The Rhône River
Since the damming of the Nile, the Rhône River is the main freshwater and sediment supplier to the Mediterranean Sea, becoming an important forcing that can drive the evolution of the biogeochemistry of this marine ecosystem.
The Rhone River is 816 km long and has a drainage area of around 97,800 km2 with strong climatic and geological heterogeneity. At 47km from the sea, the river splits into two branches so-called the Grand Rhone and the petite Rhone carrying 90% and 10% of the total discharge, respectively (Ibanez et al., 1971). The Rhone River discharge takes place in the Gulf of Lions in the Northwestern Mediterranean Sea characterized by a large shelf of 12 000 Km2. (Durrieu de Madron et al., 2000), representing a wide, progressive margin with a crescent shape continental shelf (up to 70 km wide) for a volume of 9.1 1011 m3.
Freshwater loads, particulate matter and nutrients transported by the Rhone River were determined with high frequency sampling to evaluate seasonal and inter-annual variability as well as current contribution to the marine systems. The various fractions of carbon, nitrogen, and phosphorus (dissolved, particulate, inorganic and organic) were measured daily since March 2005, at Arles, 40 km upstream of the river mouth. A particular attention was focused on flood events, responsible for the major exports of matter towards the coastal environment.
Sampling device in the Rhône River
Suspended matter :
A precise volume of Rhone water (100 to 500 ml , according to the concentration of suspended particulate matter) is filtered on a preweighted GF/C glass fiber filters to collect suspended matter. After filtration, filters were immediately dried at 60°C. After drying filters were weighted to determine the suspended particulate matter concentration of (SPMC in mg.l-1). Uncertainty of SPMC is estimated to 0.05 mg.l-1.
Inorganic and organic nutrients :
An aliquot of filtrate from SPMC is stored in polyethylene flasks for inorganic nutrients analyses. Samples for ammonium and organic nutrients analyses were stored in 50 ml glass flasks.
Inorganic nutrients (nitrate, nitrite, phosphate, silicate) were analyzed using automated colorimetric method as described in Aminot and Kérouel (2004). Ammonium concentration was determined by using the fluorescent procedure of Holmes et al. (1990). Dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) were analyzed using the wet-oxidation procedure described in Raimbault et al. (1999). The dissolved organic fractions of nitrogen (DON) and phosphorus (DOP) are obtained by subtracting inorganic forms to TDN and TDP, respectively. Concentrations are expressed as mg.l-1. Annual loading are expressed as Ton.y-1
Then, filters were used for analysis of biogenic elements. Particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate phosphorus (PP) were determined by using the wet-oxidation procedure of Raimbault et al. (1999a). Unlike nitrogen compounds, a significant part of inorganic phosphorus can be bound to suspended material, making it difficult to distinguish from particulate organic phosphorus. Moutin et al.(1998) have shown that a great part of particulate phosphorus (17 to 41%) is in fact inorganic phosphorus bound to iron or calcium. Then, the part of particulate organic phosphorus can not be truly estimated in this work. Here, PP is the sum of particulate organic phosphorus and of inorganic bound-phosphorus.
The daily fluxes (Fd) are calculated from the daily concentrations (Cd) of the different variables listed above and the concomitant daily flow rates Qd.
Fd = Qd x Cd
The annual flux was calculated by summing the daily fluxes. During periods where the pump system was stopped, daily samples were lacking, and fluxes have been calculated using the mean value for two successively samples and the mean flow rate measured during the same period. The water discharge is measured routinely by the Compagnie Nationale du Rhône (CNR, htpp://cnr.tm.fr/fr).
Water samples were collected with the help of automatic samplers installed in an observatory station placed near the gauging station. Each daily sample was the sum of 16 subsamples (150 ml) collected every 90 minutes in polyethylene flasks containing 5 ml of HgCl2 for poisoning, providing an integrated daily samples. For flow rates higher than 3000 m3.s-1, frequency sampling were increased as often as possible to one sample every 4 samples (8 samples of 150 ml each 30 minutes).
Large quantities of suspended matter are collected twice a month and during each flood event using an ultracentrifuge CEPA-Z61.