Trophic status of rivers in the Forth catchment,Scotland

1. An increase in nutrient concentrations in fresh water may pose a threat to water quality, biodiversity, and the uses to which these resources are subject. Compared with standing waters, the trophic status of running water has been neglected with few studies describing the extent of eutrophication in rivers. 2. This paper uses a range of information, including chemical analysis, subjective measures of algal abundance, and professional assessments to describe the trophic status of rivers in the Forth catchment, central Scotland. The importance of ‘low level’ data for achieving a wide geographical cover is emphasized. 3. Information collected between 1985 and 1994, now held by the Scottish Environment Protection Agency, was used to create a river classification scheme which allows the definition of trophic status using data on soluble reactive phosphorus concentration and algal abundance. Two thirds of the 3250 km of river were classified, showing that 74% of the river length had low nutrient concentrations and were defined as oligotrophic or mesotrophic. 4. Diffuse sources of phosphorus were found to be the most important factor determining the trophic status of rivers, with 71% of the river length controlled by diffuse inputs and only 7% by point sources. The importance of point sources increased with eutrophication; however, even in the most enriched category, point sources were thought to dominate the supply of nutrients in only 34% of the river length. © 1997 John Wiley & Sons, Ltd.