Fig. 8. Costs of measures to reduce 1 kg of nitrogen (a) and 1 kg of phosphorus (b) (after Gren et al. 2008) in the Oder/Odra river basin. Some measures focus on emission prevention and others focus on nutrient retention. In comparison, the nutrient-removal costs for a small 0.1-ha (1000-mē) and a large 1000-ha (10-km²) zebra mussel farm are shown.