Influence de l’organisation spatiale du paysage sur les transferts hydriques de Carbone, d'Azote et de Phosphore dans un bassin versant agricole de méso-échelle

Nitrogen (N) and phosphorus (P) enrichment of aquatic ecosystems results in their eutrophication and impacts the carbon (C) cycle. In agricultural catchments, most of the hydrologic flux of N and P originates from agricultural areas. The high spatial and temporal variability of C-N-P fluxes in headwaters (<15 km2) complicates the management and protection of water resources, but offers opportunities for optimisation of localised agro-environmental measures. A fortnightly monitoring conducted over 17 months (March 2018-July 2019) of C-N-P concentrations at 32 points, including 23 headwaters, captured this spatio-temporal variability in an agricultural catchment (Yvel River, 375 km2 - Brittany, France). The persistence of spatial patterns of water quality was demonstrated: occasional sampling allowed the ranking of sub-catchments according to their hydric exports of N and P. The link between the landscape spatial organisation and C-N-P fluxes was then investigated across two levels of organisation. Using a stochastic approach, it was shown that the proportion and type of agricultural areas on hillslopes influence N water exports, while the spatial configuration of agricultural areas is critical for P exports. The C-N-P hydrochemical signal from the hillslope is modulated in the river network. A mass balance between modelled hillslope fluxes and the measured mesoscale outlet fluxes showed that these modulations influence the temporal variability of mesoscale concentrations, but have little impact on annual fluxes.L’enrichissement en azote (N) et phosphore (P) des milieux aquatiques provoque leur eutrophisation et impacte le cycle du carbone (C). Dans les bassins versants agricoles, l’essentiel du flux hydrique de N et P provient des surfaces agricoles. La forte variabilité spatio-temporelle des flux de C-N-P à l’échelle de la tête de bassin versant (