Cycles biogéochimiques de la Mer Méditerranée : Processus et bilans

The Mediterranean Sea is characterized by various trophic regimes due to river inputs, as well as heterogeneous nitrogen to phosphorus ratios and hydrodynamic processes, in particular vertical mixing. The objective of this thesis that was performed in the framework of the MISTRALS program is the study of the biogeochemical cycles of these various regimes that composed the Mediterranean. It is mainly based on the in situ observations collected during DeWEx project and on 3D physical/biogeochemical modeling at the scale of the entire basin and at the regional of the western sub-basin. The first study is a ten-year basin scale study which allows an ecological classification, giving bioregions in function of their physical and biogeochemical properties. Southern oligotrophic areas in both eastern and western sub-basins are characterized by low efflorescence and deep nutriclines. While in the north, near Rhodes Island, in the southern Adriatic Sea and in the Liguro-Provencal sub-basin, the summer oligotrophic regime is altered annually by intense vertical dynamics, the deep convection. This process is considered as a driving force of the thermohaline circulation and entrains an enrichment of nutrients in surface layer which allows rapid and intense blooms in spring. The Mediterranean receives organic matter from the Atlantic Ocean, while, it is a source of inorganic matter for the Atlantic. In a second part of this thesis, a high resolution model on the western sub-basin was embedded in the basin model to quantify the physical and biological processes that determine the temporal variability of the nutrient stocks and their stoichiometry. The north-western Mediterranean turns from an eutrophic regime during deep convection to an oligotrophic regime after the spring bloom. During the eutrophic regime new production dominates the primary production at the surface while during the oligotrophic regime primary production is essentially associated to subsurface regenerated one. Adjacent water masses of the convection zone and west-southern regions are characterized by a dominance of regenerated production all over the year.La Méditerranée est caractérisée par une grande variété de régimes trophiques qui s’explique par les apports fluviaux, les ratios azote/phosphore particulièrement élevés dans le bassin oriental, et par les processus hydrodynamiques en particulier le mélange vertical dans les régions situées au nord. Cette thèse qui s’inscrit dans le cadre du projet MerMEX, composante du chantier MISTRALS, a pour objectif l’étude des cycles biogéochimiques en Méditerranée. Elle s’appuie en particulier sur les observations acquises dans le cadre du projet DeWEx et sur la modélisation couplée physique biogéochimie 3D. À l’échelle du bassin et à l’échelle régionale du sous-bassin occidental. La première étude à l’échelle du bassin sur une période de 10 ans a permis d’effectuer une classification de régimes biogéochimiques, en fonction de leurs caractéristiques physiques et biogéochimiques. Les zones oligotrophes présentes au sud du bassin oriental et occidental sont caractérisées par une faible efflorescence et par de grandes profondeurs de nutriclines. Tandis qu’au nord du bassin, au large de Rhodes, en mer Adriatique et dans le bassin Liguro-provençal, le régime oligotrophe estival est suivi en hiver par une dynamique verticale intense, la convection profonde. Cette dernière est considérée comme le moteur de la circulation thermohaline. Elle entraine un enrichissement des eaux de surface en sels nutritifs qui permet une efflorescence rapide et intense au printemps. L’océan Atlantique représente une source de matière organique pour la Méditerranée. Cette mer représente en revanche une source de sels nutritifs pour l’Atlantique.Ensuite, un modèle à très haute résolution a été imbriqué dans le modèle de bassin pour quantifier les processus physiques et biologiques qui déterminent la variabilité temporelle de la disponibilité et de la stœchiométrie des sels nutritifs en Méditerranée nord-occidentale. La convection profonde hivernale entraîne une homogénéisation des propriétés biogéochimiques sur la colonne d’eau, et une stœchiométrie vers des valeurs estivales. Cette région bascule d’un régime d’eutrophie lors de la convection profonde vers un régime oligotrophe après le bloom de printemps. Lors du premier régime la production primaire est dominée par une production nouvelle en surface, alors que lors du deuxième régime la production phytoplanctonique de subsurface est essentiellement régénérée. Les masses d’eau adjacentes à la zone de convection et celles situées au sud sont caractérisées par une dominance de production régénérée toute l’année. Une partie de la matière organique particulaire est exportée en dessous de la couche épipélagique. Cet export de matière organique est variable et est fortement lié à la dynamique verticale des masses d’eau

Mesure simultanée aux fréquences moyennes et cartographie de la permittivité diélectrique et de la conductivité électrique du sol

Le travail de recherche présenté dans ce mémoire de thèse porte sur l étude de la conductivité électrique et de la permittivité diélectrique du sol dans le domaine des fréquences moyennes (100 kHz ; 10 MHz), l objectif étant de développer une nouvelle méthode de cartographie simultanée de ces paramètres électriques. La conductivité électrique et la permittivité diélectrique étant sensibles aux paramètres d état du sol, et notamment à sa teneur en eau, le couplage des informations apportées par ces deux paramètres doit permettre d estimer la valeur de la teneur en eau en chaque point de mesure. Une étude de la variation des paramètres électriques en fonction de la teneur en eau, en argile, de la minéralogie des argiles impliquées ainsi que de la salinité de l électrolyte pour des fréquences comprises entre 1 kHz et 10 MHz a été entreprise en laboratoire. Dans le cas des échantillons de sable, les lois de comportement existantes prédisent correctement la réponse des paramètres électriques pour des fréquences supérieures à 2 MHz. Pour des fréquences inférieures et une teneur en argile supérieure, des effets de polarisation d interface apparaissent. Nous proposons de déterminer une relation empirique permettant de retrouver la permittivité diélectrique liée à la polarisation dipolaire à partir des mesures de conductivité électrique. En parallèle a été développé un prototype d appareil électromagnétique de mesure in situ, de type Slingram, composé de deux bobines perpendiculaires (PERP), qui fonctionne à 1,56 MHz. Les conclusions obtenues au cours des essais en laboratoire ont été testées avec succès sur deux sites expérimentaux de nature différente.The research work submitted in this PhD dissertation deals with the study of soil electrical conductivity and dielectric permittivity in the medium frequency range (100 kHz 10 MHz). The aim of the study is to develop a new mapping method where these two electrical parameters are simultaneously determined. The electrical conductivity and the dielectric permittivity are sensitive to the soil state parameters, and particularly to its water content. Coupling the information brought by the two electrical parameters leads to the estimation of soil water content at each measurement point. A study of the electrical parameters variation with water content, clay content, clay mineralogy and water salinity, using frequencies between 1 kHz and 10 MHz, was undertaken under controlled conditions. For clay free samples and frequencies higher than 2 MHz, existing theoretical models are well predicting the response of the studied material as a function of water content. In the case of lower frequencies or higher clay content, interfacial polarization arises. We propose to determine an empirical relation allowing to find the part of the dielectric permittivity linked to dipolar polarization, using electrical conductivity measurements. An in situ electromagnetic prototype was developed in parallel. It is composed of two fixed perpendicular coils (Slingram geometry) and works at 1.56 MHz. The conclusions extracted from the laboratory measurements were successfully applied to two different experimental sites.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Laboratory measurements of dielectric permittivity and electrical conductivity as a function of water content of soils in the intermediate frequency domain (100 kHz-10 MHz)

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Sources d’émission, flux et distribution spatiotemporelle des ressources nutritives

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Movement Shapes the Structure of Fish Communities Along a Cross-Shore Section in the California Current

Place: Lausanne Publisher: Frontiers Media Sa WOS:000760695700001Pelagic fish communities are shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes remains poorly understood. Here, we use a regional implementation of the APex ECOSystem Model (APECOSM), a mechanistic model of the pelagic food web, to investigate these processes in the California Current, a highly productive upwelling system characterized by vigorous mesoscale circulation. The model is coupled with an eddy-resolving representation of ocean currents and lower trophic levels, and is tuned to reproduce observed fish biomass from fisheries independent trawls. Several emergent properties of the model compare realistically with observations. First, the epipelagic community accounts for one order of magnitude less biomass than the vertically migratory community, and is composed by smaller species. Second, the abundance of small fish decreases from the coast to the open ocean, while the abundance of large fish remains relatively uniform. This in turn leads to flattening of biomass size-spectra away from the coast for both communities. Third, the model reproduces a cross-shore succession of small to large sizes moving offshore, consistent with observations of species occurrence. These cross-shore variations emerge in the model from a combination of: (1) passive offshore advection by the mean current, (2) active swimming toward coastal productive regions to counterbalance this transport, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. Our results highlight the importance of passive and active movement in structuring the pelagic food web, and suggest that a representation of these processes can help to improve the realism in simulations with marine ecosystem models

Dataset of terrestrial fluxes of freshwater, nutrients, carbon, and iron to the Southern California Bight, U.S.A.

The Southern California Bight (SCB) is an upwelling-dominated, open embayment on the U.S. West Coast and receives discharges of anthropogenically-enhanced freshwater, nutrients, carbon, and other materials. These inputs include direct point sources discharged from wastewater treatment (WWT) plants via ocean outfalls and point, non-point, and natural sources discharged via coastal rivers. We assembled a daily time series over 1971-2017 of discharges from large WWT plants ≥ 50 million gallon per day (MGD) and 1997-2017 from small WWT plants and coastal rivers. Constituents include nitrogen, phosphorus, organic carbon, alkalinity, iron, and silica. Data from research studies, several government and non-government agency databases containing discharge monitoring reports, river flow gauges, and other collateral information were compiled to produce this dataset. Predictive models and expert analysis addressed unmonitored sources and data gaps. The time series of terrestrial discharge and fluxes are provided with location of coastal discharge point or tributary. The data are deposited in a repository found in Sutula et al. [1]

The CO₂ system in the Mediterranean Sea inferred from a 3D coupled physical-biogeochemical model

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Configuration and Validation of an Oceanic Physical and Biogeochemical Model to Investigate Coastal Eutrophication in the Southern California Bight

International audienceThe Southern California Bight (SCB), an eastern boundary upwelling system, is impacted by global warming, acidification, and oxygen loss and receives anthropogenic nutrients from a coastal population of 20 million people. We describe the configuration, forcing, and validation of a realistic, submesoscale-resolving ocean model as a tool to investigate coastal eutrophication. This modeling system represents an important achievement because it strikes a balance of capturing the forcing by U.S. Pacific Coast-wide phenomena, while representing the bathymetric features and submesoscale circulation that affect the transport of nutrients from natural and human sources. Moreover, the model allows simulations at time scales that approach the interannual frequencies of ocean variability. The model simulation is evaluated against a broad suite of observational data throughout the SCB, showing realistic depiction of the mean state and its variability with satellite and in situ measurements of state variables and biogeochemical rates. The simulation reproduces the main structure of the seasonal upwelling front, the mean current patterns, the dispersion of wastewater plumes, as well as their seasonal variability. Furthermore, it reproduces the mean distributions of key biogeochemical and ecosystem properties and their variability. Biogeochemical rates reproduced by the model, such as primary production and nitrification, are also consistent with measured rates. This validation exercise demonstrates the utility of using fine-scale resolution modeling and local observations to identify, investigate, and communicate uncertainty to stakeholders to support management decisions on local anthropogenic nutrient discharges to coastal zones