Les îles d'Hyères dans le système de circulation marine et atmosphérique de la Méditerranée

International audienceThe scope of this paper is the general features of the marine and atmospheric circulations in the Mediterranean. It aims at providing keys to managers and other scientists to better understand the physical phenomena that impact the Hyères archipelago, in order to improve the environmental management if needed, and face potential risks.Cet article a pour but de présenter les grandes lignes du fonctionnement de la circulation marine et atmosphérique de la Méditerranée, afin de mieux comprendre les phénomènes physiques qui affectent les îles d'Hyères, d'optimiser la gestion de leur environnement et le cas échéant des risques potentiels. Mots-clés : Méditerranée, îles d'Hyères, Port-Cros, circulation des masses d'eau, courant, vent, état de mer, circulation atmosphérique, moyenne échelle. Abstract. The Hyères Archipelago in the marine and atmospheric circulation systems. The scope of this paper is the general features of the marine and atmospheric circulations in the Mediterranean. It aims at providing keys to managers and other scientists to better understand the physical phenomena that impact the Hyères archipelago, in order to improve the environmental management if needed, and face potential risks

How geographic distance and depth drive ecological variability and isolation of demersal fish communities in an archipelago system (Cape Verde, Eastern Atlantic Ocean)

Cape Verde is a tropical oceanic ecosystem, highly fragmented and dispersed, with islands physically isolated by distance and depth. To understand how isolation affects the ecological variability in this archipelago, we conducted a research project on the community structure of the 18 commercially most important demersal fishes. An index of ecological distance based on species relative dominance (Di) is developed from Catch Per Unit Effort, derived from an extensive database of artisanal fisheries. Two ecological measures of distance between islands are calculated: at the species level, DDi, and at the community level, DD (sum of DDi). A physical isolation factor (Idb) combining distance (d) and bathymetry (b) is proposed. Covariance analysis shows that isolation factor is positively correlated with both DDi and DD, suggesting that Idb can be considered as an ecological isolation factor. The effect of Idb varies with season and species. This effect is stronger in summer (May to November), than in winter (December to April), which appears to be more unstable. Species react differently to Idb, independently of season. A principal component analysis on the monthly (DDi) for the 12 islands and the 18 species, complemented by an agglomerative hierarchical clustering, shows a geographic pattern of island organization, according to Idb. Results indicate that the ecological structure of demersal fish communities of Cape Verde archipelago, both in time and space, can be explained by a geographic isolation factor. The analytical approach used here is promising and could be tested in other archipelago systems

Exploiting coastal altimetry to improve the surface circulation scheme over the central Mediterranean Sea

This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time

Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean

International audienceUpward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the Western Mediterranean while physical evidence of such upward advections is seldom reported. The goal of this study is to define biological and chemical criteria, which can be used to localize such upward motions zones. We use a one-dimensional, time-dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen. Simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth. In conformance with the continuity equation, boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer. Low upward advections (-<3 m d-•) led to a shallowing and sharpening of the nitracline, oxycline, and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass. By confining the phytoplankton-nutrient system in the surface mixed layer, higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer, the nitracline and the oxycline being then at the top of the pycnocline. Data collected during the Prolig 2 cruise (May 1985) on the heavy side of the Liguro-Provenqal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almer/a-Oran front during the Almofront i cruise (April 1991) in a similar situation. In both fronts, upward advections of 1-2 m d-• would be sufficient to account for the observed vertical distributions and the increased primary production. Ecological implications for the phytoplankton-nutrient system are discussed, particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts. 1. Introduction The Liguro-Provengal front [B•thoux et Prieur, 1983; Sournia et al., 1990] and the Almerfa-Oran front [Cheney and Doblar, 1982; Tintor• et al., 1988] are permanent geostrophic structures in the Western Mediterranean. Both fronts are areas of high-plankton biomass, in contrast with two adjacent oligotrophic-type systems [Lohrenz et al., 1988; Tintor• et al., 1988; Sournia et al., 1990; Prieur et al., 1993; Claustre et al., 1994b], and show net change in both the specific composition of the autotrophic [Claustre et al., 1994b] and the heterotrophic [Boucher et al., 1987; Thibault et al., 1994] communities. Several features indicate that the two fronts are sites of enhanced production in response to geostrophic frontal dynamics. In addition to the main geostrophic flow, a secondary circulation exists, i.e., the ageostrophic flow, which is characterized by enhanced vertical advections as shown for example by Dewey et al. [1991] in the California Current. It is thought that upward advections in the frontal zone upwell nutrients into the photic zone and enhance primary production [Lohrenz et al., 1988; Fideau et al., 1994]. The resulting high biomass is then downwelled by the conver-Paper number 98JC01537. 0148-0227/98/98 JC-01537509.00 gent part of the ageostrophic circulation along the isopycnals [Claustre et al., 1994a, b; Fideau et al., 1994]. However, physical evidence for upward advection in the two fronts is not well-documented (see, however, Tintor• et al. [1991] and l?iudez and Tintor• [1996]) and poorly understood. This paper defines criteria from the vertical distribution of parameters involved in primary production, which can be used to localize, with some confidence, the upward motions in such frontal zones. In a previous modeling study [Zakardjian and Prieur, 1994], two types of phytoplankton-nutrient systems resulting from high-and low-turbulent regimes (HTR and LTR, respectively) were described, with the LTR representing the ecological conditions in the waters adjacent to the Liguro-Provengal and Almerga-Oran fronts, where the vertical structure is controlled by diffuse vertical fluxes. We use a modified version of this previous one-dimensional vertical model (presented in section 2) to test the sensitivity of the LTR system when an upward advection term is added. In section 3 we describe the steady state vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen obtained with upward advection values ranging from 0 to 5 m d-•. Effects of upward advection on the vertical distribution of primary production, on phytoplankton growth, and, finally, on the productivity indexes of the water column are detailed. In section 4 we present multiparametric 27,84

Control of dormancy by lipid metabolism in Calanus finmarchicus: a population model test

International audienceThe life cycle of Calanus finmarchicus includes a prolonged dormancy phase that allows it to avoid the unfavourable environmental conditions typical of the upper ocean from late summer to early spring in the subarctic North Atlantic. Recent demographic, physiological and genetic evidence supports the hypothesis of a crucial role for lipid accumulation and metabolism in the control of dormancy. We present a stage-resolving biomass model of C. finmarchicus, implementing a mechanistic approach of the control of dormancy based on this lipid hypothesis. The dormancy process obeys 2 rules: (1) active copepodite stage Vs (C5s) enter dormancy when the ratio of lipid to total body carbon exceeds some threshold, and (2) diapausing C5s exit dormancy when their lipid storage approaches a lower threshold. We implemented the model into a 1-dimensional water column framework and compared our results to 2 consecutive years of observations of stage-specific copepodite abundances and lipid content of C5 from the Northwest Gulf of St. Lawrence, Canada. The model produced realistic phenology and temporal patterns in lipid content of C. finmarchicus in response to the observed environmental forcing. Interannual variations in the timing of entry and contributions of different generations to the overwintering stock were shown. Our results are consistent with the hypothesis of a lipid-mediated control of entrance and exit from dormancy in C. finmarchicus

Étude de la dynamique du Courant Nord au large de Toulon, à l'aide de modèle, observations in-situ et données satellites

L objectif général de cette thèse est de contribuer à l avancement de la connaissance de la variabilité du Courant Nord Méditerranéen (CN) et de ses interactions avec la dynamique côtière, en s appuyant principalement sur une configuration numérique réaliste à haute résolution de la façade méditerranéenne française, basée sur le modèle de circulation océanique NEMO et nommée GLAZUR64.La validation de cette configuration avec toutes les observations disponibles sur la période d'étude (CTD, gliders, ARGO, radar HF, ADCP, altimétrie et SST satellite) a permis d'évaluer le réalisme des simulations et leur paramétrisation, et de montrer l'apport de la haute-résolution par rapport aux configurations de bassin au 1/12 utilisées aux frontières de GLAZUR64.Enfin, l'utilisation d'un forçage océanique opérationnel a permis d'utiliser une simulation en complément des données d'une campagne en mer, pour l'étude ciblée d'un tourbillon anticyclonique associé à un méandre du CN au large de Toulon, en avril 2011 [Guihou et al., 2013].The main objective of this work is to improve our knowledge of the Northern Mediterrranean Current (NC) variability and its interactions with coastal dynamics, using high-resolution modelling of the North-Western Mediterranean Sea, based on the NEMO code and nammed GLAZUR64.The configuration has been validated using all available observations for the period of study (CTD, gliders, ARGO, HF radar, ADCP, satellite altimetry and SST), in order to assess the realism of the simulations and their parameterisations. The contribution of the high-resolution for the simulation of the mesoscale dynamics, compared to 1/12 configurations used at the boundaries is shown.Finally, the use of an operationnal oceanic boundary forcing allowed us to use a combined data-model approach to study the generation and advection of an anticyclonic eddy trapped at the coast and associated to a NC meander, flowing off Toulon in April 2011 [Guihou et al., 2013].TOULON-Bibliotheque electronique (830629901) / SudocSudocFranceF

Plankton ecosystem response to freshwater-associated bulk turbidity in the subarctic Gulf of St. Lawrence (Canada): A modelling study

International audienceWe present a three-dimensional physical–biological modelling study aiming to infer the effect of freshwater-associated bulk turbidity on the Gulf of St. Lawrence (Canada) plankton ecosystem. Bulk turbidity is parameterised using an inverse relationship derived from an extensive in situ dataset linking salinity to the diffuse attenuation coefficient of photosynthetically available radiation (PAR) due to nonchlorophyllous matter. Embedding bulk turbidity in the model led to shallowing of the photic zone in the estuarine plume in accordance with coincident observations and allowed a better discrimination between Case 1 (chlorophyll-mediated variability of the photosynthetic available radiation attenuation) and Case 2 waters. The spring bloom was delayed, and primary and secondary production rates as well as the export of biogenic matter at depth decreased in the freshwater-influenced subregions. Comparisons with literature and coincident in situ measurements showed that nitrates were over- and underestimated in the run with and without bulk turbidity, respectively. A sensitivity analysis was performed with a relatively simple but robust parameterisation of photoacclimation, i.e. the adjustment of the phytoplankton photosynthetic efficiency to local underwater light conditions. Photoacclimation allowed simulated chlorophyll and nitrate concentrations as well as lateral fluxes of nitrate to achieve the best agreement with coincident measurements and literature estimates, respectively. This study showed that accounting for the freshwater-associated bio-optical variability and phytoplankton response in terms of photosynthetic efficiency improved the model's ability to predict the plankton ecosystem dynamics and associated biogeochemical fluxes in the river-influenced Gulf of St. Lawrence

Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products

The aim of this paper is to assess the impact of numerical resolution on the simulation of the circulation in the North-Western Mediterranean Sea by comparing two realistic model configurations of 1/64 degrees and 1/16 degrees, the low resolution configuration (LRC) being also used at the high resolution configuration (HRC) open boundaries. The study mainly focuses on the Northern Current (NC), the major feature of the regional circulation in the area, influencing important processes such as meanders giving birth to eddies and NC intrusions on the shelf, strongly conditioning the interactions between coastal and off-shore waters. Both configurations use the NEMO code with the LRC covering the entire Mediterranean basin and the HRC being restricted to the North-Western Mediterranean Sea. The two numerical configurations are assessed by using hull-mounted ADCP data and geostrophic velocities derived from AVISO altimetry at key locations along the NC path. The two configurations develop a different solution regarding the current dynamics but both indicate an important variability of the NC position along its general path. The LRC simulations systematically exhibit a NC shifted too far off the coast and also spuriously displaced seaward along the Gulf of Lions shelf edge while high resolution simulations better fit the remote and in situ observations. In addition, the HRC is able to simulate more realistic coastal features such as a confined coastal jet in good agreement with the ADCP measurements. As satisfactory agreement is reached with the various observations used at different space and time scales, the development of this NEMO high resolution configuration stands as a promising experiment for various process studies or operational oriented applications. (C) 2011 Elsevier Ltd. All rights reserved

Modelling transport and stranding of jellyfish Pelagia noctiluca in the Ligurian Sea

Jellyfish bloom origins are generally sought in some biological response to the environment, leaving aside the role of transport patterns in redistributing existing populations. Here we use high resolution (1.25 km) ocean modelling to examine the role of transport in the onshore arrival and abundance of the pelagic stinging jellyfish Pelagia noctiluca on the Ligurian Sea coast. Jellyfish are modelled as Lagrangian particles with a 0-300m diel vertical migration typical of P. noctiluca. Over a year, onshore arrivals are not restricted to the summer period. Arrivals are concentrated at capes, but abundance can reach maxima in bays and in the lee of capes. Two factors impact jellyfish arrivals at the coast: the position of the Northern Current and the wind. A comparison of summer 2006 and available onshore jellyfish observations suggests a correct capture of the main stranding events by the model. These results have implications for understanding long term fluctuation

Modeling the interactions between the seasonal and diel migration behaviors of Calanus finmarchicus and the circulation in the Gulf of St. Lawrence (Canada)

The Gulf of St.-Lawrence (GSL) is a dynamic region supporting a productive pelagic ecosystem. This environment presents unique opportunities to study the interactions between the population dynamics of planktonic species and the variability of physical processes. The copepod Calanus finmarchicus is a dominant component of zooplankton biomass and abundance in the GSL We developed a 3-D coupled physical-biological numerical model in order to study the population dynamics of C. finmarchicus in the GSL for the year 1999. We coupled a life cycle model of C. finmarchicus representing the average properties of the population in terms of egg production, development, migration behavior and mortality to a regional circulation model driven by realistic atmospheric, hydrological and oceanic forcing. The distribution and abundance patterns of C. finmarchicus were sensitive to the migration behavior owing to the strong vertical and horizontal shears in the circulation. Both the timing of seasonal ontogenetic vertical migrations and the diel vertical migrations appeared to be essential to produce simulation results similar to the observations and to ensure the perennial presence of a local population in the GSL. (C) 2011 Elsevier B.V. All rights reserved