Introduction

Pollution sources to aquatic ecosystems can be categorised as point (or direct), those derived from identifiable sources such as sewage treatment plant outfalls, or diffuse, where the source of pollutant is more difficult to identify, such as surface erosion. In the former case, the effluent loads can be high; however, by virtue of a more clear relationship between source and impact, cause is more readily identifiable and solutions more readily encouraged or directed (Smol 2008). Diffuse pollution sources often create chronic symptoms of elevated pollution loads that are more difficult to establish experimentally and more difficult to identify spatially. In many instances, the drivers of these heightened releases of pollutants to receiving waters have a long history and originated from settlements and developments that extend beyond the memory of modern societies. The widespread and deep-in-time nature of diffuse sources of pollution, coupled with their nature as being, effectively, multiple point sources, renders the identification of the causes of diffuse pollution uncertain and so poses a greater challenge in terms of mitigation.

Diffuse pollutants are most often represented by sediments and solutes. Widespread vegetation clearance, catchment settlement, intensive tilling and cropping and excessive stocking rates of grazing animals all contribute to exposing surface soils to erosive forces that increase sediment loads to aquatic systems. This acts to increase sedimentation rates in streams and lakes and to increase the turbidity of the water itself.