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The contribution of agriculture to nutrient pollution in three European rivers
The flux of nutrients from land to rivers and seas has increased with time by human activities [27, 21]. This has caused ecological changes in fresh and marine waters [30, 31] and has negatively affected the quality of water for human consumption and other uses. There is a general perception that now that discharges of polluted wastewater from households and industry are being reduced, agriculture is becoming the main source of nutrient inputs to fresh and marine waters in Europe [28, 26]. The principal cause of agricultural nutrient pollution in Europe is the input of nutrients to agricultural land (fertilisers and manure) exceeding the output of nutrients from agricultural land (crop yield). The import of fodder and fertilisers from outside Europe maintain this unbal- anced system. The agricultural surplus of nutrients may potentially runoff to the aquatic environ- ment. Many studies have analysed the relation between agricultural activities and nutrient inputs to the aquatic environment [e.g. 29, 7], but only few studies have analysed this issue at large spatial and temporal scales. Such large-scale analyses are needed in order to design, monitor, and evaluate large-scale policies that aim at a reduction of nutrient levels in large river systems and coastal seas. It is not feasible to measure (diffuse) agricultural inputs of N and P to the surface water for large areas. Therefore, large-scale analyses of agricultural nutrient pollution are most often based on extrapolations from small-scale studies [32, 33], or on large-scale nutrient balances [4, 1, 6, 3]. This paper analyses the contribution of agriculture to the nitrogen (N) and phosphorus (P) loads in the Rhine, Elbe, and Po rivers. The analysis is based on an extensive geo-referenced database on nutrient sources (e.g. livestock numbers, data on wastewater treatment), physical characteristics of landscape and climate and measured river nutrient loads for the Rhine, Elbe, and Po river basins. These data cover the period 1970-1995 and have been used to validate two nutrient balance models that simulate the transfer of N and P from pollution sources to river outlets. Both models are used to quantify the contribution of agriculture to the nutrient load in the Rhine, Elbe, and Po rivers for the period 1970-1995, and to simulate the effect of the implementation of the European Directive on Nitrates from Agricultural Sources [16] on the annual average nutrient load in these rivers for the period 2015-2020. The difference in the outcome of the two models is indicative for the accuracy of our results. Moreover, the results have been compared with the re- sults of other workers, allowing for a critical evaluation of the calculated contribution of agricul- ture to nutrient pollution of European rivers and coastal seas.
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