Jean-Jacques Naudin

Evolution of particle size and concentration in the Rhône river mixing zone:: influence of salt flocculation

Abstract The salt induced flocculation phenomenon is often proposed as a major mechanism influencing the deposition of the suspended matter near the mouth of major rivers. However, depending on the particular mixing conditions between fresh water and marine water and on suspended solids (SS) reactivity, salt induced aggregation may be a minor factor controlling fast deposition of SS. This work combines field studies and laboratory experiments in order to assess the importance of salt induced flocculation in the case of the saltwedge estuary of the Rhone river. Sampling of the mixing zone has been performed in contrasted hydrodynamic conditions from a low water discharge period (500xa0m 3 xa0s −1 ) to a small flood event (2400xa0m 3 xa0s −1 ) for particle counting and salinity measurements. Through laboratory experiments, it is shown that the Rhone river particulate matter has a poor average reactivity regarding salt induced flocculation. Considering the hydrodynamics of the estuary, we show that the evolution of the concentration of larger particles (>5xa0μm) can be explained by settling and dilution. In contrast, the smallest measured fraction (2–5xa0μm) shows a more complex behavior and settling processes alone cannot explain the observed particle concentrations during the field studies. Four hypotheses are discussed for explaining the 2–5xa0μm particle concentration evolutions.

Experimental study of the Rhone plume. Part I: physics and dynamics

Abstract 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.

The effects of a strong winter storm on physical and biological variables at a shelf site in the Mediterranean

Abstract A survey involving both permanent mooring and high frequency sampling was carried in the Bay of Banyuls-sur-Mer during the fall of 1999 to assess the effect of strong and unpredictable meteorological events on the functioning of a coastal Mediterranean ecosystem. A severe winter storm took place on 12 November, which generated waves with a significant height of 7 m and a sea surface rise of about 0.5 m. The near-bottom current speed at 24xa0m reached 30 cm s –1 . This storm induced a significant increase in total suspended matter through resuspension and then a subsequent increase in gross sedimentation rates. It also resulted in an increase of the proportion of refractory particulate organic matter in the water column. It also tended to increase nutrient availability in the water column through resuspension and desorption processes. The kinetic of this increase differed among nutrients. Bacterial biomass and production were significantly enhanced by the storm. These effects were transitory and probably not due to resuspension alone. The distribution of plant pigments was modified at the immediate vicinity of the water-sediment interface due to differential resuspension and sedimentation but the storm had no effect on integrated phytoplanktonic biomass. Such a lack of response may be linked to low precipitations and/or light limitation. The storm resulted in a transitory increase of the abundance of fine particles at the water-sediment interface. These particles were coated with refractory organic matter. The storm induced a significant decrease of meiofauna abundance. The duration of the relaxation periods varied among parameters. It lasted 2 weeks for total suspended matter, surface sediment granulometry and carbohydrate contents.

Effect of mixing on microbial communities in the Rhone River plume

Abstract The biological processes involved during mixing of a river plume with the marine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buoy. Three trajectories were performed, at 2-day intervals, under different hydrological and meteorological situations. A biological uptake was evidenced from ammonium (NH 4 ) and phosphate (PO 4 ) shortage, indicating an early “NH 4 -dependent” functioning occurring before the well-known “NO 3 -based” cycle. The different ratios between NH 4 , NO 3 and PO 4 , as a function of salinity, were discussed to detail the preferential use in PO 4 and NH 4 . Salinity zones with enhanced bacterial production, high chlorophyll a concentration, as well as DOC, NH 4 and PO 4 consumption were evidenced from 20 to 35 in salinity. It was shown that the successive abundance of bacteria and phytoplankton during transfer reflected the competition for PO 4 of both communities. On the Rhone River plume, the role played by temperature, light conditions and suspended matter upon biological activity seems relatively minor compared to salinity distribution and its related parameter: nutrient availability. It can be concluded that biological uptake in the Rhone River plume was closely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the density gradient between marine and river water induced limited exchanges between the nutrient-rich freshwater and the potential consumers in the underlying marine water. Consequently, little biological activity is observed close to the river mouth. Offshore, mixing is enhanced and a balance is reached between salinity tolerance and nutrient availability to form a favourable zone for marine phytoplankton development. This can be quite far from the river mouth in case of a widely spread plume, corresponding to high river discharge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the “enhanced production zone” is drastically reduced and even bacteria could not benefit from the fast mixing regime induced.

Ultraviolet Radiation in the Rhône River Lenses of Low Salinity and in Marine Waters of the Northwestern Mediterranean Sea: Attenuation and Effects on Bacterial Activities and Net Community Production

The high content in nutrients of freshwater outflows induces highly productive and buoyant plumes spreading over marine waters (MW). As a consequence, the growth of organisms developing in these low‐salinity waters (LSW) might be potentially affected by UV‐R (280–400u2003nm). This study investigated the penetration of UV‐R and its impact on net community production (NCP) and bacterial protein (BPROTS) and DNA (BDNAS) synthesis in mesotrophic‐LSW formed from the Rhône River and in oligotrophic MW of the Northwestern Mediterranean Sea (Gulf of Lions) in May 2006. High concentrations of chlorophyll a (up to 8u2003μgu2003L−1) measured in the LSW (<37.8u2003psu, 0–10u2003m) were the main factor influencing the diffuse attenuation coefficients (Kd) of both UV‐R and photosynthetically active radiation (PAR). The mean ratio of the Kd measured between the LSW and the MW increased with wavelength from 2.4 at 305u2003nm to 2.9 at 380u2003nm and 3.1 for PAR indicating more similarity in the UV region. NCP was severely inhibited by UV‐R at the surface of the LSW, whereas no effect was measured in the surrounding MW. In contrast, BPROTS and BDNAS were affected deeper by UV‐R in the MW (up to 8u2003m depth) compared to the LSW where inhibition was only observed at the surface. Differences in response of bacteria in LSW and MW are largely explained by differences in UV‐R transparency; however, transplant experiments indicate that bacterial assemblages from the MW were also more sensitive to UV‐R than those present in the LSW. We also observed that higher activity of bacteria after nutrient additions increased their sensitivity to UV‐R during the day, but favored their recovery during the night incubation period for both LSW and MW. Results suggest that riverine and nutrient inputs may alter the effects of UV‐R on microbial activity by attenuating the UV‐R penetration and by modifying the physiology of bacteria.

Feasibility and usefulness of steady-state calculations of the mean circulation in the vicinity of the Ebro mouth. Model tests against field data

Abstract A steady-state quasi-3D finite element model is forced with in situ measured wind conditions in order to obtain an estimate of the mean circulation off the Ebro delta and to assess its consequences upon water fluxes. The model is spectral in the vertical direction and is run with a reduced number of vertical degrees of freedom (modes). The boundary conditions prescribed on the ocean contours account for the existence of a slope jet. The hypothesis is that the direct wind forcing on the upper layer and the shelf mesoscale circulation are the main mechanisms driving the flow in the area and that it should be possible to reproduce the main current features even in the vicinity of the Ebro river mouth with this relatively simple model setup, as the usual river freshwater discharge rates are rather low. This is clearly different from the case of other region of freshwater influence systems, in which the river plume dynamics and the related density currents play relevant roles. The model results are compared with surface drifter trajectories obtained during field campaigns.