From mechanical to chemical impact of anchoring in seagrasses: the premises of anthropogenic patch generation in Posidonia oceanica meadows

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20 m depth (42.6 µM and 18.8 µM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20 m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures.STARE-CAPME

Automatic solver for non-linear partial differential equations with implicit local laws: Application to unilateral contact

International audienceIn general, non-linear continuum mechanics combine global balance equations and local constitutive laws. In this work, frictionless contact between a rigid tool and a thin elastic shell is considered. This class of boundary value problems involves two non-linear algebraic laws: the first one gives explicitly the stress field as a function of the strain throughout the continuum part, whereas the second one is a non-linear equation relating the contact forces and the displacement at the boundary.Given the fact that classical computational approaches sometimes require significant effort in implementation of complex non-linear problems, a computation technique based on automatic differentiation of constitutive laws is presented in this paper. The procedure enables to compute automatically the higher-order derivatives of these constitutive laws and thereafter to define the Taylor series that are the basis of the continuation technique called asymptotic numerical method. The algorithm is about the same with an explicit or implicit constitutive relation. In the modelling of forming processes, many tool shapes can be encountered. The presented computational technique permits an easy implementation of these complex surfaces, for instance in a finite element code : the user is only required to define the tool geometry and the computer is able to obtain the higher-order derivatives

Centrality-Based Eventual Leader Election in Dynamic Networks

International audienceThis paper presents CEL, a new distributed eventual leader election algorithm for dynamic networks, which exploits topological information to improve the choice of a central leader and reduce message exchanges. The algorithm has a crosslayer neighbors detection, with a neighbor-aware mechanism, to improve the sharing of topological knowledge and elect a central leader faster. It uses a self-pruning mechanism based on topological knowledge, combined with probabilistic gossip, to improve the performance of broadcast propagation. Evaluations were conducted on the OMNeT++ environment, simulating realistic MANET with interference, collision, and messages loss. Using different parameters values, we have compared CEL to Gómez-Calzado et al. algorithm [1], on the Random Walk and the Truncated Lévy Walk mobility models. The results show better performances than [1], including fewer messages sent, shortest paths to the leader, and a more stable algorithm

Projet REPAIR: Restauration des patchs anthropiques dans l’herbier de Posidonie

REPAIR14. Life below water13. Climate actio

Structural Changes of Seagrass Seascapes Driven by Natural and Anthropogenic Factors: A Multidisciplinary Approach

Seascape ecology has been widely applied to marine habitats, including seagrass meadows, through various approaches all over the world for the past 30 years. However, these methods mainly study seagrass meadows on a single spatial scale and monitor a single driver of heterogeneity. Additionally, few assess the seascape's structural evolution. This creates gaps between the scientific data provided and those required by environmental managers and stakeholders in charge of seagrass meadow conservation. To meet their expectations, in this paper we developed a new multidisciplinary approach based on the coupling of mapping techniques, particle flux, and biometric investigations in a Mediterranean Bay, the Calvi Bay (Corsica, France), to assess the structural changes of Posidonia oceanica (L.) Delile meadows subject to disturbances. We focused our investigations on the structural characteristics, the spatial dynamics, and the particle fluxes of natural sand areas generated by bottom currents and dead matte patches which ensued from anchoring damages at 10, 15, and 20 m depth. Natural sand patches and anchoring patches differed in size, the first the largest. They also displayed different erosion-colonization dynamics. Natural sand patches were eroded at a mean speed of 12 cm.a−1 and colonized at a rate of 7 cm.a−1. Anchoring patches showed a mean erosion speed of 3.5 cm.a−1 and a colonization rate of 6.5 cm.a−1. Regarding particle fluxes, continuous meadow, and natural patch sedimentation and resuspension rates were 3.7 gDW.m−2.d−1 and 4.1 gDW.m−2.d−1 in average, respectively. In contrast, anchoring patches at 20 m depth acted as sediment traps (112.60 gDW.m−2.d−1 in winter) and showed a higher particle resuspension rate. Our results highlighted the dichotomous dynamics of seagrass seascapes influenced by natural and anthropogenic factors. Thus, the smallest anchoring patch will take about 27 years to be recolonized while the biggest requires 60 years to be covered by the plant. With an upscaling approach, together with the newest mapping tools of marine habitats, we suggest a new method to study the evolution of seagrass meadows at a large spatial scale

Non-unicité des propriétés viscoélastiques déterminées par nanoindentation conique. Cas du polypropylène

La complexité de l'analyse des données de nanoindentation lorsque les matériaux exhibent un comportement visqueux pose la question fondamentale de l'unicité des propriétés déterminées par une analyse de l'évolution temporelle de la force de pénétration et du déplacement de la pointe de l'indenteur . Ce problème de non-unicité des propriétés obtenues est souvent provoqué en pratique par une grande sensibilité aux erreurs de mesure . Cette difficulté est bien connue lors de l'analyse inverse de la courbe d'indentation élasto-plastique [1], elle est en revanche peu abordée en présence de phénomènes visqueux (viscoélastique et/ou viscoplastique, ...) comme cela peut être le cas pour les polymères. Cette étude porte sur les possibilités d'extraction de propriétés viscoélastiques (VE) d'un polypropylène (PP) par nanoindentation conique. L'objectif de ce travail est de développer une méthode d'analyse des données permettant de déterminer des propriétés VE qui soient intrinsèques et fiables (uniques et stables). Pour cela, un modèle éléments finis 2D-axisymétrique de l'essai de nanoindentation intégrant un comportement VE a été élaboré [2]. Ce modèle comporte 4 paramètres : module de Young , coefficient de Poisson , module d'anélasticité et coefficient de viscosité . Des essais de nanoindentation de charge-décharge jusqu'à une profondeur d'environ 500 nm ont été réalisés sur un échantillon de PP pour différentes vitesses de pénétration de l'indenteur (50, 500, 1000 et 5000 nm/min). Il s'avère qu'il est possible de recaler presque parfaitement le modèle numérique pour chacune des vitesses, mais que les valeurs des paramètres VE estimées sont très différentes d'une vitesse à l'autre. Il s'avère également que des combinaisons très différentes de valeurs des paramètres peuvent conduire à une évolution quasi-similaire. Ce résultat démontre la non-unicité des valeurs des 4 paramètres VE déterminés par l'exploitation d'un seul essai charge-décharge. Afin de quantifier la richesse de l'information contenue dans les données temporelles, une analyse d'identifiabilité a été menée. Basée sur le calcul d'un indice d'identifiabilité [3], elle montre qu'uniquement deux paramètres peuvent être identifiés à partir d'un essai charge -décharge et que le problème majeur est la grande sensibilité de la solution du problème inverse aux incertitudes sur les données . Pour identifier plus de 2 paramètres, il est indispensable d'enrichir l'information expérimentale. Une analyse d'identifiabilité exploitant simultanément les données de plusieurs d'essais est en cours. L'intérêt de prendre ensuite en compte un palier de maintien en charge sera quantifié par la même méthode. Cette approche basée sur l'identifiabilité paramétrique devrait permettre de concevoir un essai suffisamment riche pour déterminer des valeurs intrinsèques et fiables des 4 paramètres VE, et à moyen terme d'extraire des propriétés viscoélastiques et viscoplastiques de matériaux massifs ou en couches minces.   REFERENCE [1] J. K. Phadikar, T. A. Bogetti, et A. M. Karlsson, « On the uniqueness and sensitivity of indentation testing of isotropic materials », Int. J. Solids Struct. 50, 3242?3253, 2013. [2] M. C. Barick, « Vérification d'un modèle de simulation de l'essai de nanoindentation couplant viscoélasticité et viscoplasticité », Mémoire de stage, Institut FEMTO-ST, Besançon, 2016. [3] F. Richard, M. Villars, et S. Thibaud, « Viscoelastic modeling and quantitative experimental characterization of normal and osteoarthritic human articular cartilage using indentation », J. Mech. Behav. Biomed. Mater.24, 41?52, 2013

Topology Aware Leader Election Algorithm for MANET

National audienceThis article presents an eventual leader election algorithm for mobile dynamic networks. Each node builds knowledge of the communication graph of connected nodes, by broadcasting changes in their neighborhood. This knowledge provides the current topology of the network, used to compute the closeness centrality as the choice of the leader. Experiments were realized on PeerSim simulator, comparing our algorithm with static and dynamic flooding algorithms, on different network topologies and mobility patterns. Our algorithm improves leader stability up to 24%, sends half less messages and aims to an 8% shorter leader path

Topology Aware Leader Election Algorithm for Dynamic Networks

International audienceThis paper proposes an algorithm that eventually elects a leader for each connected component of a dynamic network where nodes can move or fail by crash. A node only communicates with nodes in its transmission range and locally keeps a global view, denoted topological knowledge, of the communication graph of the network and its dynamic evolution. Every change in the topology or in nodes membership is detected by one or more nodes and propagated over the network, updating thus the topological knowledge of the nodes. As the choice of the leader has an impact on the performance of applications that use an eventual leader election service, our algorithm, thanks to nodes topological knowledge, exploits the closeness centrality as the criterion for electing a leader. Experiments were conducted on top of PeerSim simulator, comparing our algorithm to a representative flooding algorithm. Performance results show that our algorithm outperforms the flooding one when considering leader choice stability, number of messages, and average distance to the leader

Dealing with paralogy in RADseq data: in silico detection and single nucleotide polymorphism validation in Robinia pseudoacacia L.

peer reviewedThe RADseq technology allows researchers to efficiently develop thousands of polymorphic loci across multiple individuals with little or no prior information on the genome. However, many questions remain about the biases inherent to this technology. Notably, sequence misalignments arising from paralogy may affect the development of single nucleotide polymorphism (SNP) markers and the estimation of genetic diversity. We evaluated the impact of putative paralog loci on genetic diversity estimation during the development of SNPs from a RADseq dataset for the nonmodel tree species Robinia pseudoacacia L. We sequenced nine genotypes and analyzed the frequency of putative paralogous RAD loci as a function of both the depth of coverage and the mismatch threshold allowed between loci. Putative paralogy was detected in a very variable number of loci, from 1% to more than 20%, with the depth of coverage having a major influence on the result. Putative paralogy artificially increased the observed degree of polymorphism and resulting estimates of diversity. The choice of the depth of coverage also affected diversity estimation and SNP validation: A low threshold decreased the chances of detecting minor alleles while a high threshold increased allelic dropout. SNP validation was better for the low threshold (4×) than for the high threshold (18×) we tested. Using the strategy developed here, we were able to validate more than 80% of the SNPs tested by means of individual genotyping, resulting in a readily usable set of 330 SNPs, suitable for use in population genetics applications

Patch types in Posidonia oceanica meadows around Corsica. How can we use them in seascape ecology?

The meadows formed by the Mediterranean seagrass Posidonia oceanica are subjected to various natural (e.g., water movement, light availability, sedimentation) and anthropogenic (e.g., anchoring, trawling, fish farms, explosives) phenomena that erode them and create diverse types of patches. The assemblage of the P. oceanica matrix and these patches creates particular seascapes. On the basis of this assessment, we aimed to investigate the importance of the patch type in structuring P. oceanica seascapes and to offer new prospects in the large scale studies of seagrass meadows. Five sites encompassing large P. oceanica meadows ranging from 1.86 km² to 4.42 km² along the Corsican coast (France) were considered. Eleven patch types with different sizes, shapes and origins were identified using side scan sonar images (sonograms). Five were recognized as natural and five as anthropogenic. One can be of both origins. The resolution of the sonograms allowed to detect patches of various sizes ranging from 1 m² to 111 829 m². The relation between structural characteristics of patches and the whole seascape aspect was explored using seven landscape metrics relevant for the study of meadows patchiness (patch area, mean radius of gyration, area-weighted radius of gyration, coefficient of variation of the Euclidean nearest-neighbor distance, area-weighted perimeter-area ratio, landscape division index, number of patches). Only a small number of patch types appears to play the strongest role in the characterization of the P. oceanica seascapes. Furthermore, the use of seascape structures seems to be suitable for the development of new tools like indices for the assessment of human impacts on P. oceanica meadows. In this perspective we propose a new and simple index, the Patchiness Source Index (PaSI), to estimate the origin of the patchiness (natural or anthropogenic) for a given area. A landscape approach, as well as information on patch dynamic, should be integrated in the new indices that aim to assess the state of conservation of the whole P. oceanica ecosystem