SEASONAL AND SPATIAL TRENDS OF NITROGEN AND PHOSPHORUS LOADS TO THE UPPER CATCHMENT OF THE RIVER VILAINE (BRITTANY) - RELATIONSHIPS WITH LAND-USE

Influences of urban and agricultural activities on river water quality were analysed on the upper catchment of the River Vilaine (902 km(2)) , France. Agricultural land covered 78% of the study area. Corn, sass and cereals were the main crops. Total Nitrogen (TN) and Total Phospho rus (TP) loads were determined for individual subcatchments (from 10 t o 194 km(2)) during the hydrological year 1994/95. Relationships betwe en water quality and catchment management were studied. Non-point sour ce produced 95% of TN and 90% of TP of the whole river basin. Signific ant variation in TN and TP occurred, in time and space. The greatest p art was discharged in January and February. Annual river exportation r ates ranged from 20 to 65 kg TN ha(-1) and from 1.15 to 3.05 kg TP ha( -1). Reservoirs will held almost 15% of TN and TP loads. Cyanobacteria blooms occurred in summer and autumn, with significant production of organic matter here, and downstream, in the rivers. Greatest TP river retention rate were determined downstream a town of 15 000 people, and highest TN loss occurred in the subcatchment with the highest density of cattle and the most important percent of permanent grassland.