Variability and evolution of suspended particulate matter inputs in the coastal zone : multi-scale approach in the Mediterranean basin

Les fleuves assurent le transport des matières particulaires vers la mer, introduisant ainsi du sable, des nutriments et des contaminants. Les fleuves méditerranéens sont de part leurs caractéristiques (relief accidenté, climat contrasté) naturellement riches en matières en suspension (MES), mais leurs flux sont aujourd’hui largement modifiés par la construction de barrages. Cette étude vise à comprendre la variabilité et l’évolution des apports fluviaux en MES en Méditerranée par une approche multi-échelle : (1) Dans le Golfe du Lion, les flux de MES (et nutriments associés) ont été estimés à long terme grâce à la nouvelle méthode SiRCA (Simplified Rating Curve Approach). La lithologie est le contrôle principal pour la variabilité spatiale de ces flux, ce qui attribue au Rhône le rôle dominant dans les apports totaux (95%). (2) Dans le bassin méditerranéen, une quantification complète des apports par l’ensemble des fleuves a été établie (modélisation empirique) dont 35% de MES sont aujourd’hui retenus dans les grands barrages (GB). (3) Dans le bassin du Maghreb, la retenue sédimentaire par les petits barrages a pu être estimée égale à celle des GB.Rivers ensure the transport of suspended particulate matter (SPM) to the Sea, introducing sand, nutrients and contaminants. According to their drainange basin characteristics (mountainous relief, contrasting climates), Mediterranean rivers are naturally rich in SPM, but their fluxes are largely altered today because of damming. This study aims to understand the variability and evolution of the riverine SPM inputs to the Mediterranean sea by a multiscale approach. (1) In the Gulf of Lion, total SPM fluxes (and associated nutrients) were estimated on the long term through the development of a new method SiRCA (Simplified Rating Curve Approach). The lithology is the main control for the spatial variability of these fluxes, which attributes to the Rhône River the dominant role in the overall fluxes (95%). (2) In the Mediterranean basin, empirical modeling could be established which allows for the first time a complete quantification of the inputs by all the rivers in this region (997 Mt/an). It also confirms the importance of lithology among the dominant controlling factors. By combining this model with a database of large dams in this region, it has been calculated that about 35% of SPM are today retained behind dams. (3) In the Maghreb basin, finally the role of sediment retention by small dams and hillside reservoirs has been studied. A new database on these reservoirs was created and combined with the database on large dams. This study demonstrates that small dams probably retain the same amount of sediments that large dams

Depth of the pedological profile as a conditioning factor of soil erodibility (RUSLE K-Factor) in Ecuadorian basins

International audienceSoil erodibility (K-Factor) is one of the fundamental parameters to estimate its rainfall erosion through mathematical models such as RUSLE. Carrying out an erodibility analysis at different pedological depths allows identifying what would be its susceptibility to erosion processes. Soil unit parcel data obtained by long-term field measurements are required, ensuring that the analyzed sections remain uncovered throughout observation period, investing large amounts of time and money. However, the lack of a good and extensive field database is the main limitation to apply this methodology. The objective of this work was to analyze the spatial distribution of soil erodibility in different pedological profiles, through the implementation of satellite data of soil characteristics. The methodology consisted in delimiting and analyzing the environmental characteristics of the Ecuadorian basins, obtaining the clay, silt, sand SOC contents of the analyzed depths, determining the K-Factor values and comparing them with environmental layers. Basins delimitation and environmental characteristics were extracted from regional literature; soil layer contents were obtained from SoilGrids; K-Factor calculation was made from soil characteristics using Software R and QGIS; results comparison against the elevation and land cover parameters were carried out using QGIS. The results allowed to identify very small variations between pedological profiles; determine that clay and silt are the most incident elements of K-Factor; identify that Crop and Grass are coverages that concentrate on the highest values of K-Factor as well as the highest areas. This allows the administrators of the territory to generate measures to reduce soil loss

Controls, budgets and variability of riverine sediment fluxes to the Gulf of Lions (NW Mediterranean Sea)

International audienceThe present study investigates the spatio-temporal variability of riverine sediment fluxes to the Gulf of Lions, one of the most extensive shelf regions in the Mediterranean Sea. Small coastal rivers compete here with the Rhone River, nowadays the largest Mediterranean river in terms of water discharge. Our scientific objectives were to investigate the major controls of riverine sediment yields (SY) in this area and to quantify the role of the small coastal rivers, largely ignored in previous studies, in the total sediment budgets. Another objective concerned the source identification of the Rhone sediments with regard to the major tributary contributions, and to test whether the sediment fluxes are in equilibrium in the basin. For the calculation of representative long-term fluxes, we used a Simplified Rating Curve Approach (SiRCA) which could be validated by high resolution monitoring and existing literature data. An overall of 13 drainage basins could be distinguished, covering about 86% of the study area (6 coastal rivers, the Rhone River, and 6 of its tributaries). Representative SY range from 19 to 151 t km−2 yr−1 in the investigated drainage basins. Despite their smaller basin areas and more torrential discharge regimes, SY of the coastal rivers were generally lower compared to SY of the Rhone River and its tributaries. Confrontation with the lithological, morphological and hydroclimatic basin characteristics indicate that lithology exerts the dominant control on SY in the study region. In particular, the existence of erodible sedimentary rocks in the headwater regions yields high SY. Peak values of 135 and 151 t km−2 yr−1 were observed for the Isere and Durance tributaries of the Rhone River, where badlands exist. The coastal rivers contribute on average only to slightly more than 5% of the long-term sediment inputs to the Gulf of Lions. During individual years however, their contribution can strongly increase (up to 27% in 2011). Their contribution is generally important during late spring and autumn, when flash-floods are frequent (up to 90% of the monthly sediment discharge). Summing the various tributary contributions of the Rhone River produces a sediment budget that is close to the sediment export at the river mouth. This indicates that at spatial scales, the average sediment fluxes are equilibrated in the basin. About 40% of the Rhone sediments originate from the Isere and Durance tributaries. However, fully closure of the budgets requires high SY in the lowermost basin parts. Omitting in our calculations only a few years with exceptional flooding considerably reduce these values. Trend analyses show at the same time that the sediment fluxes from the Saone and Isere tributaries decreased during the study period. At temporal scales, sediment transport from upstream to downstream was therefore not in equilibrium and the remobilisation of older sediments from the downstream basin parts may have been important

The impact of reservoir construction on riverine sediment and carbon fluxes to the Mediterranean Sea

18 pages, 8 figures, 4 tablesRiverine suspended particulate matter (SPM) is an important vector for the transfer of nutrients and contaminants to the Sea. Mediterranean rivers are naturally rich in SPM, but their fluxes have been largely altered in the past decades because of damming. Based on empirical modeling, combining literature estimates on SPM fluxes with the environmental characteristics of the corresponding drainage basins, we present for the first time a spatially explicit quantification of the natural fluxes at the level of the Mediterranean Sea and its different sub-basins. These fluxes are further combined with the evolution of major reservoir constructions during the last century in order to quantify the sediment retention by dams. Our results confirm that natural riverine sediment fluxes generally result from a combination of hydroclimatic, geomorphologic and land use parameters in the corresponding drainage basin. Among these factors, the combination of lithology and land elevation appears to be a key factor in the control of mechanical erosion rates. According to our model, the natural flux of SPM by all Mediterranean rivers cumulates to about 997 Mt yr, which corresponds to a specific SPM flux of 211 t km yr. Besides the Nile River, which holds a particular place among the Mediterranean rivers, coastal rivers (<5000 km) are particularly important for the overall SPM transport. Although they only occupy 38% of the Mediterranean drainage basin area, they export about 58% of the natural SPM flux. We further quantified the potential impact of damming on these fluxes. Using the dams which are geolocalized in the global GRandD data base, and combining them with a high resolution (5′) river routing scheme developed in this study, we find that after 2003, about 35% of the river sediments are retained by dams (26% without the Nile). These values are clearly minimum values, since GRandD is far from being complete in the Mediterranean drainage basin. According to our simulations, about 22% of the storage capacity is currently lost in these dams, and about 30% of them have a life expectancy that does not exceed 200 years. We finally also combined our modeling with a global model for the fluxes of particulate organic carbon (POC), representative for particulate organic nutrients in general. Although POC is non-linearly coupled to the fluxes of SPM, our results show that the retention of POC in dams is almost identical to the retention of sedimentsThis work was supported by the National Centre of Scientific Research (Centre national de la recherche scientifique, CNRS) through attribution of a PhD grant to MS, and by the LOICZ international project and MERMEX project under the MISTRALS programPeer Reviewe

Natural oil seep systems in the Aegean Sea

International audienceThe analysis of satellite-based Synthetic Aperture Radar Sentinel-1 data reveals the presence of an oil seepage system composed of 33 recurrent oil seeps sites in the Aegean Sea. The temporal analysis of this system shows that most oil seeps are detected during the summer period, probably due to better detection conditions during this period. The estimations of expulsion frequency computed using the summer period show that the frequency is highly variable from one seep to another with occurrence rates that reach 40% for the most active sites. Oil seeps gather among two main provinces. First, in the Limnos and Saros pull-apart basins where oil seeps are aligned along the North Anatolian Fault (NAF), which suggest that oil seep locations are mostly driven by the fault network. In the Limnos basin, detected oil seeps also coincide with the location of a gravity-driven sedimentary destabilisation complex that developed under the influence of the Limnos pull-apart basin subsidence and the transtensive strain of the NAF. Finally, oil seeps detected in the proximity of the Skyros-Edremit region seems to follow a different control considering that oil seeps appear disconnected from the main fault in the area (Skyros-Edremit)

Unravelling Climate and Anthropogenic Forcings on the Evolution of Surface Water Resources in Southern France

In the Mediterranean, climate change and human pressures are expected to significantly impact the availability of surface water resources. In order to quantify these impacts during the last 60 years (1959–2018), we examined the hydro-climatic and land use change evolution in six coastal river basins of the Gulf of Lion in southern France. By combining observed water discharge, gridded climate, mapped land use and agricultural censuses data, we propose a statistical regression model which successfully reproduces the variability of annual water discharge in all basins. Our results clearly demonstrate that, despite important anthropogenic water withdrawals for irrigation, climate change is the major driver for the detected reduction of water discharge. The model can explain 78–88% of the variability of annual water discharge in the study catchments. It requires only two climatic indices that are solely computed from monthly temperature (T) and precipitation (P) data, thus allowing the estimation of the respective contributions of both parameters in the detected changes. According to our results, the study region experienced on average a warming trend of 1.6 °C during the last 60 years which alone was responsible for a reduction of almost 25% of surface water resources

Unravelling Climate and Anthropogenic Forcings on the Evolution of Surface Water Resources in Southern France

In the Mediterranean, climate change and human pressures are expected to significantly impact the availability of surface water resources. In order to quantify these impacts during the last 60 years (1959&ndash;2018), we examined the hydro-climatic and land use change evolution in six coastal river basins of the Gulf of Lion in southern France. By combining observed water discharge, gridded climate, mapped land use and agricultural censuses data, we propose a statistical regression model which successfully reproduces the variability of annual water discharge in all basins. Our results clearly demonstrate that, despite important anthropogenic water withdrawals for irrigation, climate change is the major driver for the detected reduction of water discharge. The model can explain 78&ndash;88% of the variability of annual water discharge in the study catchments. It requires only two climatic indices that are solely computed from monthly temperature (T) and precipitation (P) data, thus allowing the estimation of the respective contributions of both parameters in the detected changes. According to our results, the study region experienced on average a warming trend of 1.6 &deg;C during the last 60 years which alone was responsible for a reduction of almost 25% of surface water resources

Anthropogenic Reservoirs of Various Sizes Trap Most of the Sediment in the Mediterranean Maghreb Basin

International audienceThe purpose of this study is to obtain a spatially explicit assessment of the impact of reservoirs on natural river sediment fluxes to the sea in the Mediterranean Maghreb Basin (MMB), a region where both mechanical erosion rates and the anthropogenic pressure on surface water resources are high. We combined modeling of riverine sediment yields (sediment fluxes divided by the drainage basin area) and water drainage intensities in a 5' × 5' grid point resolution (~10 km × 10 km) with a compilation of existing reservoirs in the area, and calculated sediment trapping based on average water residence time in these reservoirs. A total number of 670 reservoirs could be assembled from various sources (including digitization from Google maps), comprising large-scale, small-scale and hillside reservoirs. 450 of them could be implemented in our modeling approach. Our results confirm that natural sediment yields are clearly above the world average, with the greatest values for Morocco (506 t km −2 year −1), followed by Algeria (328 t km −2 year −1) and by Tunisia (250 t km −2 year −1). Including dams in the downstream routing of suspended sediments to the sea reduces the natural sediment flux in the entire MMB to the sea from 96 to 36 Mt km −1 year −1 , which corresponds to an average sediment retention of 62%. Trapping rates are highest in the Tunisian basin part, with about 72%, followed by the Algerian (63%) and the Moroccan basin parts (55%). Small reservoirs and hillside reservoirs are quantitatively important in the interception of these sediments compared to large reservoirs. If we only considered the dams included in the widely used Global Reservoir and Dam (GRanD) database which comprises mainly large reservoirs sediment retention behind dams would account for 36% of the natural suspended particulate matter (SPM) flux to the Mediterranean Sea. Our data reveal negative correlation between sediment retention and natural erosion rates among the three Maghreb countries, which can be explained by the greater difficulties to build dams in steep terrains where natural sediment yields are high. Although the lowest sediment retention rates are found in the Moroccan part of the MMB, it is probably here where riverine sediment starvation has the greatest impacts on coastline dynamics. Understanding the impact of dams and related water infrastructures on riverine sediment dynamics is key in arid zones such as the MMB, where global warming is predicted to cause important changes in the climatic conditions and the water availability

Beachrocks of the last low sea level, substrate of the Great Amazon Reef system along the outer Guiana shelf

The Great Amazon Reef System is a living biogenic mesophotic reef ecosystem that has been recently described along the shelf break of Brazil. An oceanographic cruise was carried out in 2019 along the outer edge of the French Guiana Shelf. A side-scan sonar survey was conducted to locate reef outcrops and allowed twelve in situ 80- to 120-m depth dives and sampling of the reef rocks and peripheral sands. The majority of the hard rocks are composed of biological concretions. However, several fragments revealed the inside presence of sandstone clasts. These clasts, more or less enveloped by biogenic coatings, probably represent destroyed clasts of underlying or neighboring beachrock banks. Their dominant cement is micritic (high-magnesian calcite); the intergranular or extragranular porous field was later filled with low-magnesian sparry calcite. The sand or gravel that accumulated near the barrier mainly consists of the blunt debris of coastal fauna and flora associated with different carbonate or ferruginous neoformed ooids. At 104-m depth, ooids extracted from dive 11 samples dated from the start of MIS2 (27,370 cal yr BP) and attest to the presence of a significant coastal accumulation. At this same site, cementing did not take place until about 3500 years later (23,990 cal yr BP). The cement of a nearby beachrock indicates a much more recent age (16,170 cal yr BP). Lastly, the age of 4100 yr BP measured on the barnacles attached to the top of the reef attests to the late Holocene reef's biological activity

The Mediterranean Region as a Paradigm of the Global Decoupling of N and P Between Soils and Freshwaters

The global socio-economic and agricultural expansion is accompanied by large inputs of nitrogen (N) and phosphorus (P) on land and by a serious alteration of the water cycle and water quality. The Mediterranean basin represents a paradigmatic region to study the entangled nutrient and water challenges because the region, where many of the world's climatic and socio-economic gradients are present, is predicted to suffer severe water stress in the coming decades yet at the same time agricultural intensification and population are increasing in many rim countries. We here describe the biogeochemical budgets of N and P in 549 river basins for the 2000–2009 period, analyzing how the climatic gradient and water management practices affect the fluxes of N and P and their stoichiometric ratios. Average land inputs are 3,600 kg N km−2 yr−1 and 470 kg P km−2 yr−1, with a significant variation between basins (>100 times) closely related to the stage of agricultural intensification. Moreover, the combination of aridity and water regulation can strongly alter the final balances, not only by changing the export of nutrients by rivers (riverine export is ca. 10% for N and 8% for P in arid basins), but also decoupling the N:P ratios between terrestrial and freshwater compartments