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The "DYNAMIT " project was set up to study the agricultural impact on the biogeochemical cycling of organic matter (OM) in natural waters and its fate in the freshwater - seawater mixing zone. Three main reasons are behind this choice: (i) agricultural activities are obviously a major threat to human water security and biodiversity, (ii) the composition of the organic matter is still poorly characterized and (iii) its behavior in the coastal aquatic system is far from being understood. This project was developed on Brittany since this is the first agricultural region in France, the largest agricultural exporter countries of the world and the second leading agricultural power of the European Union.
The Penzé River (NW Brittany, France), was sampled during one hydrological year from 05/01/2012 to 09/01/2013 with an 8 day frequency at Prat Guen (3°54′42.5″ W–48°31′18.8″N) 1 km from the village of Saint Thégonnec, outside the turbid area and upstream of the salty zone of the estuary. We have examined the dynamics and sources of reduced sulfur, humic substances (HS) and DOC in this river system affected by agricultural practices. New differential pulse cathodic stripping voltammetry method (DP-AdCSV) (Pernet-Coudrier et al., 2013), was successfully applied to measure glutathione-like compounds (GSHs), thioacetamide-like compounds (TAs) and a liquid chromatography coupled to organic detector (LC-OCD-OND) to analyze HS and DOC at high frequency. The streamflow-concentration patterns, principal components analysis and flux analysis allowed discrimination of the source of each organic compound type. Surprisingly, the two RSS and HS detected in all samples, displayed different behavior. As previously shown, manuring practice is the main source of DOC and HS in this watershed where agricultural activity is predominant (Waeles et al., 2013). The HS were then transferred to the river systems via runoff, particularly during the spring and autumn floods, which are responsible of >60% of the annual flux. TAs had a clear groundwater source and may be formed underground, whereas GSHs displayed two sources: one aquagenic in spring and summer probably linked to the primary productivity and a second, which may be related to bacterial degradation. High sampling frequency allowed a more accurate assessment of the flux values which were 280 tC y-1 for DOC representing 20 kgC ha-1 y-1. HS, TAs and GSHs fluxes represented 60, 13, and 4% of the total annual DOC export, respectively (Marie et al., 2015).

Marie, L ; Pernet-Coudrier,B ; Waeles,M ; Gabon,M ; Riso,R (2015).Dynamics and sources of reduced sulfur, humic substances and dissolved organic carbon in a temperate river system affected by agricultural practices. Sci.Total Environ. 537, 23 - 32.
Pernet-Coudrier, B., Waeles, M., Filella, M., Quentel, F., Riso, R.D., 2013. Simple and simultaneous determination of glutathione, thioacetamide and refractory organic matter in natural waters by DP-CSV. Sci. Total Environ. 463–464, 997–1005.
Waeles, M., Riso, R., Pernet-Coudrier, B., Quentel, F., Durrieu, G., Tissot, C., 2013. Annual cycle of humic substances in a temperate estuarine system affected by agricultural practices. Geochim. Cosmochim. Acta 106, 231–246.

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