Rhone River Research Articles

Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). II. Investigating the effects of high trophic levels on nutrients and plankton dynamics and associated feedbacks

The end-to-end OSMOSE-GoL model parameterized, calibrated and evaluated for the Gulf of Lions ecosystem (Northwestern Mediterranean Sea) has been used to investigate the effects of introducing two-ways coupling between the dynamics of Low and High Trophic Level groups.The use of a fully dynamic two-ways coupling between the models of Low and High Trophic Levels organisms provided some insights in the functioning of the food web in the Gulf of Lions. On the whole microphytoplankton and mesozooplankton were found to be preyed upon by High Trophic Levels planktivorous groups at rates lower than 20% and 30% of their respective natural mortality rates, but these relatively low rates involved some important alterations in the infra-seasonal and annual cycles of both High and Low Trophic Levels groups. They induced significant changes in biomass, fisheries landings and food web interactions by cascading effects. Spatial differential impacts of High Trophic Levels predation on plankton are less clear except in areas in which primary productivity is high. Higher predation rates on plankton groups were encountered within the area of the Rhone river’s influence and in areas associated to the presence of mesoscale eddies in the Northwestern part of the Gulf of Lions, especially. Generally, the pressure of the High Trophic Levels predation was the highest in areas of highest biomass whatever the plankton group considered.The two-ways coupling between Low and High Trophic Levels models revealed both bottom-up and top-down controls in the ecosystem with effects on planktivorous species similar to those observed in the field. The use of the end-to-end model enabled to propose a set of potential mechanisms that may explain the observed decrease in small pelagic catches by the French Mediterranean artisanal fisheries over the last decade.

Réévaluation des apports moyens de matières en suspension de l'Arve au Rhône

L'Arve est le principal contributeur au flux de matières en suspension (MES) du Rhône entre le lac Léman et Lyon. Une évaluation fiable des apports moyens de MES de l'Arve est cruciale pour la connaissance et la gestion des flux sédimentaires sur ce secteur du Rhône et à travers ses aménagements hydroélectriques. Cette étude vise à réévaluer les flux moyens de l'Arve en suspension fine, en croisant les données historiques avec les flux de MES de référence pour les années 2012 à 2017 issus du suivi turbidimétrique en continu à la station de Genève Bout-du-Monde de l'Observatoire des sédiments du Rhône. Ce jeu de données a permis d'évaluer les deux méthodes de calcul des flux mises en œuvre par le Service hydrologique national suisse pour les périodes avec des mesures discontinues des MES : la méthode des débits classés et la courbe de tarage sédimentaire à deux segments. En combinant au mieux ces méthodes, le flux moyen interannuel entre 1965 et 2017 est estimé à 0,68 Mt/an avec un écart-type de 0,29 Mt/an. Cette valeur est cohérente avec le flux moyen mesuré par la station turbidimétrique entre 2012 et 2017 (0,63 Mt) et avec un suivi historique pour l'année 1890 (0,71 Mt) ; en revanche, elle est nettement inférieure à l'estimation précédemment proposée par l'étude globale Rhône (2000), comprise entre 1,0 et 3,6 Mt/an, qui inclut sans doute la suspension graduée de sables et limons grossiers, très mal connue. À partir de relevés bathymétriques et granulométrique des dépôts, nous avons estimé que la retenue de Verbois intercepte approximativement 0,1 Mt/an de particules grossières issues de l'Arve et environ la moitié des apports de MES.

An efficient representation of glacier dynamics in a semi-distributed hydrological model to bridge glacier and river catchment scales

Glacierised river catchments have been shown to be highly sensitive to climate change, while large populations depend on the water resources originating from them. Hydrological models are used to aid water resource management, yet their treatment of glacier processes is either rudimentary in large-scale applications or linked to fully distributed glacier models that prevent larger model domains. Also, data scarcity in mountainous catchments has hampered the implementation of physically based approaches over entire river catchments. A fully integrated glacier dynamics module was developed for the hydrological model SWIM (SWIM-G) that takes full account of the spatial heterogeneity of mountainous catchments but keeps in line with the semi-distributed disaggregation of the hydrological model. The glacierised part of the catchment is disaggregated into glaciological response units that are based on subbasin, elevation zone and aspect classes. They seamlessly integrate into the hydrological response units of the hydrological model. Robust and simple approaches to ice flow, avalanching, snow accumulation and metamorphism as well as glacier ablation under consideration of aspect, debris cover and sublimation are implemented in the model, balancing process complexity and data availability. The fully integrated model is also capable of simulating a range of other hydrological processes that are common for larger mountainous catchments such as reservoirs, irrigation agriculture and runoff from a diverse soil and vegetation cover. SWIM-G is initialised and calibrated to initial glacier hypsometry, glacier mass balance and river discharge. While the model is intended to be used in medium to large river basins with data-scarce and glacierised headwaters, it is here validated in the data-scarce catchment of the Upper Aksu River, Kyrgyzstan/NW China and in the relatively data-abundant catchment of the Upper Rhone River, Switzerland.

Organic matter isotopes reveal enhanced rainfall activity in Northwestern Mediterranean borderland during warm substages of the last 200 kyr

In this study we use bulk sediment organic matter stable isotopes (δ13Corg and δ15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (δ13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ∼3‰ values in δ15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represent 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel deposition.

The sediment budget and dynamics of a delta‐canyon‐lobe system over the Anthropocene timescale: The Rhone River delta, Lake Geneva (Switzerland/France)

AbstractDeltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep‐sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon‐levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year−1 (distal lobe) to 0·0737 m year−1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon‐lobe systems leading to important distal sediment accumulation zones.

Flood hazard assessment of the Rhône River revisited with reconstructed discharges from lake sediments

Abstract Accurate flood hazard assessments are crucial for adequate flood hazard mapping and hydraulic infrastructure design. The choice of an acceptable and cost-effective solution for such assessments depends upon the estimation of quantiles for different characteristics of floods, usually maximum discharges. However, gauge series usually have a limited time length and, thereby, quantile estimates associated to high return periods are subject to large uncertainties. To overcome this limitation, reconstructed flood series from historical, botanical or geological archives can be incorporated. In this study, we propose a novel approach that i) combines classic series of observations with paleodischarges (of the Rhone River) reconstructed from open lake sediments (Lake Bourget, Northwestern Alps, France) and ii) propagates uncertainties related to the reconstruction method during the estimation of extreme quantiles. A Bayesian approach is adopted in order to properly treat the non-systematic nature of the reconstructed flow data, as well as the uncertainties related to the reconstruction method. While this methodology has already been applied to reconstruct maximum discharges from historical documents, tree rings or fluvial sediments, similar applications need to be tested today on open lake sediments as they are one of the only archives that provide long and continuous paleoflood series. Reconstructed sediment volumes being subject to measurement errors, we evaluate and account for this uncertainty, along with the uncertainty related to the reconstruction method, the parametric uncertainty, and the rating-curve errors for systematic gauged flows by propagating these uncertainties through the modeling chain. Reconstructed maximum discharges appear to largely overcome values of observations, reaching values of approximately 2,600, 4,200, 2,450 and 2,500 m3/s in 1689, 1711, 1733 and 1737 respectively, which correspond to historically-known catastrophic floods. Extreme quantiles are estimated using direct measurements of maximum discharges (1853-2004) only and then combined to the sedimentary information (1650-2013). The comparison of the resulting estimates demonstrates the added value of the sedimentary information. In particular, the four historical catastrophic floods are very unlikely if only direct observations are considered for quantile estimations.

Using 3D modeling and remote sensing capabilities for a better understanding of spatio-temporal heterogeneities of phytoplankton abundance in large lakes

Lake biological parameters show important spatio-temporal heterogeneities. This is why explaining the spatial patchiness of phytoplankton abundance has been a recurrent ecological issue and is an essential prerequisite for objectively assessing, protecting and restoring freshwater ecosystems. The drivers of these heterogeneities can be identified by modeling their dynamics. This approach is useful for theoretical and applied limnology. In this study, a 3D hydrodynamic model of Lake Geneva (France/Switzerland) was created. It is based on the Delft3D suite software and includes the main tributary (Rhone River) and two-dimensional high-resolution meteorological forcing. It provides 3D maps of water temperature and current velocities with a 1 h time step on a 1 km horizontal grid size and with a vertical resolution of 1 m near the surface to 7 m at the bottom of the lake. The dynamics and the drivers of phytoplankton heterogeneities were assessed by combining the outputs of the model and chlorophyll-a concentration (Chl-a) data from MERIS satellite images between 2008 and 2012. Results highlight physical mechanisms responsible for the occurrence of seasonal hot-spots in phytoplankton abundance in the lake. At the beginning of spring, Chl-a heterogeneities are usually caused by an earlier onset of phytoplankton growth in the shallowest and more sheltered areas; spatial differences in the timing of phytoplankton growth can be explained by spatial variability in thermal stratification dynamics. In summer, transient and locally higher phytoplankton abundances are observed in relation to the impact of basin-scale upwelling.

Des flux d'eau aux flux de matières en suspension et de contaminants associés : gestion d'un réseau de stations hydro-sédimentaires sur le Rhône

La Base de Données pour les Observatoires en Hydrologie (BDOH) est un outil de bancarisation qui permet de combiner les mesures de débit, de matières en suspension (MES) et de contaminants particulaires issues d'un même réseau de stations pour calculer des chroniques de flux instantanés et cumulés. Cette application est utilisée dans le cadre de l'Observatoire des Sédiments du Rhône (OSR) pour estimer les flux de MES et de contaminants associés (Hg, PCB,...) en transit dans le Rhône et ses principaux affluents. Cependant, avant le calcul de ces chroniques, les données mères lacunaires doivent être reconstituées. Un test de sensibilité a été mené afin d'évaluer l'impact de différentes méthodes d'estimation des chroniques de concentrations en MES et en Hg particulaire sur les calculs de flux instantanés et cumulés dans le Rhône aux deux stations principales de l'OSR (Jons et Arles). Les résultats montrent que, quelle que soit la méthode d'estimation utilisée, les flux annuels de MES et de Hg particulaire ne diffèrent pas plus de 5 %. A l'échelle mensuelle, en revanche, les différentes méthodes génèrent des variabilités plus élevées (26-51 %). En perspective, ces premiers résultats seront complétés par l'évaluation des incertitudes quantitatives sur les calculs de flux particulaires.

Deltaic and Coastal Sediments as Recorders of Mediterranean Regional Climate and Human Impact Over the Past Three Millennia

AbstractDeltaic and shallow marine sediments represent unique natural archives to study the evolution of surface coastal ocean water properties as compared to environmental changes in adjacent continents. Sea surface temperatures (SSTs) and higher plant biomarker records were generated from the Rhone and Var River deltaic sediments (NW Mediterranean Sea), and three sites in the South Adriatic Sea (Central/Eastern Mediterranean Sea), spanning all or part of the past three millennia. Because of the high sediment accumulation rates at all core sites, we were able to produce time series at decadal time scale. SSTs in the Gulf of Lion and the convection area of the South Adriatic Sea indicate similar cold mean values (around 17 °C) and pronounced cold spells, reflecting strong wind‐driven surface water heat loss. However, they differ in the rate of postindustrial warming, which is steeper in the Gulf of Lion. The three Adriatic Sea SST records are notably different reflecting different hydrological influence from nearshore to open sea sites. The compositional features of higher plant n‐alkanes in the Rhone and Var delta sediments and inferred vegetation types show differences consistent with the latitudinal extension of the drainage basins of both river streams. In the Adriatic Sea, both coastal and open sea sediments indicate enhanced land‐derived material over the past 500 years, which is not seen in the NW Mediterranean record. We suggest that increased erosion as the result of changes in land use practices is the most likely cause for this trend.

Total and methyl-mercury seasonal particulate fluxes in the water column of a large lake (Lake Geneva, Switzerland)

Concentrations and fluxes of total and methylmercury were determined in surface sediments and associated with settling particles at two sites in Lake Geneva to evaluate the sources and dynamics of this toxic contaminant. Total mercury concentrations measured in settling particles were different throughout the seasons and were greatly influenced by the Rhone River particulate inputs. Total mercury concentrations closer to shore (NG2) ranged between 0.073 ± 0.001 and 0.27 ± 0.01μg/g, and between 0.038 ± 0.001 and 0.214 ± 0.008μg/g at a site deeper in the lake (NG3). Total mercury fluxes ranged between 0.144 ± 0.002 and 3.0 ± 0.1μg/m2/day at NG2, and between 0.102 ± 0.008 and 1.32 ± 0.08μg/m2/day at NG3. Combined results of concentrations and fluxes showed that total mercury concentrations in settling particles are related to the season and particle inputs from the Rhone River. Despite an observed decrease in total mercury fluxes from the coastal zone towards the open lake, NG3 (~ 3km from the shoreline) was still affected by the coastal boundary, as compared to distal sites at the center of the lake. Thus, sediment focusing is not efficient enough to redistribute contaminant inputs originating from the coastal zones, to the lake center. Methylmercury concentrations in settling particles largely exceeded the concentrations found in sediments, and their fluxes did not show significant differences with relation to the distance from shore. The methylmercury found associated with settling particles would be related to the lake's internal production rather than the effect of transport from sediment resuspension.

Morphological resilience to flow fluctuations of fine sediment deposits in bank lateral cavities

Abstract Lateral cavities are built in the banks of rivers for several purposes: to create harbors, to capture sediment, to keep a central navigable channel (i.e., Casiers de Girardon in the Rhone river) or to promote the formation of aquatic habitats if a limited amount of sediment is captured, providing hydraulic and morphologic diversity (i.e., the case of Japanese Wandos). This work is focused on this latter purpose: promotion of hydraulic and morphologic diversity. In these scenarios, an increase in the flow discharge in the main channel may, however, re-mobilize the deposit of sediment inside these lateral embayments and cause a sudden increase of the sediment concentration and turbidity in the main channel. It is thus of interest to characterize the resistance and resilience of these sedimentary deposits when the main channel is subjected to high flow or flushing events. Laboratory tests were carried out for five different normalized geometries of the cavities installed in the banks of an open channel and for five hydrographs with different levels of unsteadiness. Water depth, sediment deposit mass, sediment concentration and area covered by the settled sediments were recorded throughout each experiment. Although sediment deposits established at equilibrium before the flushing events are different depending on the geometry of the cavities, generally, they are recovered after being flushed by the high flow events. It is shown that the resistance and resilience of the sediment deposits are strongly dependent on the flow field and the mass exchange between the main channel and the cavities. This mass exchange is governed by the geometry of the cavities and the magnitude of the hydrographs applied.

Modeling of PCB trophic transfer in the Gulf of Lions; 3D coupled model application

3D coupled modeling approach is used for the PCB dispersion assessment in the Gulf of Lion and its transfer to zooplankton via biogeochemical processes. PCB budgets and fluxes between the different species of PCB: dissolved, particulate, biosorbed on plankton, assimilated by zooplankton, which are governed by different processes: adsorption/desorption, bacteria and plankton mortality, zooplankton excretion, grazing, mineralization, volatilization have been estimated. Model outputs were compared with the available in situ data.It was found that the Rhone River outflows play an important role in the organism contamination in the coastal zone, whereas the atmospheric depositions are rather more important in the offshore zones. The transfer of the available contaminant to bacteria and phytoplankton species is mainly related to the biomass present in the water column. Absorption fluxes (grazing) to zooplankton are rather higher than the passive sorption fluxes, which are themselves also linked to the sorption coefficient.

Temporal Variability of Sea Surface Temperature Patterns in the Gulf of Lions During Heavy Precipitation Events in The Cevennes-Vivarais Region

The Cévennes-Vivarais region in southern France frequently suffers from Heavy Precipitation Events (HPEs), especially during the autumn season. The northwestern Mediterranean Sea is a source of heat and moisture for these HPEs, with strong air sea exchanges, which are mainly controlled by the near-surface wind intensity and the Sea Surface Temperature (SST). The aim of this study is to characterize the SST structures, location and variability related to HPEs. Indeed, the Gulf of Lion is characterized by a cyclonic circulation with three main oceanic features: (1) the Northern Current (NC), (2) the Balearic Front (BF), (3) and the deep Convective Cell (CC). The MEDidterranean ReanalYsiS (MEDRYS1V2), an ocean reanalysis at 1/12˚ resolution was used over the 2000-2011 period to identify the NC, BF and CC oceanic feature's locations and for the calculation of an SST index. Then, an unsupervised cluster analysis method, using Principle Component Analysis for dimension reduction and the Silhouette for clustering, was used 20 in order to determine the most typical periods. The Local Moran's I (LMI) spatial statistical method, was used to highlight the areas of significant temporal variability of SST, considering the periods defined with the clustering method. For each period [early autumn (August-September), October and late autumn (November-December)], the LMI values, only considering the HPE initial stage, are extracted and compared to the averaged LMI values. The results highlight that in August-September, the Rhone river outflow have the most significant effect on SST variability on average, same as during HPEs. In October, for the HPEs initial stage, the largest variability seems to be related to the effects of the Mistral and Tramontane wind. In later autumn, there is a southward displacement of the BF and CC patterns and during the HPEs initial stage, the most significant SST variability is found near the center of the CC.

Neanderthal selective hunting of reindeer? The case study of Abri du Maras (south-eastern France)

Monospecific exploitation of reindeer by Neanderthals is a common behaviour in the Upper Pleistocene of Western Europe. However, reindeer-dominated assemblages have largely been reported from regions of northern Germany and southwestern France, with few examples noted in southeastern France, where faunal assemblages yield most of the time a variety of other large ungulates such as red deer, horse and diverse bovids. Here, we present multi-strand (bio-and eco-) archaeological datasets from the site of Abri du Maras (level 4.1), situated at the mouth of the Ardeche and Rhone rivers, a new example of a reindeer-dominated Neanderthal site in southeastern France. Dated to the beginning of the MIS 3, the zooarchaeological assemblage is dominated by reindeer (88% of the NISP, representing 16 individuals) but also includes horse, bison, giant deer (Megaloceros giganteus), red deer, ibex and lagomorphs. The combination of zooarchaeological, cementochronological and tooth microwear analyses evidence a single species-dominated spectrum, with catastrophic mortality and repeated autumnal deaths. This integrated approach provides an extensive picture of human subsistence behaviour, pointing to short-term hunting episodes of reindeer herds in an exceptional context of a quasi-exclusive Neanderthal accumulation. The high number of individuals and selective butchery may correspond with a cooperative and planned mass hunting strategy. The multidisciplinary approach undertaken here also incorporating paleontological, charcoal, ecological and isotopic analyses places the archaeological and zooarchaeological data within a broader regional palaeoenvironmental framework, providing valuable landscape-contextual information. The zooarchaeological data suggest a subsistence behaviour different from other Neanderthal reindeer-dominated assemblages often connected with specialised butchery or hunting sites.

Signatures of Late Pleistocene fluvial incision in an Alpine landscape

Uncertainty regarding the relative efficacy of fluvial and glacial erosion has hindered attempts to quantitatively analyse the Pleistocene evolution of alpine landscapes. Here we show that the morphology of major tributaries of the Rhone River, Switzerland, is consistent with that predicted for a landscape shaped primarily by multiple phases of fluvial incision following a period of intense glacial erosion after the mid-Pleistocene transition (∼0.7 Ma). This is despite major ice sheets reoccupying the region during cold intervals since the mid-Pleistocene. We use high-resolution LiDAR data to identify a series of convex reaches within the long-profiles of 18 tributary channels. We propose these reaches represent knickpoints, which developed as regional uplift raised tributary bedrock channels above the local fluvial baselevel during glacial intervals, and migrated upstream as the fluvial system was re-established during interglacial periods. Using a combination of integral long-profile analysis and stream-power modelling, we find that the locations of ∼80% of knickpoints in our study region are consistent with that predicted for a fluvial origin, while the mean residual error over ∼100 km of modelled channels is just 26.3 m. Breaks in cross-valley profiles project toward the elevation of former end-of-interglacial channel elevations, supporting our model results. Calculated long-term uplift rates are within ∼15% of present-day measurements, while modelled rates of bedrock incision range from ∼1 mm/yr for low gradient reaches between knickpoints to ∼6–10 mm/yr close to retreating knickpoints, typical of observed rates in alpine settings. Together, our results reveal approximately 800 m of regional uplift, river incision, and hillslope erosion in the lower half of each tributary catchment since 0.7 Ma.

A study of the Würm glaciation focused on the Valais region (Alps)

Abstract. During the Last Glacial Maximum (LGM), the glaciation in the European Alps reached maximum ice extent. We already simulated the steady states of the Alpine ice coverage for several climate drivers in Becker et al. (2016) and heighten in this article such studies for the Swiss Valais region. To this end, we employ the Parallel Ice Sheet Model (PISM), which combines the shallow ice approximation (SIA) with basal sliding elements of the shallow shelf approximation (SSA), and subject this model to various external driving mechanisms. We further test the sensitivity of this kind of the ice coverage in the Valais region to a temporally constant climate and to monotonic ice sheet build-up from inception to steady state as well as to the Dye 3 temperature driving during the past 120 000 years. We also test differences in the precipitation patterns exerted to the northern and southern catchment areas of the Rhone and Toce rivers to possible transfluence changes in ice from the northern to the southern catchment areas and vice versa. Moreover, we study the effect of the ice deformability and estimate the removal up to 1000 m of sediment in the Rhone Valley and study the removal of rock hindering the flow through the valley cross section at the knee of Martigny. All these studies took place because of a discrepancy in the ice height prediction of the modelled ice sheet with its geomorphologically reconstructed counterpart with proxy data obtained by Bini et al. (2009) as well as a difference in ice height between the two of up to 800 m. Unfortunately, all the scenarios in the model do not sufficiently reduce this discrepancy in the height prediction and the geomorphological reconstruction. The model results have discovered an ice dynamical discrepancy with the land map in Bini et al. (2009).

Tritium in river waters from French Mediterranean catchments: Background levels and variability

Tritium background levels in various environmental compartments are deeply needed in particular to assess radiological impact, especially in river systems where most of releases from nuclear facilities are performed. The present study aims to identify the main environmental factors that influence tritium background levels in rivers at the regional scale. 41 samples were collected from 2014 to 2016 along 17 small rivers in the south of France. All were located out of the influence of direct releases from nuclear facilities. Tritiated water (HTO) concentrations measured in water samples ranged from 0.12±0.11 to 0.86±0.15BqL−1 and HTO concentrations in rains were modelled between 2015 and 2016 over the study period referring to time series acquired from 1963 to 2014 at Thonon-les-Bains monitoring station. The results of tritium concentrations in rivers studied present a significant variability and are more than twice lower than forecasted values in rain. Multiple linear regressions allowed identifying that HTO concentration in rains, watershed area and altitude were the main tested parameters that are linked to the variability of HTO concentrations in the studied rivers. Finally, HTO fluxes delivered to the Mediterranean Sea by French coastal rivers out of influence of nuclear releases were estimated. The results highlight that those account for around 1% of HTO exported while 99% are transferred by the nuclearized Rhone River.

The environment of the last hunters-gatherers and first agro-pastoralists in the western Mediterranean region, between the Rhone and the Northern Apennines (7th - 6th millennium cal. BCE): Attractiveness of the landscape units and settlement patterns

In the north western Mediterranean, in the area between the Rhone River and the Northern Apennines, the last Mesolithic societies (Castelnovian) and the first Neolithic societies (Impressed Ware or Impressa) coexisted during the first half of the 6th millennium cal. BCE (Before Common Era). Linking the two settlement distribution patterns (mainly high lands and low lands for the Castelnovian versus Mediterranean coastal areas for the Impressa) to their specific environmental backgrounds during that period of coexistence enables us to document the attractiveness of the various available landscape units as a function of the subsistence practices (hunting, fishing and gathering versus agro-pastoralism). Pollen and charcoal data from 41 archaeological sites along with contemporaneous natural (off archaeological sites) sequences (hereafter referred to as “off-site sequences”) from three windows (Provence/Western Liguria, the middle Rhone valley/Prealps and Southern Alps, Eastern Liguria/Northern Apennines) were examined in order to reconstruct the vegetal landscape in the surroundings of the Mesolithic and Neolithic settlements between 6500 and 5400 cal. BCE. The importance of environmental versus cultural factors in the settlement preferences of both groups is discussed in order to document our reflection concerning non-consensual issues, such as the existence of interaction or avoidance behaviours or the sharing (or not) of parts or all of the territory and of its natural resources. The results notably highlight the expansion of fir forests that, based on ecological and accessibility criteria, could be considered as rather inauspicious for settlement and hunting as well as for pastoral activities. This expansion may have influenced the settlement patterns of both cultural complexes, leading populations to locate their settlements principally within landscape units that remained clear of extensive fir forests. It appears that, despite being motivated by the prevailing subsistence activities, the choice of an area of land for settlement is deeply guided by various other cultural factors which are less directly dependent upon natural resources. Thus, in an area providing a large range of possibilities, the landscape in which the groups establish themselves could be considered as just one of many cultural characteristics.

The environmental legacy of historic Pb-Zn-Ag-Au mining in river basins of the southern edge of the Massif Central (France).

The main rivers (Aude, Orb, Herault) that discharge into the Gulf of Lions and the west bank tributaries of the Rhone River including the Gardon have former non-ferrous metal mines in their upper drainage basin. Using unpublished data and data from the literature, this study provides an integrated overview of the contamination of water and sediment along the continent-sea continuum and of its impacts on the biota and on human health. In the upper part of these basins, water and stream sediments are enriched in metal(-loids) compared to median European concentrations. Arsenic is the main contaminant in the rivers Aude and Gardon d'Anduze, Sb in the Orb and Gardon d'Alès, and Tl in the Herault river. A rapid reduction in dissolved and particulate concentrations was systematically observed along the river due to dilution and precipitation. The high concentrations of metal(-loid)s observed suggest that the former mining activity still represents a potential threat for the environment, but the lack of high temporal resolution monitoring, especially during Mediterranean floods, prevents accurate assessment of metal fluxes from these rivers to the Mediterranean Sea. Studies dedicated to the impacts on human health are too rare, given that studies have shown a higher rate of arsenic-specific cancer near Salsigne mine in the Aude River basin and cases of saturnism in children in the upper Herault River basin. These studies underline the need to take environmental health issues into consideration not only in these watersheds but around the entire Mediterranean basin, which harbors numerous metalliferous ores that have been mined for millennia.

Occurrence of α, ω-dicarboxylic acids and ω-oxoacids in surface waters of the Rhone River and fluxes into the Mediterranean Sea

The Mediterranean Sea is a semi-enclosed marine environment surrounded by densely populated areas. This ecosystem is under strong anthropogenic pressure at present. Riverine waters are important input pathways of water-soluble organic compounds that potentially contribute to the dissolved organic carbon (DOC) pool. Here, we report the first ever measurements of bi-functional carboxylic acids such as α, ω -dicarboxylic acids (or diacids) and related polar compounds ω -oxoacids (or ketoacids) (BCAs=α, ω-dicarboxylic acids+ω -oxoacids), along with the DOC levels in Rhone River waters. Surface water samples were collected from February 2006 to June 2009 approximately 50km above the Rhone River mouth, which is the main supplier of freshwater to the Mediterranean Sea. The BCA concentrations averaged 32.4±15.3µgl−1, and exhibited a wide range of values from 13.2µgl−1 (Spring 2008) to 71.2µgl−1 (winter 2007). The contribution of carbon from BCAs to the DOC pool (BCA-C) accounted for 0.28 to 1.42% of the DOC. Although no seasonal trend was evident in the studied period, our results showed that the highest BCA concentrations did not always follow high water discharges. α,ω -dicarboxylic acids are the most abundant compound class (20.7±10.6µgl−1), which is followed by ω -oxoacids (11.7±6.2µgl−1). The Rhone River was estimated to deliver between 1.37 and 16.4 t d−1 of BCA to the Gulf of Lions. Assuming a mean water discharge of ∼1790m3s−1, a broad estimate suggests a loading of 750–4000 t yr−1 BCAs to the northwestern Mediterranean Sea. Our results indicated that glyoxylic acid (ωC2) was the most abundant BCA followed by oxalic acid (C2di) and fumaric acid (trans configuration of unsaturated C4 diacid). This result indicates that there is a different molecular distribution between the aquatic and atmospheric compartments, including rainwaters and aerosols, for which previous studies highlighted a predominance of oxalic acid, which was followed by malonic and/or succinic acid. Runoff and riverine biological process (rather than photochemical oxidation reactions) are hypothesized to control the BCA occurrence and molecular distribution in the Rhone River and thus their inputs to the coastal NW Mediterranean Sea, although the environmental implications of this stock of BCA are still unknown.