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 agricult ural river of southwestern France, the Save River, in order to identify the ir nonpoint and/or point-source origin. For this purpose, TP and NI concent rations were measured and the fluxes calculated along the whole river at 27 sampling sites during three different seasons; Pearson correlation coeffic ient and partial correlation were systematically calculated for each sampli ng period between TP or NI loads and the environmental variables; i.e. numb er of inhabitants, forest, crop and pasture surfaces. In Spring and Winter, concentrations and fluxes of NI and TP increased from the headwaters to th e mouth of the Save River. During the dry season, in Summer, concentrations and fluxes of NI and TP were strongly influenced by instream biological pr ocesses 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 appe ars 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 rainies t season, i.e. in Spring, phosphorus fluxes were also related to forests wh ich are mainly located in the steep slope upstream part of the drainage bas in. (C) 1998 Elsevier Science Ltd. All rights reserved.