Physique et chimie du milieu marin: circulation des masses d'eau

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Sicherheitsaspekte bei der Lagerung von Schüttgütern-Untersuchung für die Bedingungen des thermischen Durchgangs

Sicherheitsaspekte bei der Lagerung von Schüttgütern-Untersuchung für die Bedingungen des thermischen Durchgang

Effective thermal conductivity measurements by the transient hot-wire method: investigation of the interpretation procedure of experimental data

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Sécurité des stockages de matériaux en grains - Influence de la conductivité thermique

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Seasonal dynamics and stoichiometry of the planktonic community in the NW Mediterranean Sea: a 3D modeling approach

International audienceThe origin of the high N:P ratios in the Mediterranean Sea is one of the remaining important questions raised by the scientific community. During the last two decades it was observed that the inorganic ratio NO 3 :PO4 ratio in major Mediterranean rivers including the Rhone River has dramatically increased, thereby strengthening the P-limitation in the Mediterranean waters (Ludwig et al, 2009, The MerMex group, 2011) and, as a result, increasing the anomaly in the ratio NO 3 :PO4 of the Gulf of Lions (GoL) and in all the western part of NW Mediterranean. The N:P ratios in seawater and in the metabolic requirements for plankton growth are indeed of particular interest, as these proportions determine which nutrient will limit biological productivity at the base of the food web and may select plankton communities with distinct biogeochemical function (Deutsch &Weber, 2012). In this context, an in the same spirit as the study of Parsons & Lalli (2002), an interesting question is whether high NO 3 :PO4 ratios in sea water can favor dead-end gelatinous food chains to the detriment of chains producing fish or direct food for fish. More generally, we aim at characterizing the impact of changes in the NO 3 :PO4 ratio on the structure of the planktonic food web in the Mediterranean Sea. Coupled physical-biogeochemical modeling with the Eco3M-MED biogeochemical model (Baklouti et al., 2006a,b, Alekseenko et al., 2014) coupled with the hydrodynamic model MARS3D (Lazure&Dumas, 2008) is used to investigate the impact of Rhone River inputs on the structure of the first levels of the trophic web of the NW Mediterranean Sea. The fact that the model describes each biogenic compartment in terms of its abundance (for organisms), and carbon, phosphorus, nitrogen and chlorophyll (for autotrophs) contents means that the intracellular quotas and ratios of each organism can be calculated at any time. This provides information on the intracellular status of organisms, on the elements that limit their growth and ultimately enhances our understanding of the functioning of this planktonic food-web. The present work consisted in running two different scenarios (low and high NO 3 :PO4 in the Rhone River). The lower ratio is the one presently found in the Rhône river outputs while the higher ratio is twice the lower one.The study focused on a one-year period (2010) since the model outputs during this period have already been partially validated (Alekseenko et al., 2014). At this stage, we first explore the spatial and temporal dynamics of the carbon stocks, in living and non-living compartments as well as related carbon fluxes. Results showed that, after one year of simulation, the change in NO 3 :PO4 of Rhone River mostly impacts organisms in the shelf zone of GoL, and especially the lowest trophic levels during the spring phytoplankton bloom. The increase in NO 3 :PO4 decreases primary production and bacterial production rates, thereby decreasing food availability for zooplankton which population growth decreases. During the spring phytoplankton bloom period, the decrease in Chl-a induced by the high NO 3 :PO4 Rhone ratio has an order of value comparable to the Chl-a mean level observed in the GoL. In the case of the scenario with high NO 3 :PO4 ratio, bacterial production increases after the spring bloom, what, in turn, increases the development of ciliates. The impact of NO 3 :PO4 scenarios on the mesozooplankton and jellyfish compartments considered in the model is overall low after a one-year simulation, but, probably due to their longer life cycle comparing to smaller organisms, this should be investigated at longer time scales

Seasonal dynamics and stoichiometry of the planktonic community in the NW Mediterranean Sea: a 3D modeling approach

International audienceThe origin of the high N:P ratios in the Mediterranean Sea is one of the remaining important questions raised by the scientific community. During the last two decades it was observed that the inorganic ratio NO 3 :PO4 ratio in major Mediterranean rivers including the Rhone River has dramatically increased, thereby strengthening the P-limitation in the Mediterranean waters (Ludwig et al, 2009, The MerMex group, 2011) and, as a result, increasing the anomaly in the ratio NO 3 :PO4 of the Gulf of Lions (GoL) and in all the western part of NW Mediterranean. The N:P ratios in seawater and in the metabolic requirements for plankton growth are indeed of particular interest, as these proportions determine which nutrient will limit biological productivity at the base of the food web and may select plankton communities with distinct biogeochemical function (Deutsch &Weber, 2012). In this context, an in the same spirit as the study of Parsons & Lalli (2002), an interesting question is whether high NO 3 :PO4 ratios in sea water can favor dead-end gelatinous food chains to the detriment of chains producing fish or direct food for fish. More generally, we aim at characterizing the impact of changes in the NO 3 :PO4 ratio on the structure of the planktonic food web in the Mediterranean Sea. Coupled physical-biogeochemical modeling with the Eco3M-MED biogeochemical model (Baklouti et al., 2006a,b, Alekseenko et al., 2014) coupled with the hydrodynamic model MARS3D (Lazure&Dumas, 2008) is used to investigate the impact of Rhone River inputs on the structure of the first levels of the trophic web of the NW Mediterranean Sea. The fact that the model describes each biogenic compartment in terms of its abundance (for organisms), and carbon, phosphorus, nitrogen and chlorophyll (for autotrophs) contents means that the intracellular quotas and ratios of each organism can be calculated at any time. This provides information on the intracellular status of organisms, on the elements that limit their growth and ultimately enhances our understanding of the functioning of this planktonic food-web. The present work consisted in running two different scenarios (low and high NO 3 :PO4 in the Rhone River). The lower ratio is the one presently found in the Rhône river outputs while the higher ratio is twice the lower one.The study focused on a one-year period (2010) since the model outputs during this period have already been partially validated (Alekseenko et al., 2014). At this stage, we first explore the spatial and temporal dynamics of the carbon stocks, in living and non-living compartments as well as related carbon fluxes. Results showed that, after one year of simulation, the change in NO 3 :PO4 of Rhone River mostly impacts organisms in the shelf zone of GoL, and especially the lowest trophic levels during the spring phytoplankton bloom. The increase in NO 3 :PO4 decreases primary production and bacterial production rates, thereby decreasing food availability for zooplankton which population growth decreases. During the spring phytoplankton bloom period, the decrease in Chl-a induced by the high NO 3 :PO4 Rhone ratio has an order of value comparable to the Chl-a mean level observed in the GoL. In the case of the scenario with high NO 3 :PO4 ratio, bacterial production increases after the spring bloom, what, in turn, increases the development of ciliates. The impact of NO 3 :PO4 scenarios on the mesozooplankton and jellyfish compartments considered in the model is overall low after a one-year simulation, but, probably due to their longer life cycle comparing to smaller organisms, this should be investigated at longer time scales

Dynamisches Verhalten von durchgehenden Reaktionen in Anhäufungen granulierter staubförmiger Stoffe

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Modeling the Influence of Wind and Rivers on Current, Salinity and Temperature over the French Guiana Continental Shelf during the Rainy Season

This paper deals first with the formulation of a three-dimensional numerical model intended to determine the spatial and temporal evolution of the oceanic circulation in coastal zones under the effects of various oceanic and meteorological constraints. Simulations are based on the Mobeehdycs model which was developed through collaborative work of several laboratories. Then, the paper presents the results of the application of the model to the continental shelf of French Guiana under academic but realistic climatic conditions. The application required the adaptation of the model and the use of appropriate techniques for solving the equations accounting for the peculiarities of the local constraints. The application is of importance as, because of the lack of systematic observations, the current, salinity and temperature fields at the site are poorly known. A better knowledge of these fields is recognized as of fundamental interest for a characterization of the site from the biological and the ecological viewpoints. The results clearly show the effects of the external forcing (wind and rivers) on the fields evolution, at the surface and with respect to the depth. The time scales of these evolutions as well as their mutual influence are identified. Finally, the results agree, at least qualitatively with some of the few observational results available at present

Transient self-heating in storage of bulk reactive solids: modeling and analysis

International audienceControlling heat transfer in the storage of reactive material (cereals, coal, peroxides, etc.) is important to operate under safe conditions. Many parameters, such as initial storage temperature, ambient temperature, heap volume, and effective thermal conductivity, are involved in the thermal behavior of those systems. The authors present a model to estimate the heat transfer and therefore the runaway hazard in the storage of reactive solids

Impact of periodic nutrient input rate on trophic chain properties

Marine ecosystems are characterized by a strong influence of hydrodynamics on biological processes. The associated models involve the coupling of physical to biological models and therefore require a large number of state variables. The consequent high complexity limits our capacity to perform a complete and detailed study and even prevents any complete mathematical study of these models. It is also difficult to disentangle among all the processes involved, which ones actually drive the system at any moment. Hydrodynamics, among other consequences, affect the way under which the nutrients are supplied to marine ecosystems. The variability of nutrient input rate in marine systems generally results from runs-off in coastal systems and from physical processes (wind forcing and hydrodynamics) in open ocean. This paper is devoted to the study of the effects of the nutrient input rate variability on the dynamics and the functioning of trophic chains. In this context, we aim to provide an understandable study, based on simplified system models. We consider a periodic nutrient input rate and analyze how this variability modifies some system properties: its dynamics, its functioning and its structure. The dynamics is obtained by numerical simulations and when possible, enlighten by already published mathematical results. The functioning is measured by the time averaged state variables during the simulation period, and their variability. The structure concerns the number of surviving populations, a proxy of specific biodiversity. We show how these properties can be affected and provide some conditions under which the modifications can occur. We also highlight that, even if the physical process is the main driving force in the global dynamics, the choice of the biological model is important to understand the biological response of the system to physical forcing