Origine et devenir du phosphore dans le continuum aquatique de la Seine des petits bassins amont à l'Estuaire (rôle du phosphore échangeable sur l'eutrophisation)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Origin and fate of phosphorus in the Seine watershed (France): Agricultural and hydrographic P budgets

International audiencePhosphorus (P) sources (point and diffuse) in the human-impacted Seine basin (64,840 km2) were evaluated for the year 2000. An agricultural P budget showed that fertilizers represented 59% (20–25 kgP ha−1) of P inputs to the soils. A P surplus (2.5 kg P ha−1 y−1) contributed to P enrichment of the agricultural soils whose stocks amounted to 1800–5000 kgP ha−1. A hydrographic P budget showed that runoff (0.39–0.51 kgP ha−1) dominated the diffuse sources. These losses represented a very low percentage (0.01%) of the P stocks in soils and contributed to 19–25% of the total P inputs to the drainage network. Point sources remained the main source of P (75–81%), particularly in the downstream urbanized zone. Phosphorus retention in the river drainage network accounted for 9–15% of the total P inputs, indicating that it must not be ignored in large river P budget calculations. The Seine basin exported 8000 tP y−1 (44% as particulate P) to its estuary. The annual mean particulate P in suspended sediment at the outlet (2.9 gP kg−1) was fourfold greater than in headwaters and in rural zones. The similar increase of the particulate inorganic P/particulate organic P ratio in suspended sediment along the river continuum clearly indicated the increasing pressure of point sources. The close relation between P content of suspended sediment during the high-flow period and the P content of agricultural soils resulted in proposing a novel method to calculate the PP losses from runoff

Typical features of particulate phosphorus in the Seine estuary (France)

International audienceDuring the 2001–2002 hydrological cycle, 8,000 tons P year−1 (44% as particulate phosphorus) originating from the Seine basin entered the Seine estuary. P content in suspended sediments (SS) is 2.9 g P kg−1 (80% as inorganic form) at Poses (the upstream limit of the Seine estuary). During the transfer from Poses to Caudebec (the limit of saline water intrusion), Particulate Inorganic Phosphorus (PIP) in SS decreases by 40% whereas Particulate Organic Phosphorus (POP) remains stable. This decrease is explained as the result of (i) a dilution by SS poorer in P, originating from storage zones (mudflats) within the estuary, and (ii) a loss of P by sedimentation, especially in the Rouen harbour where 15% to 20% of SS are yearly trapped and extracted. Downstream, in the turbidity maximum of the estuary, P content in SS is twice as low as at Poses (1.5 g P kg−1, 70% as inorganic form). PIP content is fairly homogeneous both vertically and throughout an annual survey. On the other hand, POP varies by season. Higher POP content is observed during vernal period with phytoplankton accumulation. In a salinity gradient from 0 to 30 (PSS78), PIP content further decreases by 30%. POP does not vary much in this gradient, leading to the assumption that PIP is submitted to desorption in response to the dilution by marine waters (poor in orthophosphates). Using 32P isotopic method, we establish a mathematical formulation of P exchangeable between the suspended solid and water phases of the Seine estuary. Particulate P in the turbidity maximum zone is shown to represent a possible source of dissolved P, available for algal growth in the Seine Bight. Instead of playing a role of nutrient retention, the turbidity maximum zone of the Seine estuary could favour coastal eutrophication

Phosphorus budget in the Marne Watershed (France): urban vs. diffuse sources, dissolved vs. particulate forms

International audienceWe evaluated the P sources (point, diffuse), through a nested watershed approach investigating the Blaise (607 km2), dominated by livestock farming, the Grand Morin (1202 km2), dominated by crop farming, and the Marne (12,762 km2), influenced by both agriculture and urbanization. Fertilizers account for the main P inputs (>60%) to the soils. An agricultural P surplus (0.5–8 kg P ha–1 year–1) contributes to P enrichment of the soil. The downstream urbanized zone is dominated by point sources (60%, mainly in dissolved forms), whereas in the upstream basin diffuse sources dominate (60%, mostly particulate). Among the diffuse sources (losses by forests, drainage and runoff), losses by runoff clearly dominate (>90%). P retention in the alluvial plain and the reservoir represents 15–30% of the total P inputs. Dissolved and particulate P fluxes at the outlet of the Marne are similar (340 and 319 tons of P year–1, respectively). The Blaise sub-basin receives P from point and diffuse sources in equal proportions, and retention is negligible. The Grand-Morin sub-basin, influenced by the urbanized zone receives, as does to the whole Marne basin, 60% of P inputs as point sources. The total particulate phosphorus in suspended sediments averaged 1.28 g P kg–1, of which about 60% are inorganic and 40% organic P. Particulate phosphorus exchangeable in 1 week and 1 year (32P isotopic method) accounts for between almost 26% and 54% of the particulate inorganic phosphorus in the suspended sediment and might represent an important source of dissolved P, possibly directly assimilated by the vegetation

Processus fondamentaux conditionnant les apports de sédiments fins dans les retenues - optimisation des méthodes de mesure et modélisation statistique

Le transport de Matières En Suspension (MES) dans les cours d eau est le résultat des processus d érosion des sols et de transport des particules à l échelle du bassin versant. La dynamique des flux de MES est étudiée dans un contexte alpin, pour deux bassins versants méditerranéens (l Asse et la Bléone), deux bassins de haute montagne (le Ferrand et la Romanche, dans le massif de l Oisans, Alpes françaises) et un bassin complexe, l Isère à Grenoble. Les flux de MES sont calculés à partir des chroniques de débit et de concentration, reconstituées à partir des enregistrements haute fréquence de turbidité. Ces enregistrements permettent de mettre en évidence les processus hydrométéorologiques responsables des apports de MES dans le système fluvial. La variabilité temporelle des flux de MES dans les cours d eau est mise en évidence et décrite à différentes échelles temporelles (journalière, saisonnière ou annuelle) et le régime de transport de MES des bassins versants est caractérisé à l aide d indicateurs statistiques traduisant la dynamique des flux de MES à l échelle du bassin versant. Un modèle conceptuel à réservoir permettant de reproduire les flux journaliers de MES est présenté. Après calage et validation du modèle sur les chroniques mesurées, il est utilisé afin de reconstituer de longues chroniques réalistes de flux de MES non mesurées, à partir de données de débit et de pluie historiques. Plusieurs scénarios valables sont établis pour chaque bassin versant.Suspended Particle Matter (SPM) transport in rivers results from soil erosion processes and particles transport through the watershed. SPM dynamics is analyzed in an alpine context over two Mediterranean watersheds (the Asse and the Bléone), two mountainous watersheds characterized by snow-melt regime (the Ferrand and the Romanche, French Alps) and a larger rainfall/snow-melt regime watershed, the Isère at Grenoble. High frequency turbidity records are converted to SPM chronicles using a relation between suspended sediment concentration and turbidity. SPM concentration and water discharge data permit to identify fundamental hydro meteorological processes responsible for SPM transport in the fluvial system. SPM fluxes temporal variability is highlighted and described for different time steps, from daily to yearly. Some statistical indicators are calculated and used to describe SPM fluxes dynamics and to characterize SPM transport regime. A statistical tank model simulating daily SMP fluxes is then presented. This model is calibrated and validated using measured SPM fluxes chronicles. Using calibrated parameters set and historical water discharge and rainfall data, the model can simulate SPM fluxes over longer periods. For each watershed, various parameters sets are used to simulate different SMP transport scenarios over the last 30 years.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Nutrient dynamics and control of eutrophication in the Marne River system: modelling the role of exchangeable phosphorus

International audienceThe Marne River (12,762 km2) is one of the main tributaries of the Seine river, upstream from the City of Paris. Its population counts 2 million inhabitants of which 70% are concentrated in the downstream part of the basin. It has a high industrial activity and an intensive agriculture. The Marne River water supplies drinking water production to a large number of Parisians. A diversion reservoir (350×106 m3) is used mainly to sustain summer low flows. On the basis of field observations and experimental studies, a hydrological and ecological model of this river system was built using the Riverstrahler approach (Billen, G., Garnier, J., Hanset, Ph., 1994. Modelling phytoplankton development in whole drainage networks: the riverstrahler model applied to the seine river system. Hydrobiologia 289, 119–137; Garnier, J., Billen, G., Coste, M., 1995. Seasonnal succession of diatoms and Chlorophyceae in the drainage network of the river Seine: observations and modelling. Limnol. Oceanogr. 40, 750–765). The modelling strategy was to consider separately nine sub-basins and the main branch, to which was coupled a model of the reservoir. The point and diffuse sources of nutrients were analysed over more than 10 years (1991–2001) taking into account the role of exchangeable phosphorus (Némery (2003). Origine et devenir du phosphore dans le continuum aquatique de la Seine, des petis basins à l'estuaire. Rôle du phosphore échangeable sur l'eutrophisation. Thèse Univ. Paris 6, p. 258; Némery, J., Garnier, J., Morel, C., 2004. Phosphorus budget in the Marne watershed (France): urban vs. diffuse sources, dissolved vs. particulate forms. Biogeochemistry (in press)). The model was validated through its ability to reproduce available water quality observations. Different realistic scenarios of future reduction of phosphorus load were tested, in various hydrological conditions (dry and wet years). Phytoplankton development can be slightly reduced by a further 85% abatement of phosphorus in all the wastewater treatment plants of the basin, but a reduction both of diffuse and point phosphorus sources would be necessary to further decrease eutrophication

Assessment of suspended sediment transport in four alpine watersheds (France): influence of the climatic regime

International audienceHigh-frequency water discharge and suspended sediment concentration (SSC) databases were collected for 3 years on four contrasted watersheds: the Asse and the Bléone (two Mediterranean rainfall regime watersheds) and the Romanche and the Ferrand (two rainfall-snowmelt regime watersheds). SSCs were calculated from turbidity recordings (1-h time step), converted into SSC values. The rating curve was calculated by means of simultaneous SSC measurement taken by water sampling and turbidity recording. Violent storms during springtime and autumn were responsible for suspended sediment transport on the Asse and the Bléone rivers. On the Ferrand and the Romanche, a large share of suspended sediment transport was also caused by local storms, but 30% of annual fluxes results from snowmelt or icemelt which occurred from April to October. On each watershed, SSC up to 50 g l-1 were observed. Annual specific fluxes ranged from 450 to 800 t km-2 year-1 and 40-80% of annual suspended sediment fluxes occurred within 2% of the time. These general indicators clearly demonstrate the intensity of suspended sediment transport on these types of watersheds. Suspended sediment fluxes proved to be highly variable at the annual scale (inter-annual variability of specific fluxes) as well as at the event scale (through a hysteresis loop in the SSC/Q relationship) on these watersheds. In both cases, water discharge and precipitations were the main processes involved in suspended sediment production and transport. The temporal and spatial variability of hydro-meteorological processes on the watershed provides a better understanding of suspended sediment dynamics

Increase in surface runoff in the central mountains of Mexico : lessons from the past and predictive scenario for the next century

The hydrological response of a medium scale mountainous watershed (Mexico) is analysed over half a century. The hydrograph separation highlights an increasing surface runoff contribution since the early 1970's. This increase is attributed to land use changes while the meteorological forcing (rains) remains statistically stable over the same period. As a consequence, the intensity of annual extreme floods has tripled up over the period of survey, increasing flood risks in the region. The paper ends with a climatic projection over the 21st century. The decrease of precipitation and the increase of temperature should accentuate the trend engaged since the 1970's by reducing groundwater resources and increasing surface-runoff and associated risks

N, P, Si budgets for the Red River Delta (northern Vietnam) : how the delta affects river nutrient delivery to the sea

The Red River Delta (RRD) (Vietnam), a region experiencing rapid population growth, industrialization, and economic development, concentrates 54% of the population of the whole Red River watershed in less than 10% of the basin area. Our study aimed at understanding and quantifying the processes by which the delta affects the nutrient fluxes coming from the upstream watershed before they reach the sea. A comprehensive budget of nitrogen (N), phosphorus (P), and silica (Si) fluxes associated with natural and anthropogenic processes in the terrestrial and hydrological system of the delta was established for five sub-basins of the delta for the period 2000-2006, based on official statistical data, available measurements, and our own sampling campaigns and enquiries. The results show that anthropogenic inputs of N and P brought into the delta area are higher than the amounts delivered by the river from the upstream watershed. However, the amounts of these two elements ultimately delivered to the coastal zone from the delta are lower than the amounts carried by the upstream river, showing extremely efficient retention of both the soils and the delta's drainage network. For Si (taking into account both dissolved and amorphous solid forms), the retention is much lower. High retention of N and P and low retention of Si in the delta area have up to now protected the coastal zone from severe eutrophication problems