Spatial changes in the modalities of N and P inputs in a rural river network

Nitrates (NI) and total phosphorus (TP) fluxes were analysed in an agricultural river of southwestern France, the Save River, in order to identify their nonpoint and/or point–source origin. For this purpose, TP and NI concentrations were measured and the fluxes calculated along the whole river at 27 sampling sites during three different seasons; Pearson correlation coefficient and partial correlation were systematically calculated for each sampling period between TP or NI loads and the environmental variables; i.e. number of inhabitants, forest, crop and pasture surfaces. In Spring and Winter, concentrations and fluxes of NI and TP increased from the headwaters to the mouth of the Save River. During the dry season, in Summer, concentrations and fluxes of NI and TP were strongly influenced by instream biological processes and artificial water supply. During wet seasons, i.e. in Winter and Spring, in spite of the same spatial dynamics of TP and NI fluxes, it appears that nitrate fluxes entering the river originated mainly from croplands (i.e. diffuse pollution) whereas phosphorus fluxes were highly related to the number of inhabitants (i.e. point source pollution). During the rainiest season, i.e. in Spring, phosphorus fluxes were also related to forests which are mainly located in the steep slope upstream part of the drainage basin.