International audienceThis paper focuses on soluble reactive phosphorus (SRP) dynamics along a 225 km stretch of the Seine River, including the Paris urban area, for the 2007–2011 period. The impact of hydro-sedimen-tary processes on SRP concentrations and fluxes is estimated under various hydrological conditions. Sorp-tion interaction parameters between SRP and sus-pended matter are experimentally determined on river water samples and are included in a hydro-ecological model. Simulated concentrations are compared to weekly measurements at 11 monitoring stations. The introduction of sorption in the model reduces the root mean square error of simulated SRP concentrations by 20 % and allows the simulation of particulate inorganic P (PIP) accumulation in the system. With these ameliorations, the model constitutes a reliable manage-ment tool, which is compatible with the requirements of new regulations as the European Water Framework Directive. P mass balances are assessed upstream and downstream the major waste water treatment plant of the Paris urban area. P fluxes in the system are mainly driven by hydrological conditions and sediment-related processes. While SRP is the predominant P form during low flow, PIP accounts for more than 70 % of the total P during high flow. Moreover, SRP sorption fluxes are of the same order of magnitude as biotic fluxes affecting SRP concentrations. According to the model, and based on all the available data, 75 % of the SRP release by the river bed sediments occurs during high flow periods, and PIP exchanges at the sediment–water interface are more than 4 times higher during high flow periods than during low flow periods

-In the early 2000s, most of the effluents from the Paris agglomeration (12 million inhabitants) were still discharged after only a standard activated sludge treatment of carbon, thus releasing a high ammonium load. This triggered the growth of nitrifying bacteria in the river and caused a deficit of dissolved oxygen concentration 200 km downstream, at the entrance of the estuarine area. From 2007 on, improvements of the wastewater treatment plant (WWTP) helped to nitrify most of the ammonium, and to denitrify 70% of the nitrate produced since 2012. This paper documents the effect of these recent changes in the treatment of Paris effluents, based on measurements of nitrogen species concentrations and nitrifying activities along longitudinal profiles of the lower Seine River. These observations are put into perspective with respect to previous observations, and a modeling approach is developed to evidence the role of ammonium and nitrifying organisms released with the WWTP effluents into the receiving water body. Today, a larger biomass of ammoniumoxidizing microbes is being discharged by the WWTP, but the microorganisms released do not grow, possibly because of the low substrate in the river. The same is true for nitrite-oxidizing microbes, which explains the persistence of nitrite often discharged with the effluents because of incomplete nitrification and/or denitrification in the treatment line. Taking into account the development of microbial populations of nitrifying organisms is essential to explain the observed spatial and temporal pattern of the nitrogen species.

Phosphorus (P) plays a strategic role in agricultural production as well as in the occurrence of freshwater and marine eutrophication episodes throughout the world. Moreover, the scarcity and uneven distribution of minable P resources is raising concerns about the sustainability of long-term exploitation. In this paper we analyze the P cycle in anthropic systems with an original multiscale approach (world region, country, and large basin scales) in two contrasting world regions representative of different trajectories in socioeconomic development for the 1961-2009 period: Europe (EU-27)/France and the Seine River Basin, and Asia (ASEAN-8)/Vietnam and the Red River Basin. Our approach highlights different trends in the agricultural and food production systems of the two regions. Whereas crop production increased until the 1980s in Europe and France and has stabilized thereafter, in ASEAN-8 and Vietnam it began to increase in the 1980s and it is still rising today. These trends are related to the increasing use of fertilizers, although in European countries the amount of fertilizers sharply decreased after the 1980s. On average, the total P delivered from rivers to the sea is 3 times higher for ASEAN-8 (300 kg P km À2 yr À1) than for EU-27 countries (100 kg P km À2 yr À1) and is twice as high in the Red River (200 kg P km À2 yr À1) than in the Seine River (110 kg P km À2 yr À1), with agricultural losses to water in ASEAN-8 3 times higher than in EU-27. Based on the P flux budgets, this study discusses early warnings and management options according to the particularities of the two world regions, newly integrating the perspective of surface water quality with agricultural issues (fertilizers, crop production, and surplus), food/feed exchanges, and diet, defining the so-called water-agro-food system.