Implementation and assessment of a model including mixotrophs and the carbonate cycle (Eco3M_MIX-CarbOx v1.0) in a highly dynamic Mediterranean coastal environment (Bay of Marseille, France) – Part 1: Evolution of ecosystem composition under limited light and nutrient conditions

Many current biogeochemical models rely on an autotrophic versus heterotrophic food web representation. However, in recent years, an increasing number of studies have begun to challenge this approach. Several authors have highlighted the importance of protists capable of combining photoautotrophic and heterotrophic nutrition in a single cell. These mixotrophic protists are known to play an important role in the carbon cycle. Here, we present a new biogeochemical model that represents the food web using variable stoichiometry. It contains the classic compartments such as zooplankton, phytoplankton, and heterotrophic bacteria and a newly added compartment to represent two types of mixotrophic protists: non-constitutive mixotrophs (NCMs) and constitutive mixotrophs (CMs). We demonstrate that the model correctly reproduces the characteristics of NCMs and CMs and proceed to study the impact of light and nutrient limitation on planktonic ecosystem structure in a highly dynamic Mediterranean coastal area, namely the Bay of Marseille (BoM, France), paying special attention to the dynamics of mixotrophic protists in these limiting conditions. In addition, we investigate the carbon, nitrogen, and phosphorus fluxes associated with mixotrophic protists and showed the following: (i) the portion of the ecosystem in terms of the percentage of carbon biomass occupied by NCMs decreases when resources (nutrient and prey concentrations) decrease, although their mixotrophy allows them to maintain a carbon biomass almost as significant as the copepod one (129.8 and 148.7 mmol C m−3, respectively), as photosynthesis increases as a food source, and (ii) the portion of the ecosystem in terms of the percentage of carbon biomass occupied by CM increases when nutrient concentrations decrease due to their capability to ingest prey to supplement their N and P needs. In addition to providing new insights regarding the conditions that lead to the emergence of mixotrophs in the BoM, this work provides a new tool to perform long-term studies and predictions of mixotroph dynamics in coastal environments under different environmental forcings.</p

Sedimentary impact of a storm on the Mediterranean sand beach of Sète: a numerical approach by hydrodynamic and sedimentary modelling

International audienceno abstrac

Sedimentary impact of a storm on the Mediterranean sand beach of Sète: a numerical approach by hydrodynamic and sedimentary modelling

International audienceno abstrac

Erosion of particulate inorganic and organic matter in the Gulf of Lion

Critical shear stress of erosion and erosion rate of particulate inorganic and organic matter were measured in a flume at three muddy stations. Critical shear stress ranged between 0.022 and View the MathML source. At the deepest site, annual erosion of particulate organic nitrogen and phosphorus could exceed net deposition fluxes, showing the importance of erosion processes. Erosion may modify total system mineralisation rates by introducing sedimentary particulate organic matter into the water column and thus this process must be taken into account in studies of biogeochemical cycle

Modélisation de la dynamique de sédimentaire de la plage de Sète. Le rechargement sédimentaire des barres sableuses sous-marines, une solution pour lutter contre l'érosion ?

International audienceno abstrac

Évaluer l’Impact de la métropole Aix-Marseille sur l’Acidification de la baie de Marseille et les conséquences sur les microorganismes marins, approche par Modélisation (IAMM)- Résumé exécutif

La baie de Marseille est un environnement côtier méditerranéen complexe soumis à de nombreux apports externes et affecté par divers processus hydrodynamiques. Compte tenu de cette complexité, sa réaction face au changement global est particulièrement difficile à anticiper. Dans cette étude, une approche par modélisation couplée physique biogéochimie, idéale dans ce type d'environnement, est utilisée pour envisager l'évolution des propriétés de la baie pendant les 30 prochaines années, pour un scénario de changement climatique optimiste (RCP 2.6) et pessimiste (RCP 8.5). Les tendances obtenues montrent une augmentation de la température (comprise entre 0.07 et 0.25°C pour les 30 ans) et une diminution du pH (comprise entre -0.02 et -0.05 unité pH pour les 30 ans) dans les eaux de surface de la baie. Cependant, ces résultats doivent être utilisés avec précaution car ils sont entachés par différents biais : (i) une sous-estimation de la température en été, et une sous-estimation globale de la pCO2 marine, et (ii) une forte dépendance au forçage atmosphérique choisi qui est connu pour être dans la fourchette basse des projections des écarts à la normale de température de l'atmosphère et de leur variabilité

Morphodynamique de la plage de Sète sous l'incidence d'une houle de tempête: expérimentation et modélisation

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Field and laboratory measurements of sediment erodibility: A comparison

The erosion of fine-grained cohesive sediments is a ubiquitous phenomenon in marine and estuarine environments. Critical shear stress and erosion rates are predominant variables in transport mechanics. This study presents a comparison between in situ and laboratory measurements of sediment erodibility. The main goal was to assess the reliability of critical shear stress of erosion determined in a laboratory flume. The laboratory experiment allowed us to define the erodibility of sediments sampled at a reference station in the Gulf of Fos. This site is frequently influenced by Rhône river inflows. Two different values of critical shear stress of erosion were determined, the lowest (τc1 = 0.02 ± 0.002 N m− 2) related to the erosion of the fluff layer, and the highest (τc2 = 0.05 ± 0.005 N m− 2) to a more consolidated layer or to coarser particles. These values of critical shear stress were very close to commonly in situ based estimates of fluff critical shear stresses, which ranged between 0.02 and 0.05 N m− 2. The comparison with acoustic backscatter data recorded during a field cruise in autumn attested in a qualitative way to the reliability of the critical shear stress of erosion determined in our laboratory flume. The erosion coefficients determined during this study, varying between 3.4 10− 6 and 7.5 10− 6 kg m− 2 s− 1, were used to calculate in situ erosion rates. Between 6 and 9 October, the mean erosion rate at the reference station was 20 g m− 2 h− 1 and maximum values reached 65 g m− 2 h− 1. The calculated erosion rates were used to predict in situ variations in suspended sediment concentration (SPM). The predicted increase in SPM was 14 mg l− 1, close to the measured increase in SPM (10 mg l− 1). The results from this initial attempt to compare in situ and laboratory erosion measurements are encouraging but further investigation is needed. Potential organic matter fluxes determined in our flume were around 1.7 mmol PON m− 2 h− 1 and 115 μmol POP m− 2 h− 1. An estimation based on these results showed that the organic material introduced into the water column by erosion amounted to between 12 and 200% of the daily pelagic primary production. This is considerable and should be considered in biogeochemical studies of coastal systems

Déformation de la houle pendant une période de tempête sur une plage sableuse à barres: Expérimentation et modélisation

International audienceno abstrac