This paper deals with water quality assessment and recommendations for a classification system based on nitrogen (N) and phosphorus (P) concentrations. In order to evaluate the influence of agricultural intensity, climate and hydrology on water quality, the long-term data (from 1995 to 2009) collected in three Latvian diffuse pollution monitoring sites (Berze, Mellupite and Vienziemite) were analysed. Measurements were carried out within areas where agriculture was the main source of diffuse nutrient loading at four spatial scales, i.e. experimental plot, drainage field, small catchment and river. The available long-term data series shows large variations in nutrient concentrations, depending on the intensity of agricultural production system and the scale of measurements. The concentrations of total N are higher at the plot scale, decreasing when the spatial scale of measurements increase. The proposed simplified classification system (five classes) was based upon the assumption that good chemical status for rivers in agricultural areas represent concentrations of total N < 1.5 mg L−1 and total P < 0.075 mg L−1, while in small catchments total N < 2.5 mg L−1 and total P < 0.050 mg L−1 and in subsurface drainage water total N < 5.5 mg L−1 and total P < 0.020 mg L−1.

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