The map shows the mean annual concentrations of total ammonium (TA) as mg/L N-NH4in European rivers measured at WISE SoE River monitoring stations from 1992 till 2011. All data are annual means. For most countries these are based on measurements over the whole year, but for some countries the annual mean actually represents only the growing season (due to ice cover in winter).The purpose of the map is to provide an overview of the annual concentrations of the total ammonium/ammonium in rivers across Europe and to enable viewer to make comparisons of the values in European countries or individual monitoring sites. NH4 is the empirical formula of the ammonium ion which, in water, is an important nitrogenous fertilizer for aquatic plants. In excess ammonium can give rise to problems because it contributes indirectly to BOD by reducing Dissolved Oxygen concentrations in water. It also forms ammonium hydroxide in water and is extremely toxic to fish and aquatic life at elevated pH levels.Total ammonium concentrations are normally increased as a result of discharges from waste water treatment plants, industrial effluents and agricultural runoff. It exerts a demand on oxygen in water as it is transformed to oxidised forms of nitrogen. In addition it is toxic to aquatic life at certain concentrations in relation to water temperature, salinity and pH. It can also pollute water used for drinking and bathing. In less detailed scale data is aggregated by country and pie charts are displayed. In more detailed scale individual stations appear in shape of triangles. Historical data (from 1992 on) can be displayed and viewed with time slider tool.