Réponses fonctionnelles des écosystèmes estuariens dans le contexte du changement global : le cas du réseau trophique de l'estuaire de la Gironde

The philosophical and technical changes associated with the emergence of our "modern" civilizationcaused numbers of physical, chemical and biological changes those led to changes in the functioning of"Nature". The necessity to understand this functioning has led to question the reconfiguration process ofthe various eco-sociosystems’ components and features under the influence of Global Change. TheGironde estuary, one of the largest estuaries in Western Europe, has shown since three decades, signsof this Global Change. In this context, this study highlights the functional trajectory of this estuarineecosystem during the last thirty years. Three aspects of its functioning are described, analyzed anddiscussed. First, the study of the inter-annual fish community evolution highlighted three periods withdistinct functioning. The exploration of the seasonal patterns of the fish and zooplankton dynamicsduring each period shows phenological changes able to cause, in some cases, prey-predator mismatch.This leads to question the stability of trophic interactions and the trophic capacity of the ecosystem forfish. Finally, within each of these three periods, the estuarine food web was modeled within a holisticframework. The comparison of the functional properties of each model highlighted an increase in thestress of the Gironde estuary and questioned the sustainability of its nursery function for Bay of Biscaymarine stocks. Four forecasting scenarios summarize conclusions of this study and let see possiblefutures of this ecosystem.Les changements philosophiques et techniques qui ont accompagné l’avènement de notre civilisation «moderne » laissent dans leurs sillages un cortège de bouleversements physiques, chimiques etbiologiques à l’échelle du globe. Aujourd’hui les preuves sont nombreuses pour affirmer que cechangement global modifie le fonctionnement de la « Nature ». La nécessité d’appréhender et decomprendre ce fonctionnement a conduit à interroger les processus de reconfiguration des diversescomposantes et fonctionnalités des éco-sociosystèmes sous l’influence des changements globaux.Concentrant aujourd’hui près de 75 % de la population humaine, les écosystèmes estuariens et côtierssont particulièrement vulnérables, de plus en plus exploités et contaminés et leur biodiversité estlargement impactée. L’estuaire de la Gironde, un des plus grands estuaires d’Europe de l’Ouest, montredepuis au moins trois décennies des signes de ce changement global. C’est dans ce contexte que cettethèse décrit la trajectoire fonctionnelle de l’écosystème estuarien au cours des trente dernières années.Trois aspects du fonctionnement y sont décrits, analysés et discutés. L’étude de la dynamiqueinterannuelle du cortège ichtyologique a permis, tout d’abord, de mettre en avant trois périodes defonctionnement distinctes dans les dernières décennies. L’exploration des rythmes saisonniers despoissons et de leurs proies zooplanctoniques dans chacune d’elle a ensuite permis de montrer desmodifications de la phénologie de ces espèces à même d’engendrer, pour certaines, desdésynchronisations temporelles entre proies et prédateurs questionnant, par conséquent, la stabilité desrelations trophique et la capacité trophique du milieu. Enfin, un modèle holistique du réseau trophique aété réalisé pour chacune des trois périodes. La comparaison des propriétés de chacun d’eux a permisde conclure à une augmentation du stress de l’estuaire de la Gironde et à remettre en question sacapacité à durablement jouer son rôle de nourricerie pour les stocks de poissons marins du Golfe deGascogne. Quatre scénarii d’évolutions prospectifs synthétisent les conclusions de cette étude etdonnent à voir des avenirs possibles de cet écosystème

Functional responses of estuaries ecosystems in the global change context : case of the Gironde estuary food web

Les changements philosophiques et techniques qui ont accompagné l’avènement de notre civilisation «moderne » laissent dans leurs sillages un cortège de bouleversements physiques, chimiques etbiologiques à l’échelle du globe. Aujourd’hui les preuves sont nombreuses pour affirmer que cechangement global modifie le fonctionnement de la « Nature ». La nécessité d’appréhender et decomprendre ce fonctionnement a conduit à interroger les processus de reconfiguration des diversescomposantes et fonctionnalités des éco-sociosystèmes sous l’influence des changements globaux.Concentrant aujourd’hui près de 75 % de la population humaine, les écosystèmes estuariens et côtierssont particulièrement vulnérables, de plus en plus exploités et contaminés et leur biodiversité estlargement impactée. L’estuaire de la Gironde, un des plus grands estuaires d’Europe de l’Ouest, montredepuis au moins trois décennies des signes de ce changement global. C’est dans ce contexte que cettethèse décrit la trajectoire fonctionnelle de l’écosystème estuarien au cours des trente dernières années.Trois aspects du fonctionnement y sont décrits, analysés et discutés. L’étude de la dynamiqueinterannuelle du cortège ichtyologique a permis, tout d’abord, de mettre en avant trois périodes defonctionnement distinctes dans les dernières décennies. L’exploration des rythmes saisonniers despoissons et de leurs proies zooplanctoniques dans chacune d’elle a ensuite permis de montrer desmodifications de la phénologie de ces espèces à même d’engendrer, pour certaines, desdésynchronisations temporelles entre proies et prédateurs questionnant, par conséquent, la stabilité desrelations trophique et la capacité trophique du milieu. Enfin, un modèle holistique du réseau trophique aété réalisé pour chacune des trois périodes. La comparaison des propriétés de chacun d’eux a permisde conclure à une augmentation du stress de l’estuaire de la Gironde et à remettre en question sacapacité à durablement jouer son rôle de nourricerie pour les stocks de poissons marins du Golfe deGascogne. Quatre scénarii d’évolutions prospectifs synthétisent les conclusions de cette étude etdonnent à voir des avenirs possibles de cet écosystème.The philosophical and technical changes associated with the emergence of our "modern" civilizationcaused numbers of physical, chemical and biological changes those led to changes in the functioning of"Nature". The necessity to understand this functioning has led to question the reconfiguration process ofthe various eco-sociosystems’ components and features under the influence of Global Change. TheGironde estuary, one of the largest estuaries in Western Europe, has shown since three decades, signsof this Global Change. In this context, this study highlights the functional trajectory of this estuarineecosystem during the last thirty years. Three aspects of its functioning are described, analyzed anddiscussed. First, the study of the inter-annual fish community evolution highlighted three periods withdistinct functioning. The exploration of the seasonal patterns of the fish and zooplankton dynamicsduring each period shows phenological changes able to cause, in some cases, prey-predator mismatch.This leads to question the stability of trophic interactions and the trophic capacity of the ecosystem forfish. Finally, within each of these three periods, the estuarine food web was modeled within a holisticframework. The comparison of the functional properties of each model highlighted an increase in thestress of the Gironde estuary and questioned the sustainability of its nursery function for Bay of Biscaymarine stocks. Four forecasting scenarios summarize conclusions of this study and let see possiblefutures of this ecosystem

Functional responses of estuaries ecosystems in the global change context : case of the Gironde estuary food web

Les changements philosophiques et techniques qui ont accompagné l’avènement de notre civilisation «moderne » laissent dans leurs sillages un cortège de bouleversements physiques, chimiques etbiologiques à l’échelle du globe. Aujourd’hui les preuves sont nombreuses pour affirmer que cechangement global modifie le fonctionnement de la « Nature ». La nécessité d’appréhender et decomprendre ce fonctionnement a conduit à interroger les processus de reconfiguration des diversescomposantes et fonctionnalités des éco-sociosystèmes sous l’influence des changements globaux.Concentrant aujourd’hui près de 75 % de la population humaine, les écosystèmes estuariens et côtierssont particulièrement vulnérables, de plus en plus exploités et contaminés et leur biodiversité estlargement impactée. L’estuaire de la Gironde, un des plus grands estuaires d’Europe de l’Ouest, montredepuis au moins trois décennies des signes de ce changement global. C’est dans ce contexte que cettethèse décrit la trajectoire fonctionnelle de l’écosystème estuarien au cours des trente dernières années.Trois aspects du fonctionnement y sont décrits, analysés et discutés. L’étude de la dynamiqueinterannuelle du cortège ichtyologique a permis, tout d’abord, de mettre en avant trois périodes defonctionnement distinctes dans les dernières décennies. L’exploration des rythmes saisonniers despoissons et de leurs proies zooplanctoniques dans chacune d’elle a ensuite permis de montrer desmodifications de la phénologie de ces espèces à même d’engendrer, pour certaines, desdésynchronisations temporelles entre proies et prédateurs questionnant, par conséquent, la stabilité desrelations trophique et la capacité trophique du milieu. Enfin, un modèle holistique du réseau trophique aété réalisé pour chacune des trois périodes. La comparaison des propriétés de chacun d’eux a permisde conclure à une augmentation du stress de l’estuaire de la Gironde et à remettre en question sacapacité à durablement jouer son rôle de nourricerie pour les stocks de poissons marins du Golfe deGascogne. Quatre scénarii d’évolutions prospectifs synthétisent les conclusions de cette étude etdonnent à voir des avenirs possibles de cet écosystème.The philosophical and technical changes associated with the emergence of our "modern" civilizationcaused numbers of physical, chemical and biological changes those led to changes in the functioning of"Nature". The necessity to understand this functioning has led to question the reconfiguration process ofthe various eco-sociosystems’ components and features under the influence of Global Change. TheGironde estuary, one of the largest estuaries in Western Europe, has shown since three decades, signsof this Global Change. In this context, this study highlights the functional trajectory of this estuarineecosystem during the last thirty years. Three aspects of its functioning are described, analyzed anddiscussed. First, the study of the inter-annual fish community evolution highlighted three periods withdistinct functioning. The exploration of the seasonal patterns of the fish and zooplankton dynamicsduring each period shows phenological changes able to cause, in some cases, prey-predator mismatch.This leads to question the stability of trophic interactions and the trophic capacity of the ecosystem forfish. Finally, within each of these three periods, the estuarine food web was modeled within a holisticframework. The comparison of the functional properties of each model highlighted an increase in thestress of the Gironde estuary and questioned the sustainability of its nursery function for Bay of Biscaymarine stocks. Four forecasting scenarios summarize conclusions of this study and let see possiblefutures of this ecosystem

Abrupt shifts in the Gironde fish community: an indicator of ecological changes in an estuarine ecosystem

For decades, global climate change has directly and indirectly affected the structure and function of ecosystems. Abrupt changes in biodiversity have been observed in response to linear or sudden modifications to the environment. These abrupt shifts can cause long-term reorganizations within ecosystems, with communities exhibiting new functional responses to environmental factors. Over the last 3 decades, the Gironde estuary in southwest France has experienced 2 abrupt shifts in both the physical and chemical environments and the pelagic community. Rather than describing these shifts and their origins, we focused on the 3 inter-shift periods, describing the structure of the fish community and its relationship with the environment during these periods. We described fish biodiversity using a limited set of descriptors, taking into account both species composition and relative species abundances. Inter-shift ecosystem states were defined based on the relationship between this description and the hydro-physico-chemical variables and climatic indices defining the main features of the environment. This relationship was described using generalized linear mixed models on the entire time series and for each inter-shift period. Our results indicate that (1) the fish community structure has been significantly modified, (2) environmental drivers influencing fish diversity have changed during these 3 periods, and (3) the fish-environment relationships have been modified over time. From this, we conclude a regime shift has occurred in the Gironde estuary. We also highlight that anthropogenic influences have increased, which re-emphasizes the importance of local management in maintaining fish diversity and associated goods and services within the context of climate change

EStimating Contaminants tRansfers Over Complex food webs (ESCROC): An innovative Bayesian method for estimating POP's biomagnification in aquatic food webs

International audiencePollution greatly impacts ecosystems health and associated ecological functions. Persistent Organic Pollutants (POPs) are among the most studied contaminants due to their persistence, bioaccumulation, and toxicity potential. Biomagnification is often described using the estimation of a Trophic Magnification Factor (TMF). This estimate is based on the relationship between contamination levels of the species and their trophic level. However, while the estimation can be significantly biased in relation to multiple sources of uncertainty (e.g. species physiology, measurement errors, food web complexity), usual TMF estimation methods typically do not allow accounting for these potential biases. More accurate and reliable assessment tool of TMFs and their associated uncertainty are therefore needed in order to appropriately guide chemical pollution management. The present work proposes a relevant and innovative TMF estimation method accounting for its many variability sources. The ESCROC model (EStimating Contaminants tRansfers Over Complex food webs), which is implemented in a Bayesian framework, allows for a more reliable and rigorous assessment of contaminants trophic magnification, in addition to accurate estimations of isotopes trophic enrichment factors and their associated uncertainties in food webs. Similar to classical mixing models used in food web investigations, ECSROC computes diet composition matrices using isotopic composition data while accounting for contamination data, leading to more robust food web descriptions. As a demonstration of the practical application of the model, ESCROC was implemented to revisit the trophic biomagnification of 5 polyfluoroalkyl substances (PFAS) in a complex estuarine food web (the Gironde, SW France). In addition to the TMF estimate and 95% confidence intervals, the model provided biomagnification probabilities associated to the investigated contaminants

ESCROpath , a Bayesian mixing model to quantify diets and trophic flows in aquatic food webs

International audienceFood-web modelling is a key tool to provide a global and comprehensive knowledge on community structure, biodiversity and ecosystem processes and functioning. In particular, it allows computing integrative and holistic indices describing food web characteristics, topology and functioning. However, one of the main sources of uncertainty in most food-web models is the estimation of diet matrices. In the present work, we propose an innovative approach that combines (a) a Bayesian mixing model using both isotopes and contaminants as chemical tracers with (b) classical mass-balance equations. This dual approach allows the simultaneous estimation of diet composition, isotopic enrichment, contaminant biomagnification, and contaminants and biomass flows in the whole food web. This original model named ESCROpath also provides food-web indices derived from ecological network analysis (ENA). As a case study, the approach was implemented in the Gironde estuarine food web (SW France) for which isotopes, contaminants and trophic data exist. Two sets of priors were constructed accounting for more or less uncertainty in trophic parameter estimates. Outputs were compared with previous published Ecopath results. A constrained calibration led to very similar outputs as Ecopath (which shows that the method is able to find the initial set of parameters if it is forced to do so), whereas a free calibration led to slight differences in trophic parameters and ENA indice estimations (which shows that the Ecopath solution was not fully optimal). Quite different diet matrices and estimations of flows distribution within the food web can thus be obtained. ESCROpath is an original flexible food-web modelling tool that, for the first time, makes it possible to go from a model mainly built on an ‘estimate’ of the parameters based on expert knowledge (which constitutes the main criticism formulated against Ecopath) to a statistical Bayesian framework for the estimation of the trophic parameters. It thus provides a very integrated framework for food-web modelling by estimating simultaneously trophic parameters, diet compositions and trophic enrichment/magnification factors. By doing this, it notably provides reliable and robust uncertainty estimations for output parameters

Study area in the Gironde estuary, southwest France.

<p>Study area in the Gironde estuary, southwest France.</p

Magnitude of overlap modification between predator and prey: Values extracted from S4 Fig indicate the magnitude of overlap changes between predator/prey couples.

<p>Dark-grey boxes show the undocumented situations of predatory/prey interactions. Light-grey boxes show the situations without changes in predator/prey overlap.</p

Synthesis of predator/prey interaction results: symbols refer to decision tree in Fig 2.

<p>Dark-grey boxes show the undocumented situations of predatory/prey interactions?. Light-grey boxes show the situations without changes in predator/prey overlap.</p

Example of four modifications of yearly distribution patterns among periods: the four situations illustrate the different types of phenological changes accounted for in this study.

<p>The red arrows correspond to peak of abundance of species for the period 1 to 3 (noted: P1, P2, P3). A) The annual peak of E. encrasicolus abundance shiftedf earlier in the year for the period two and three. B) The annual peak of A. regius abundance shifted later in the year for the period three. C) The annual peak of E. affinis shifted earlier in the year for the period two and returned near the first situation during the third period. D) The annual peak of S. sprattus shifted later in the year for the period two and returned near the first situation at the third period (note: for this species, only the first peak of presence in the estuary was considered).</p