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Abstract.A coupled hydrodynamic-biogeochemical modelling is developed to address main mechanisms that drive the particulate organic carbon (POC) deposition in the Gulf of Lions (NW-Mediterranean). Low-salinity water (LSW, salinity <37.5) lenses detached from the Rhone River plume under specific wind conditions tend to favour the biological productivity and provide a good opportunity for validating a planktonic ecosystem modelling. A specific calibration dedicated to river plume ecosystems is then proposed and validated using in situ measurements within such LSW lens (BIOPRHOFI cruise -May 2006) and on the Gulf of Lions. During spring 2006, the POC deposition is maximal on the prodelta area and within the coastal area in the Gulf of Lions. Organic detritus mostly contribute to the total POC deposition (82-92 %) whereas the contribution of living organisms (microphytoplankton) appears lower than 17 %. Exploring both influences of terrestrial inputs from the Rhone River and planktonic ecosystems on the POC deposition on the shelf, we estimated that the contribution of terrestrial POM inputs to the total POC deposition is lower than 17 % at the shelf scale during the study period, with maxima during peak discharges of the Rhone River. The main deposition area of terrestrial POC is found in the vicinity of the river mouth in agreement with sediment data. On the other hand, a remarkable influence of marine biological processes on the POC deposition is highlighted further on the shelf (from 60 to 80 m depth). A tight feedback between zooplankton and POM contents in the water column is proposed to explain Correspondence to: P. A. Auger (pierre-amael.auger@aero.obs-mip.fr) the control of POC deposition by zooplankton: terrestrial POM inputs would favour the development of living organisms through photosynthesis and grazing processes increasing the retention of organic matter within the food web. By favouring the development of large-sized zooplankton, LSW lenses may have paradoxically a negative impact on the carbon deposition on the shelf. In the same way, peak discharges of the Rhone River finally increase the gradient of POC deposition between the prodelta and the offshore area in the Gulf of Lions. The biogenic elements from the Rhone River are then exported further offshore through advection of zooplankton communities on the Gulf of Lions shelf.

The complicated dynamic processes occurring when fluvial waters mix with marine waters control the nature and the fluxes of materials exported by rivers to the sea. Understanding these processes is of primary importance in evaluating budgets. In wide-open estuarine situations these processes take place under the influence of an intense turbulence induced by tides. Conversely. the Rhone waters spread into the Mediterranean Sea in the form of an easily distinguishable buoyant plume often extending far offshore from the mouth of the river. The aim of this study is to describe the dynamic and hydrological fields on the basis of eulerian VHF radar mapping of surface currents coupled with lagrangian in situ physical or geochemical measurements. This paper focuses mainly on physical processes. Data analysis provides an insight into the typical scales of variability of the phenomena, either vertically or horizontally. It is shown that morphological fluctuations can occur (mainly in orientation and offshore extent) according to wind and outflow forcing conditions, and that the vertical structure variations can range from an almost unaltered two-layer distribution to an evolving and deepening mixed layer situation, or even to a more complex superimposed multi-layered structure. The simultaneous examination of radar maps and lagrangian drifter tracking allows the main dynamic tendencies of the Rhone plume to be sketched out. 0 Elsevier, Paris river plume / VHF radar I Rhone I dynamics RCsumC-&de expCrimentale du panache du RMne. Partie I : physique et dynamique. La zone de mClange entre les eaux d'un fleuve et les eaux marines est le s5ge de processus dynamiques complexes qui conditionnent la nature et les flux de mat&es apportCes au milieu marin. La comprehension de ces mdcanismes est primordiale pour effectuer des bilans. Contrairement aux estuaires oti la dilution est induite par une turbulence intense entretenue par les flux entrant et sortant de mar&e, les eaux du Rhi,ne s'e'panchent en MCditerranCe sous forme d'un panache flottant sur de grandes distances au large de l'embouchure. L'objet du prCsent travail est d'apprkhender les champs dynamiques et hydrologiques de cette structure au moyen de cartographies de vitesses eulkriennes de courant superficiel par radar VHF et au moyen de mesures lagrangiennes in situ physiques et gCochimiques. Cet article est consacrk aux processus physiques. L'analyse des don&es permet de cerner la variabilite du phenomkne, tant horizontale que verticale. Les fluctuations de morphologie (orientation et extension vers le large) du panache en fonction des conditions m6tCorologiques et de debit fluvial sont mises en Cvidence, ainsi que les variations de sa structure verticale, allant d'une structure bicouche pratiquement non altCrCe B une couche bien mClangCe, de caractiristiques en Cvolution et d'Cpaisseur croissante, ou & une superposition multicouche plus complexe. L'examen simultanC des cartes de courant et des don&es physiques Iagrangiennes permet de degager les grandes tendances du fonc...