How much can changes in the agro-food system reduce agricultural nitrogen losses to the environment? Example of a temperate-Mediterranean gradient.

Ammonia (NH3) volatilization, nitrous oxide (N2O) emissions, and nitrate (NO3-) leaching from agriculture cause severe environmental hazards. Research studies and mitigation strategies have mostly focused on one of these nitrogen (N) losses at a time, often without an integrated view of the agro-food system. Yet, at the regional scale, N2O, NH3, and NO3- loss patterns reflect the structure of the whole agro-food system. Here, we analyzed at the resolution of NUTS2 administrative European Union (EU) regions, N fluxes through the agro-food systems of a Temperate-Mediterranean gradient (France, Spain, and Portugal) experiencing contrasting climate and soil conditions. We assessed the atmospheric and hydrological N emissions from soils and livestock systems. Expressed per ha agricultural land, NH3 volatilization varied in the range 6.2-44.4 kg N ha-1 yr-1, N2O emission and NO3 leaching 0.3-4.9 kg N ha-1 yr-1 and 5.4-154 kg N ha-1 yr-1 respectively. Overall, lowest N2O emission was found in the Mediterranean regions, where NO3- leaching was greater. NH3 volatilization in both temperate and Mediterranean regions roughly follows the distribution of livestock density. We showed that these losses are also closely correlated with the level of fertilization intensity and agriculture system specialization into either stockless crop farming or intensive livestock farming in each region. Moreover, we explored two possible future scenarios at the 2050 horizon: (1) a scenario based on the prescriptions of the EU-Farm-to-Fork (F2F) strategy, with 25% of organic farming, 10% of land set aside for biodiversity, 20% reduction in N fertilizers, and no diet change; and (2) a hypothetical agro-ecological (AE) scenario with generalized organic farming, reconnection of crop and livestock farming, and a healthier human diet with an increase in the share of vegetal protein to 65% (i.e., the Mediterranean diet). Results showed that the AE scenario, owing to its profound reconfiguration of the entire agro-food system would have the potential for much greater reductions in NH3, N2O, and NO3- emissions, namely, 60-81% reduction, while the F2F scenario would only reach 24-35% reduction of N losses.

Phosphorus management in cropping systems of the Paris Basin: From farm to regional scale.

The sustainability of phosphorus (P) fertilization in cropping systems is an important issue because P resources on earth are limited and excess P in soils can lead to ecological damage such as eutrophication. Worldwide, there is an increasing interest in organic farming (OF) due to its good environmental performance. However, organic cropping systems are suspected of generating negative P budgets, which questions their ability to provide sustainable P management. The design of agricultural systems at a broader scale also largely influences the shape of the P cycle and the possibility of its recycling to cropland. In this context, the aim of this study was to assess the relative influence of (i) OF versus conventional farming (CF) practices and (ii) the structure of agro-food systems at the regional scale, on P cycling and availability on cropland. For this purpose, we examined P budgets and soil P status of 14 organic and conventional cropping systems in commercial farms located in the Paris Basin. Available P was analyzed using two different methods: resin P and Olsen P. The results revealed no significant differences between CF and OF in available P stocks. Phosphorus budgets were always negative and significantly lower in CF systems, indicating that P was mined from soil reserves. In parallel, we estimated P budgets over cropland in all French regions for two distinct periods, 2004-2014 and 1970-1981, and showed that specialized intensive cropping systems in the Paris Basin led to a high, positive P budget in the latter period. However, this trend was reversed in the 2004-2014 period due to a sharp reduction of the mineral fertilizer application rate. The shift from very high P budgets to much lower and sometimes negative P budgets would not be a threat for agriculture due to the current high level of Olsen P in these regions, which was consistent with our measurements at the plot scale. Overall, these results suggest that OF would not lead to more P deficiency than CF. Instead, they emphasize that sustainable P management not only depends on farmers' choices but mainly on the structure and specialization of agro-food systems.

Modelling of faecal indicator bacteria (FIB) in the Red River basin (Vietnam).

Many studies have been published on the use of models to assess water quality through faecal contamination levels. However, the vast majority of this work has been conducted in developed countries and similar studies from developing countries in tropical regions are lacking. Here, we used the Seneque/Riverstrahler model to investigate the dynamics and seasonal distribution of total coliforms (TC), an indicator of faecal contamination, in the Red River (Northern Vietnam) and its upstream tributaries. The results of the model showed that, in general, the overall correlations between the simulated and observed values of TC follow a 1:1 relationship at all examined stations. They also showed that TC numbers were affected by both land use in terms of human and livestock populations and by hydrology (river discharge). We also developed a possible scenario based on the predicted changes in future demographics and land use in the Red River system for the 2050 horizon. Interestingly, the results showed only a limited increase of TC numbers compared with the present situation at all stations, especially in the upstream Vu Quang station and in the urban Ha Noi station. This is probably due to the dominance of diffuse sources of contamination relative to point sources. The model is to our knowledge one of the first mechanistic models able to simulate spatial and seasonal variations of microbial contamination (TC numbers) in the whole drainage network of a large regional river basin covering both urban and rural areas of a developing country.

Nesting nitrogen budgets through spatial and system scales in the Spanish agro-food system over 26 years.

Along its route through the agro-food system nitrogen (N) can be wasted, heightening diverse environmental problems. Geopolitical instabilities affect prices of N fertilisers and livestock feed, challenging production systems and increasing their need to reduce N waste. The analysis of N flows is essential to understanding the agroenvironmental performance of agro-food systems to detect leakages and to design strategies for reducing N pollution while producing feed and food. Sectorial analyses can mislead conclusions, prompting the need for integrated approaches. We present a multiscale analysis of N flows for the 1990-2015 period to identify both the strengths and weaknesses of the Spanish agro-food system. We constructed N budgets at three system scales, namely crop, livestock and the agro-food system, and at two spatial scales: national and regional (50 provinces). The big picture shows a country with increasing crop (575 to 634 GgN/yr) and livestock (138 to 202 GgN/yr, edible) production and nitrogen use efficiency improvements, especially for certain crop and livestock categories. Nevertheless, this falls short of reducing agricultural surpluses (812 GgN/yr) and external dependency, which is closely related to the externalisation of certain environmental impacts (system NUE, from 31 % to 19 % considering externalisation). The regional picture shows the contrasted operation between provinces, assigned to three agro-food system categories: fuelled by synthetic fertiliser (29 provinces), grassland inputs to livestock (5 provinces) or the net import of feed (16 provinces). Regional specialisation on crop or livestock production was reinforced, hampering good recirculation of N through livestock feed from regional cropland and their N fertilisation by regional livestock excretion. We conclude that pollution and external dependency need to be further reduced in Spain. To do so, the big picture of the full system is paramount but must be adapted to the regional particularities.

Nitrogen inputs by irrigation is a missing link in the agricultural nitrogen cycle and related policies in Europe.

Irrigation, one of the 28 agri-environmental indicators defined in the European Common Agricultural Policy, is often neglected in agricultural nitrogen (N) budgets, while it can be a considerable source of N in irrigated agriculture. The annual N input from irrigation water sources (NIrrig) to cropping systems was quantified for Europe for 2000-2010 at a resolution of 10 × 10 km, accounting for crop-specific gross irrigation requirements (GIR) and surface- and groundwater nitrate concentration. GIR were computed for 20 crops, while spatially explicit nitrate concentration in groundwater was derived using a random forest model. We show that although GIR were relatively stable (46-60 km3 yr-1), the Nirrig in Europe increased over the 10-year period (184 to 259 Gg N yr-1), approximately 68 % of which occurred in the Mediterranean region. The main hotspots appeared in areas with both high irrigation requirements and high groundwater nitrate concentration, reaching up to averaged values of 150 kg N ha-1 yr1. These were mainly located in Mediterranean Europe (Greece, Portugal and Spain) and to a lesser extent in Northern Europe (The Netherlands, Sweden and Germany). By not including NIrrig, environmental and agricultural policies are underestimating the real extent of N pollution hotspots in European irrigated systems.

Unravelling nutrient fate and CO2 concentrations in the reservoirs of the Seine Basin using a modelling approach.

Reservoirs are active reactors for the biogeochemical cycling of carbon (C) and nutrients (nitrogen: N, phosphorus: P, and silica: Si), however, our in-depth understanding of C and nutrient cycling in reservoirs is still limited by the fact that it involves a variety of closely linked and coupled biogeochemical and hydrological processes. In this study, the updated process-based Barman model was applied to three reservoirs of the Seine Basin during 2019-2020, considering the variations of carbon dioxide (CO2) concentrations and key water quality variables. The model simulations captured well the observed seasonal variations of water quality variables, although discrepancies remained for some variables. According to the model, we found that: (1) the three reservoirs are autotrophic ecosystems and showed high removal efficiency of dissolved inorganic carbon and nutrients during 2019-2020; (2) phytoplankton assimilation, benthic denitrification, precipitation and dissolution of calcium carbonate, and gas exchange at the water-air interface are the dominant processes for water quality variations in reservoirs; (3) based on scenarios results, trophic state and mean water depth of reservoir would impact the biogeochemical processes and the retention efficiency of nitrate and dissolved silicate. Finally, we expect that the successful application of Barman model in the reservoirs of the Seine Basin could provide a useful tool for simulating reservoir water quality changes and thus evaluating the impacts of reservoirs on downstream water quality.

Crop production and nitrogen use in European cropland and grassland 1961-2019.

This paper presents EuropeAgriDB v1.0, a dataset of crop production and nitrogen (N) flows in European cropland 1961-2019. The dataset covers 26 present-day countries, detailing the cropland N harvests in 17 crop categories as well as cropland N inputs in synthetic fertilizers, manure, symbiotic fixation, and atmospheric deposition. The study builds on established methods but goes beyond previous research by combining data from FAOSTAT, Eurostat, and a range of national data sources. The result is a detailed, complete, and consistent dataset, intended as a basis for further analyses of past and present agricultural production patterns, as well as construction of scenarios for the future.

Potential denitrification and nitrous oxide production in the sediments of the Seine River Drainage Network (France).

To investigate bottom sediment denitrification at the scale of the Seine drainage network, a semi-potential denitrification assay was used in which river sediments (and riparian soils) were incubated for a few hours under anaerobic conditions with non limiting nitrate concentrations. This method allowed the nitrous oxide (N(2)O) concentration in the headspace, as well as the nitrate, nitrite, and ammonium concentrations to be determined during incubation. The rates at which nitrate decreased and N(2)O increased were then used to assess the potential denitrification activity and associated N(2)O production in the Seine River Basin. We observed a longitudinal pattern characterized by a significant increase of the potential rate of denitrification from upstream sectors to large downstream rivers (orders 7-8), from approximately 3.3 to 9.1 microg NO(3)(-)-N g(-1) h(-1), respectively, while the N(2)O production rates was the highest both in headwaters and in large order rivers (0.14 and 0.09 N(2)O-N g(-1) h(-1), respectively) and significantly lower in the intermediate sectors (0.01 and 0.03 N(2)O-N g(-1) h(-1)). Consequently, the ratio N(2)O production:NO(3) reduction was found to reach 5% in headstreams, whereas it averaged 1.2% in the rest of the drainage network, an intermediate percentage being found for the riparian soils. Finally, the ignition loss of sediments, together with other redundant variables (particulate organic carbon content: g C 100 g(-1) dry weight [dw], moisture: g water 100 g(-1) dw, sediment size <50 mum: g material size <50 mum 100 g(-1) dw) were found to control these activities. However, the biodegradability of organic matter must be measured to better understand the factor controlling denitrification and its associated N(2)O production.

Nutrient transfer in three contrasting NW European watersheds: the Seine, Somme, and Scheldt Rivers. A comparative application of the Seneque/Riverstrahler model.

An understanding of the ecological functioning of an aquatic continuum on a multi-regional scale relies on the ability to collect suitable descriptive information. Here, the deterministic Seneque/Riverstrahler model, linking biogeochemistry with the constraints set by geomorphology and anthropogenic activities, was fully implemented to study the Seine, Somme, and Scheldt Rivers. Reasonable agreement was found between calculated and observed nutrient fluxes for both seasonal and inter-annual variations along the networks. Nutrient budgets underline: i) a clear partition of diffuse and point sources with respect to the specific activities of the watersheds, ii) the importance of riparian retention, responsible for 25-50% of nitrogen retention, iii) the role played by benthic processes, resulting in the retention of up to 45% of the phosphorus and 35% of the silica entering the river systems. Nutrient ratios confirmed that fluxes to the Eastern Southern Bight of the North Sea are imbalanced, supporting the potential for undesirable algal blooms.

Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852-2014): From traditional agriculture to conventional intensive systems.

France was a traditionally agricultural country until the first half of the 20th century. Today, it is the first European cereal producer, with cereal crops accounting for 40% of the agricultural surface area used, and is also a major country for livestock breeding with 25% of the European cattle livestock. This major socioecological transition, with rapid intensification and specialisation in an open global market, has been accompanied by deep environmental changes. To explore the changes in agricultural GHG emissions over the long term (1852-2014), we analysed the emission factors of N2O from field experiments covering major land uses, in a gradient of fertilisation and within a range of temperature and rainfall, and used CH4 emission coefficients for livestock categories, in terms of enteric and manure management, considering the historical changes in animal excretion rates. We also estimated indirect CO2 emissions, rarely accounted for in agricultural emissions, using coefficients found in the literature for the dominant energy consumption items (fertiliser production, field work and machinery, and feed import). From GHG emissions of ~30,000 ktons CO2 Eq yr-1 in 1852, reaching 54,000 ktons CO2 Eq yr-1 in 1955, emissions more than doubled during the 'Glorious thirties' (1950-1980), and peaked around 120,000 ktons CO2 Eq yr-1 in the early 2000s. For the 2010-2014 period, French agriculture GHG emissions stabilised at ~114,000 ktons CO2 Eq yr-1, distributed into 49% methane (CH4), 22% carbon dioxide (CO2) and 29% nitrous oxide (N2O). A regional approach through 33 regions in France shows a diversity of agriculture reflecting the hydro-ecoregion distribution and the agricultural specialisation of local areas. Exploring contrasting scenarios at the 2040 horizon suggests that only deep changes in the structure of the agro-food system would double the reduction of GHG emissions by the agricultural sector.

Nitrous oxide emissions from denitrifying activated sludge of urban wastewater treatment plants, under anoxia and low oxygenation.

Batch experiments were made to better understand the mechanisms of N2O emissions from activated sludge in denitrifying conditions found in urban WWTPs, i.e. under anoxic and low oxygenation conditions. The results showed that in completely anoxic conditions, denitrification, related to a periplasmic nitrate reductase activity, is the major producer of N2O (100% of the N2O production), whereas the nitrate ammonifying activity is not significant. In a gradient of low oxygenation, the highest N2O emissions (49.7+/-3.8 microg N2O-N/g SS/h on average) occurred at a dissolved-oxygen concentration of around 0.3mg O2/L. Below 0.3mg O2/L, heterotrophic denitrification appeared to be the major process responsible for the N2O emission (100% at zero oxygenation). From 0.4 to 1.1mg O2/L, N2O emissions were due to two processes: (i) heterotrophic denitrification that represented about 40% of the N2O production, and (ii) autotrophic nitrifier denitrification that accounted for about 60%. The N2O emissions from activated sludge represented on average 0.4% of reduced NO3(-) in anoxic conditions. The N2O emissions associated with denitrification of entire nitrogen load would amount to 155 T N2O-N/year, if all the Paris wastewater was treated by a process using activated sludge.

A report card and quality indicators for the Seine estuary: from scientific approach to operational tool.

The scientific teams from the interdisciplinary Seine-Aval (SA) research program and the SA's operational pole, GIPSA (Groupement d'Intérêt Public Seine-Aval) have worked together to create a report card designed to help the Estuary Council (Conseil de l'Estuaire) revitalize its original functions: maintaining functional links between the various estuarine ecosystems, comprehending and managing the estuary's natural habitats and biological populations, and monitoring and improving the physical-chemical quality of the estuarine waters. The report card will be able to synthesize the information obtained from several system performance variables and available operational indicators. This approach, intended to guide the estuary managers, is the oeuvre of several scientific teams; it is particularly important in the context of the Water Framework Directive because it facilitates the elaboration of a group of relevant indicators, which can then be used as operational tools. A report card will provide decision-makers (e.g., political authorities; national, regional and local institutions and industries) with the key indicators for evaluating the system and predicting changes in terms of selected objectives, such as the preservation and restoration of the estuary's environmental functionalities. The final objective of the research is to choose among the available indicators to approximate potential ecological risks. Integrating the socio-economical data will perhaps lead to setting risk acceptability thresholds for the different uses of the Seine estuary. In the end, collaboration between the scientists, the managers, and the GIPSA operational pole will be essential to produce a viable report card about the environmental status of the Seine estuary. To illustrate the research now under way, this article presents the results for three actions undertaken, concerning: (i) physical indicators (i.e., an inventory of the estuary first as a whole, and then section by section); (ii) benthic indicators (i.e., seven indices which show a moderate EcoQ for the lower part of the estuary); and (iii) a eutrophication indicator (i.e., an indicator for coastal eutrophication potential (ICEP), which helps to limit the nutrient fluxes (N or P) that exceed the silica flux delivered by the Seine network, based on the Redfield ratios for algal propagation).

Long trend reduction of phosphorus wastewater loading in the Seine: determination of phosphorus speciation and sorption for modeling algal growth.

The lower Seine River is severely affected by the release of the treated wastewater from the 12 million inhabitants of the Paris agglomeration. Whereas urban effluents were the major source of phosphorus pollution in the late 1980s, the ban on polyphosphates from detergents in 1991 considerably reduced the phosphorus (P) loading to the Seine River and was followed in 2000 by the implementation of phosphorus treatment in the largest wastewater treatment plant of Paris conurbation (Seine Aval). Phosphorus discharged to the rivers from domestic wastewater was reduced by 80 %, significantly decreasing phytoplankton biomass in the large branches of the Seine River. Considering that phosphorus treatment (the use of ferric salts in the P treatment line) might change the adsorption of ortho-phosphates on suspended matter, we experimentally studied again their sorption processes in these new conditions. We found parameters of the Langmuir equation (Pac = 0.003 mgP mgSS-1; Kps = 0.04 mgP L-1) that significantly differed from the values previously considered for modeling of the whole Seine, especially for Kps (Pac = 0.0055 mgP mgSS-1; Kps = 0.7 mgP L-1). Using the Seneque-Rivertrahler modeling approach, we showed a better agreement between P observations and simulations with the new P sorption parameters, with slight effect on the simulation of the development of phytoplankton in the water column.

Two contrasted future scenarios for the French agro-food system.

Narratives of two prospective scenarios for the future of French agriculture were elaborated by pushing several trends already acting on the dynamics of the current system to their logical end. The first one pursues the opening and specialization characterizing the long-term evolution of the last 50 years of most French agricultural regions, while the second assumes a shift, already perceptible through weak signals, towards more autonomy at the farm and regional scales, a reconnection of crop and livestock farming and a more frugal human diet. A procedure is proposed to translate these qualitative narratives into a quantitative description of the corresponding nutrient fluxes using the GRAFS (Generalized Representation of Agro-Food Systems) methodology, thus allowing a comprehensive exploration of the agronomical and environmental performance of these two scenarios. The results show that the pursuit of the opening and specialization of French agriculture, even complying with regulations regarding reasoned fertilization, would result in considerable environmental burdens namely in terms of water pollution. The scenario generalizing organic farming practices, reconnection of crop and livestock farming systems and a demitarian human diet makes it possible to meet the future national food demand while still exporting significant amounts of cereals to the international market and ensuring better groundwater quality in most French regions.

Organic market gardening around the Paris agglomeration: agro-environmental performance and capacity to meet urban requirements.

Organic market gardening is often promoted by urban municipalities as a way to resource part of the food supply, creating new social links and protecting groundwater resources. The agronomical and environmental performance of six commercial organic market gardening farms supplying vegetables in Paris were evaluated and compared with other vegetable production systems. When expressed in terms of protein production, the yield of these systems appears rather low compared with the productive capacity of open-field organic cropping systems where vegetable production is inserted into rotation with other crops. Moreover, the requirement of producing infiltrated water meeting the drinking water standards seriously limits the allowable rate of fertilisation, thus limiting production. The data reported herein show that to supply the amount of vegetables required by the Paris agglomeration (12 million inhabitants) only by organic market gardening, 160,000-205,000 ha, i.e. 28-36 % of the agricultural area of the surrounding Ile-de-France region, would be required. We conclude that organic market gardening is only one of several other farming systems which can contribute to a re-localised supply of vegetables to large cities.

How can water quality be improved when the urban waste water directive has been fulfilled? A case study of the Lot river (France).

The Lot river, a major tributary of the downstream Garonne river, the largest river on the Northern side of the Pyrenees Mountains, was intensively studied in the 1970s. A pioneering program called "Lot Rivière Claire" provided a diagnosis of water quality at the scale of the whole watershed and proposed an ambitious program to manage nutrient pollution and eutrophication largely caused by urban wastewater releases. Later on, the implementation of European directives from 1991 to 2000 resulted in the nearly complete treatment of point sources of pollution in spite of a doubling of the basin's population. At the outlet of the Lot river, ammonium and phosphate contamination which respectively peaked to 1 mg N-NH4 L-1 and 0.3 mg P-PO4 L-1 in the 1980s returned to much lower levels in recent years (0.06 mg N-NH4 L-1 and 0.02 mg P-PO4 L-1), a reduction by a factor 15. However, during this time, nitrate contamination has regularly increased since the 1980s, from 0.5 to 1.2 mg N-NO3 L-1 in average, owing to the intensification of agriculture and livestock farming. Application of the Riverstrahler model allowed us to simulate the water quality of the Lot drainage network for the 2002-2014 period. We showed that, with respect to algal requirements, phosphorus and silica are well balanced, but nitrogen remains largely in excess over phosphorus and silica. This imbalance can be problematic for the ecological status of the water bodies. Using the model, for simulating various scenarios of watershed management, we showed that improvement of urban wastewater treatment would not result in any significant change in the river's water quality. Even though arable land occupies a rather limited fraction of the watershed area, only the adoption of better farming practices or more radical changes in the agro-food system could reverse the trend of increasing nitrate contamination.

Nutrient inputs and hydrology together determine biogeochemical status of the Loire River (France): Current situation and possible future scenarios.

The Grafs-Seneque/Riverstrahler model was implemented for the first time on the Loire River for the 2002-2014 period, to explore eutrophication after improvement of wastewater treatments. The model reproduced the interannual levels and seasonal trends of the major water quality variables. Although eutrophication has been impressively reduced in the drainage network, a eutrophication risk still exists at the coast, as shown by the N-ICEP indicator, pointing out an excess of nitrogen over silica and phosphorus. From maximum biomass exceeding 120 µgChla l-1 in the 1980's, we observed decreasing maximum values from 80 to 30 µgChla l-1 during the period studied. Several scenarios were explored. Regarding nutrient point sources, a low wastewater treatment scenario, similar to the situation in the 1980's, was elaborated, representing much greater pollution than the reference period (2002-2014). For diffuse sources, two agricultural scenarios were elaborated for reducing nitrogen, one with a strict application of the agricultural directives and another investigating the impact of radical structural changes in agriculture and the population's diet. Although reduced, a risk of eutrophication would remain, even with the most drastic scenario. In addition, a pristine scenario, with no human activity within the basin, was devised to assess water quality in a natural state. The impact of a change in hydrology on the Loire biogeochemical functioning was also explored according to the effect of climate change by the end of the 21st century. The EROS hydrological model was used to force Riverstrahler, considering the most pessimistic SRES A2 scenario run with the ARPEGE model. Nutrient fluxes all decreased due to a >50% reduction in the average annual discharge, overall reducing the risk of coastal eutrophication, but worsening the water quality status of the river network. The Riverstrahler model could be useful to help water managers contend with future threats in the Loire River, at the scale of its basin and at smaller nested scales.

Production vs. respiration in river systems: an indicator of an "ecological status".

The Riverstrahler model of ecological functioning of large drainage networks validated on the Seine river system has been used for calculating the seasonal variations of Production and Respiration at various spatial scales (e.g. according to river orders). Based on the measurements of biological processes, the P/R ratio has led to an evaluation of the "ecological functioning", beyond the notion of "good ecological status". Furthermore, the effects, on the P/R ratio, of the geomorphological and climate factors characterizing the Hydro-Eco-Regions (HER) of the Seine watershed have been quantitatively explored with the model. Whereas one finds a typical upstream-downstream pattern of the P/R ratio variations under the traditional rural conditions that prevailed in the Seine basin until the end of the 18th century, this pattern is strongly affected by the changes in urban populations and the implementation of wastewater collection and treatment, more than by the specificity of the physical factors characterizing the different HER. We have also found that autotrophy (a P/R ratio >1) might leads to eutrophication symptoms when P exceeds 1-2 mg C m(-2) d(-1) and that heterotrophy of the system (P/R ratio< 1) would reveal organic pollution when R exceeds 1-2 mg C m(-2) d(-1), stocks and fluxes of organic matter being expressed in carbon unit. Consequently, the P/R ratio appears as a good indicator of the perturbations caused by human activities in the watershed. The Riverstrahler model is able to quantify this effect.

Fecal bacteria in the rivers of the Seine drainage network (France): sources, fate and modelling.

The Seine river watershed (France) is a deeply anthropogenically impacted area, due to the high population density, intense industrial activities and intensive agriculture. The water quality and ecological functioning of the different rivers of the Seine drainage network have been extensively studied during the last fifteen years within the framework of a large French multidisciplinary scientific program (PIREN Seine program). This paper presents a synthesis of the main data gained in the scope of this program concerning the microbiological water contamination of the rivers of the Seine drainage network. The more common indicator of fecal contamination (fecal coliforms) was mainly used; some complementary works used E. coli and intestinal enterococci as alternative fecal indicators. Point sources (outfall of wastewater treatment plants) and non point sources (surface runoff and soil leaching) of fecal pollution to the rivers of the watershed were quantified. Results showed that, at the scale of a large urbanised watershed as the Seine basin, the input of fecal micro-organisms by non-point sources is much lower than the inputs by point sources. However, the local impact of diffuse non-human sources (especially surface runoff of pastured fields) can be of major importance on the microbiological quality of small headwater rivers. Fecal contamination of the main rivers of the Seine watershed (Seine, Marne, Oise rivers) was studied showing high level of microbiological pollution when compared to European guidelines for bathing waters. The strong negative impact of treated wastewater effluents outfall on the microbiological quality of receiving rivers was observed in different areas of the watershed. Once released in rivers, culturable fecal bacteria disappeared relatively rapidly due to mortality (protozoan grazing, lysis) or loss of culturability induced by stress conditions (sunlight effect, nutrient concentration, temperature). Mortality rates of E. coli were studied in different types of rivers within the watershed showing, in summer conditions, no major difference in the mortality rates in small and large rivers. As a result of these studies, a module describing the dynamics of fecal bacteria has been developed and embedded within a hydro-ecological model describing the functioning of the rivers of the whole watershed (the SENEQUE model). Once validated, such a model can be used for testing predictive scenarios and thus can be a very useful tool for the management of microbiological water quality at the scale of the whole basin.

SENEQUE: a multi-scaling GIS interface to the Riverstrahler model of the biogeochemical functioning of river systems.

The Riverstrahler model describes the biogeochemical functioning of an entire river system, from 100 to 100,000 km(2) or more, taking into account the constraints set by the morphology of the drainage network, the meteorological/hydrological conditions, and the inputs of material from point and non-point sources in the watershed. This tool has been applied for research purpose to several river systems differing in terms of hydrological regime and anthropogenic influences. In order to improve its capabilities and its generic dimension, as well as to develop a user-friendly interface allowing its transfer to non-specialist users including managers, the model has been coupled to a GIS interface. This gives the user the possibility to visualize the available geospatial database, to select the best geographical representation of the drainage network, to automatically prepare the corresponding input files required for the model, to pilot the model calculation and to visualize the results. The coupling with a GIS interface has considerably improved the capabilities of the Riverstrahler model. The code of the model is now entirely generic and can be run on any river system for which a suitable database is available. Its spatial resolution can be adapted to the requirement of the relevant problem, from the highest level, where each elementary watershed is individualized, to the lower level, where the whole basin is idealized as one basin with tributaries of each order having the same characteristics. As an illustration of the new potentialities offered by the coupling of Riverstrahler with a GIS through the SENEQUE interface, the results of a same modeling scenario are compared at different spatial resolutions. For the first time, with on-line coupling to a geodatabase, the effect of increasing the spatial resolution of the drainage network representation on the performance of the Riverstrahler model has been examined. At the outlet of the basin, the water quality results were found invariant to a large degree, whatever the details of its representation in the calculations. This result justifies the use of a low resolution representation of the upstream watershed when results are required only at the outlet of the basin.