Many rivers have become more eutrophic over the past 40 years [7–9]. It is estimated that some 70% of total nitrogen in inland waters is from diffuse sources such as agricultural runoff, precipitation and urban waste, while 43% of soluble reactive phosphorus (SRP) is from agriculture, 24% from domestic/human sewage waste and 19% from industrial detergents [10]. Changes in agricultural methods, (e.g. increased use of artificial fertilizers, a switch to autumn-sown cereals, and the ploughing of permanent pastures) have led to a six-fold increase in the amount of nitrogen in many lowland waterways, while heavily fertilized soils result in significant leaching of phosphorus.