely related taxon of M. persicae associated with unsampled plants. We found a sharp differentiation between individuals from peach and herbaceous plants. Individuals from herbaceous hosts were separated into two genetic clusters, one more strongly associated with tobacco. The 4-loci resistance genotypes showed a strong association with the four genetic clusters, indicative of barriers to the spread of resistance. However, we found a small number of clones with resistant alleles on multiple host-plant species, which may spread insecticide resistance between crops. The 7-year survey revealed a rapid turn-over of aphid genotypes as well as the emergence, frequency increase and persistence of clones with resistance to several families of insecticides. This study highlights the importance of considering landscape-scale population structure to identify the risk of emergence and spread of insecticide resistance for a particular crop

Geomorphic influence on intraspecific genetic differentiation and diversity along hyporheic corridors

International audience1. The hyporheic zone of rivers potentially acts as a dispersal corridor for ground-water organisms because it provides a spatially continuous interstitial habitat between isolated aquifers. Yet, the degree to which it can facilitate the move- ment of organisms has been hypothesized to vary in response to change in sedi- ment regime, which determines channel morphology.2. In this study, we used microsatellite markers to test for a relationship between the genetic structure and diversity of the minute interstitial isopod Proasellus walteri and channel morphology along three nearby hyporheic corridors differing widely in their sediment regime. We predicted that genetic diversity would decrease and genetic structuring would increase as sediment supply-limited channels would become prominent features in the river corridor. The reason is that such channels have fewer and less suitable sedimentary habitats for migra- tion because they lack large depositional bedforms such as gravel bars.3. Using genotypic data from seven microsatellite loci for a total of 713 individuals distributed among 25 demes, we found that demes had on average more alleles and were less differentiated in the river showing the most extensive alluvial depos- its and shortest length of sediment supply-limited channels. Population clusters were also of greater size, reaching up to 30 km in length. The longitudinal pattern of genetic differentiation in this sediment-rich river was best explained by hydro- logic distance and the longitudinal pattern of allelic richness was bell-shaped, as expected under a stepping-stone model with symmetrical migration.4. The length of sediment supply-channels was more important than hydrologic dis- tance in explaining the longitudinal distribution of genetic differentiation in the two other corridors facing a sediment shortage. Allelic richness decreased mono- tonically upstream in the most sediment-poor river. This correlates with the expan- sion further downstream of sediment supply-limited channels in this river, which is likely to decrease animal movement and hence gene flow among demes.5. Thisstudyprovidesthefirstevidencethatthedegreetowhichthehyporheiczone facilitates the movement of groundwater organisms varies greatly among rivers of contrasted geomorphology. Extending the application of riverscape genetics across a range of interstitial taxa and geomorphic settings holds much promise for assessing the contribution of the hyporheic zone to the dispersal of groundwater organisms

Map showing the location of sampling sites.

<p>Dark, light and very light grey patterns show the River catchments colonized by <i>Prosaelluswalteri</i>_T058, <i>P</i><i>. walteri</i>_T059, and <i>P</i><i>. walteri</i>_T060 (focal taxa), respectively. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0076213#pone-0076213-t001" target="_blank">Table 1</a> for site codes.</p