Large rivers, especially large lowland rivers, often have their nutrient concentrations modified by industrial emissions, sewage and agricultural effluents, and they also are likely to have their flow regime regulated by dams. Because their catchments include a substantial land area, each large river has its individual climate and geology.
For these reasons it is difficult to generalize, and interested readers should consult studies of specific rivers (e.g. Whitton, 1975b; Davies and Walker, 1986; Dodge, 1989; and detailed descriptions of individual river systems in the Monographiae Biologicae series by W.Junk, The Hague). Even to summarize data for a given river is problematic, because values change greatly along a river’s length, seasonally, and between cleaner and polluted sections.
Meybeck (1982) provides a comprehensive review of the various forms (dissolved and particulate, organic and inorganic) of carbon, nitrogen and phosphorus in world rivers. Natural levels are estimated from small streams in the temperate zone and major rivers of the tropics and subarctic, where anthropogenic inputs are thought to be minimal. Nutrient concentrations from these systems are very similar to the average content of rain, supporting the view that they represent an unpolluted state. Natural levels of dissolved phosphorus are very low, around 0.01mg1-1 for PO4-3 and 0.025mg1-1 for total dissolved phosphate, which includes the organic form. Natural levels of DIN (including ammonium, nitrate and nitrite) are also low compared with rivers affected by human activities, about 0.12mg1-1,and nitrate is the major fraction. Ammonia averages15﹪, and nitrite about 1﹪, of DIN. BecauseNH4+ is usually the preferred form of inorganic nitrogen, data on nitrate alone or total inorganic nitrogen may be of limited usefulness.