Properties of fluorescent dissolved organic matter in the Gironde Estuary

Fifty-year sedimentary record of heavy metal pollution (Cd, Zn, Cu, Pb) in the Lot River reservoirs (France).

A cross-system comparison of organic carbon origin and behaviour in nine European estuaries is presented. The study sites display a very large range of hydrological and environmental conditions. The watershed of the respective estuaries were characterized by plotting the total organic carbon (TOC) in the rivers versus the inhabitants/discharge ratio. This allows to distinguish four types of watershed with regard to anthropogenic forcing and organic carbon levels: polluted by sewage inputs (Scheldt and to a much lesser extent, Ems, Sado and Thames), decontaminated (Elbe and Rhine), pristine (Gironde and Douro) and eutrophized (Loire and Scheldt). In the estuarine zone, dissolved organic carbon (DOC) almost always decreased linearly with increasing salinity. Exceptions were: the Scheldt, where a net consumption of sewage-derived DOC was observed, the Gironde, where a net production of DOC occurred in the maximum turbidity zone (MTZ) and the Sado and Ems, where DOC was supplied from large intertidal areas. By contrast, a large fraction of the riverine particulate organic carbon (POC) was mineralized in all the estuaries, except the Douro, where residence time of waters is only a few days. A fraction of POC appeared however refractory and accumulated in the MTZs, where terrestrial soil-derived material dominates (Elbe, Ems, Loire, Gironde and Sado). In the marine regions of most estuaries, autochthonous POC was present during spring and summer. The analysis of all river and estuarine data allows estimation of the loss of continental POC occurring in each estuary. It decreases in the following order: Scheldt⪢Thames>Ems=Sado=Loire>Gironde>Elbe>Rhine>Douro, which almost corresponds to the anthropogenic pressure in the respective watersheds. Two major variables appear to control the intensity of this mineralization: the origin of the POC, the lability increasing with pollution, and the residence time of particles in the estuarine zone.

Behaviour of Organic Carbon in Nine Contrasting European Estuaries

The daily variability of river suspended sediment concentration (Cs) and related yield (Y) is studied at 60 global stations. The data set covers natural conditions (e.g. pre-reservoir data), ranging from the humid tropics to subarctic and arid regions, located in all types of relief (yearly runoff q* from 0.1 to 55 l s � 1 km � 2 ). Basin area ranges from 64 km 2 to 3.2 million km 2 . Survey lengths range from 1 to 20 years with a median of 3 years. Median values (Cs50, q50, Y50) and discharge-weighted averages for Cs* and Y* range from 5 to 29000 mg l � 1 and 10 to 5000 kg km � 2 day � 1 , respectively. A set of indicators of variability are proposed for sediment concentration, water and sediment discharges including mean to median ratios (Cs*/Cs50, Y*/Y50), the percentage of sediment flux discharged in 2% of time (Ms2), the percentage of time necessary to carry half of the sediment flux (Ts50), and quantiles of Cs, q and Y distributions corresponding to the discharge-weighted averages. Since most of the sediment flux is discharged in less than 25% of the time, ‘‘truncated rating curves’’ metrics are proposed between the Cs vs. q relationship for periods of high flux. Temporal variability decreases with increasing basin size, lake abundance, and is higher for basins influenced by glaciermelt and snowmelt. The least variable sediment flux regimes are noted for the Mississippi at its mouth, the Rhone Lacustre, the St. Lawrence and the Somme, a medium-sized French phreatic river. The most variable flux regimes were for small- to mediumsized basins (i.e. <1000 to 10000 km 2 ) such as steep Andean Bolivian basins, Thai basins, the Eel (CA) and Walla Walla (OR) rivers. A proposed global scale typology is based on six classes key variability indicators. D 2003 Elsevier Science B.V. All rights reserved.

Global variability of daily total suspended solids and their fluxes in rivers

The study of source‐to‐sink systems relates long‐term variations in sediment flux to morphogenic evolution of erosional–depositional systems. These variations are caused by an intricate combination of autogenic and allogenic forcing mechanisms that operate on multiple time scales – from individual transport events to large‐scale filling of basins. In order to achieve a better understanding of how these mechanisms influence morphological characteristics on different scales, 29 submodern source‐to‐sink systems have been investigated. The study is based on measurements of morphological parameters from catchments, shelves and slopes derived from a ∼1 km global digital elevation model dataset, in combination with data on basin floor fans, sediment supply, water discharge and deposition rates derived from published literature. By comparing various morphological and sedimentological parameters within and between individual systems, a number of relationships governing system evolution and behaviour are identified. The results suggest that the amount of low‐gradient floodplain area and river channel gradient are good indicators for catchment storage potential. Catchment area and river channel length is also related to shelf area and shelf width, respectively. Similarly to the floodplain area, these parameters are important for long‐term storage of sediment on the shelf platform. Additionally, the basin floor fan area is correlative to the long‐term deposition rate and the slope length. The slope length thus proves to be a useful parameter linking proximal and distal segments in source‐to‐sink systems. The relationships observed in this study provide insight into segment scale development of source‐to‐sink systems, and an understanding of these relationships in modern systems may result in improved knowledge on internal and external development of source‐to‐sink systems over geological time scales. They also allow for the development of a set of semi‐quantitative guidelines that can be used to predict similar relationships in other systems where data from individual system segments are missing or lacking.

Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systems

Ten-year observation of the Gironde tributary fluvial system: fluxes of suspended matter, particulate organic carbon and cadmium

Using a databank, grouping results on daily measurements of Total Suspended Matter (TSM) and Particulate Organic Carbon (POC) on the lower Garonne river at La Reole (upstream limit of the dynamic tide) over the last 20 years, we studied seasonal variations in particulate organic carbon concentrations and fluxes, using TSM concentration classes. The results show seasonal variations in POC concentrations and fluxes, and especially a strong impact of flood events. A qualitative and quantitative hypothesis on the differentiation of POC origin is suggested. On every time scale (daily, seasonally or yearly), this hypothesis allows the estimation of POC inputs from the three different pools: soil, litter and autochthonous production. The Garonne River exports 78 000 tons of POC annually, 54% from soil (0.8 t km-2 yr-1), 38% from litter (about 0.55 t km-1 yr-1) (km of permanent stream) and 8% from autochthonous production. Most of this material is probably mineralized in the Gironde estuary.

Seasonal variation and origin of Particulate Organic Carbon in the lower Garonne River at La Reole (southwestern France)

The aim of this study is to determine, by applying a direct method, dissolved inorganic nitrogen (DIN) inputs which reach a coastal lagoon: the Arcachon Bay (France). Yearly monitoring of the different sources (groundwater, surficial water and atmospheric water) allows the annual DIN discharge and the variations of DIN discharges throughout the year to be quantified. The Arcachon Bay received 870 t-N in the year 1996. Study of the two main hydrologic seasons (high and low water-table periods) has shown that 78% of this annual discharge occurs during the wet period (from December to April). River DIN inputs account for more than 90% of the total annual discharge and during the wet period they account for 95% of total DIN inputs. Rivers are therefore the most important source of DIN for the Arcachon Bay and thus the principal factor in the Bay's eutrophication. The reasons for this are an important water discharge (more than 74% of the total freshwater discharge) and relatively high DIN concentrations (95 μM on average) in comparison with groundwater and atmospheric water (33 and 43 μM, respectively). As for the atmospheric source, it accounts for 9% of the annual DIN discharge with wide variations throughout the year since it accounts for 5% in the wet period and 26% in the dry period (May to November). This fluctuation is due to a rise in the DIN atmospheric flux which increases from 32 to 49 t-N. Further, this study proves that, nowadays, the groundwater which flows into the Arcachon Bay is not DIN contaminated. Combined with the values mentioned above, the low groundwater discharge (less than 3% of the total annual discharge) yields a slight contribution (less than 1%).

Study of Annual and Seasonal Dissolved Inorganic Nitrogen Inputs into the Arcachon Lagoon, Atlantic Coast (France)

The specific yield of the Nivelle River (238 km2) was estimated in the range of 75 t·km−2·yr−1 in 1996 and 1997 with a high-frequency monitoring. This reliable result allowed us to assess the contribution of small mountainous rivers to the sediment fluxes into the Bay of Biscay: 1.9 Mt·yr−1, i.e. 52 % of the total input (3.7 Mt·yr−1). Moreover, whereas the Gironde River has a larger contributive area (75 000 km2), its suspended sediment input into the ocean is only 42 % of the total flux (1.5 Mt·yr−1). Therefore, we emphasize the importance of small mountainous river TSM inputs in sedimentological and biogeochemical processes occurring on the continental margins.

Assessment of suspended matter input into the oceans by small mountainous coastal rivers: the case of the Bay of Biscay

We report here the first data set on wet deposition of heavy metals in the southwestern French coastal zone. In this region, there are two major sensitive coastal ecosystems: the Gironde Estuary and the Arcachon Lagoon. Chemical analyses of heavy metals were carried out by ICP-MS. Annual mean concentrations of the dissolved fraction in precipitation were 0.2, 3.4, 4.3, 8.1 and 30 μg L-1for Cd, Ni, Cu, Pb and Zn, respectively. In terms of annual fluxes, these numbers are of the same order of magnitude as the fluxes measured in southeastern France, but are higher than those measured in western Brittany. When extrapolated to the entire Bay of Biscay, the annual wet dissolved fluxes of Cd, Ni, Cu, Pb and Zn are respectively 7, 110, 140 340 and 1440 t yr-1. According to available data in the literature, the regional Cd, Cu, Pb and Zn atmospheric fluxes for the Bay of Biscay are of the same order of magnitude as riverine inputs (Loire and Gironde). On a daily or weekly time scale, we observed a strong variability of elemental fluxes: up to 20% of the annual dissolved flux may occur in a rain event shorter than 3.5 days. Although elements display generally parallel variations with time, they sometimes follow independent behaviours (e.g. Pb and Cd), suggesting that they may derive from different geographical and/or pollution sources.

E. Maneux

Papers

Ten-year observation of the Gironde tributary fluvial system: fluxes of suspended matter, particulate organic carbon and cadmium

Seasonal variation and origin of Particulate Organic Carbon in the lower Garonne River at La Reole (southwestern France)

Study of Annual and Seasonal Dissolved Inorganic Nitrogen Inputs into the Arcachon Lagoon, Atlantic Coast (France)

Assessment of suspended matter input into the oceans by small mountainous coastal rivers: the case of the Bay of Biscay

Temporal Patterns of the Wet Deposition of Zn, Cu, Ni, Cd and Pb: The Arcachon Lagoon (France)