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Dr. Chantal Gascuel-Odoux

National Institute for Agricultural Research (INRA), Rennes, France

Understanding the evolution of nitrogen and phosphorus fluxes at different levels (national, regional and critical sites), with regards to current inputs and legacy.

Dr. Chantal GascuelDr. Chantal Gascuel
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At national level, one of the major challenge is to assess the areas at risk regarding eutrophication issue of continental and marine aquatic ecosystems. For this challenge, two conceptual models, Nut-ting-N and Nutting-P, calibrated over the highest frequency data sets of the national monitoring, and using different data base characterizing by indicators, land use, hydro systems and climate as retention factors, have been developed. These models allow the water agencies to estimate the N and P delivery over all the terrestrial water bodies, as well the part of domestic and agricultural sources. Indicators on eutrophication risks have been developed. These tools are now used for national reporting of the WFD. The current challenge is to estimate the uncertainty according the monitoring.

At regional level, all the data bases are generally more detailed, so that trends and sources can be evaluated. This issue will be illustrated on phosphorus. From different indicators, a general decrease of dissolved phosphorus fluxes related to the domestic source decreasing have been shown, but an increase of agricultural sources has also been points out due to soil phosphorus storage in agricultural sources. The riparian areas have been hypothesis as the source main areas. An effort has been consequently done to improve total as well extractible phosphorus soil content. As the total phos-phorus is well estimated, the extractible phosphorus is poorly estimated due to a lack of information of agricultural practices at plot level.

On sites already affected by eutrophication, modelling is used to help territories in remediation. This issue will be illustrated on nitrogen, on the case of green algae development on some bay of the Brit-tany coast, attributed to nitrogen fluxes. After a long period of reducing nitrogen inputs by optimizing fertilization, which have induced a decrease in N concentration (from 35 to 26 NO3 mg/l), the next step to reach a target of around 10 mg/l to avoid coastal eutrophication, is changing land use and agricultural systems. To help the communities in such strong changes, participative scenario and modelling scenarios have been used and will be presented, the main one being based on grassland development. The conclusion is that a multiple and diverse solution, discussed farm per farm have to developed.

Finally, a collective scientific expertise has been asked by agricultural and French ministries as well the French biodiversity agency dealing with water issue. 45 experts in ecology, hydrology and biochemistry, juridical, economic and social sciences have been analyzed 4500 scientific papers and written a synthesis (in French and English) to build next regulations on a scientific basis.


  • Moreau P, Ruiz L, Vertès F, Baratte C, Delaby L, Faverdin P, Gascuel-Odoux C, Piquemal B, Ramat E, Salmon-Monviola J and Durand P (2013) CASIMOD’N: An agro-hydrological distributed model of catchment-scale nitrogen dynamics integrating farming system decisions. Agricultural Systems. 118, 41-51.
  • Durand P., Moreau P., Salmon-Monviola, J., Ruiz L., Vertes F., Gascuel-Odoux C. Modeling the interplay between N cycling processes and mitigation options of nitrate pollution in farming catchments. Journal of Agricultural Science.
  • Dupas R., C. Gascuel-Odoux, N. Gilliet, C. Grimaldi, G. Gruau. (2015) Distinct export dynamics for dissolved and particulate phosphorus reveal independent transport mechanisms in an arable headwater catchment. Hydrological Processes 29,(14), 3162-3178.
  • Dupas R., G. Gruau, S. Gu, G. Humbert, A. Jaffrezic, C. Gascuel-Odoux. (2015) Groundwater control of biogeochemical processes causing phosphorus release from riparian wetlands. Water Research 84, 307-314.