Seasonal and spatial trends of nitrogen and phosphorus loads to the upper catchment of the river Vilaine (Brittany): Relationships with land use

Abstract

Influences of urban and agricultural activities on river water quality were analysed on the upper catchment of the River Vilaine (902 km2), France. Agricultural land covered 78% of the study area. Corn, grass and cereals were the main crops. Total Nitrogen (TN) and Total Phosphorus (TP) loads were determined for individual subcatchments (from 10 to 194 km2) during the hydrological year 1994/95.Relationships between water quality and catchment management were studied. Non-point source produced 95% of TN and 90% of TP of the whole river basin. Significant variation in TN and TP occurred, in time and space. The greatest part was discharged in January and February. Annual river exportation rates 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.