Transport of nutrients in rivers: Investigations on the Po River (Italy)

The identification of the characteristics of transport of phosphorus compounds under different river flows, and the study of the ratio of liquid to solid phase phosphorus loads was possible by sampling Po River waters at the basin closing section. Our observations show that the concentrations of total solid matter transported are determined not only by the extent of water discharge and the succession of different river flows; they also depend on whether contributions are mainly from the Alps or the Apennines. Total phosphorus and organic matter concentrations in suspended sediments decrease as turbidity and discharge increase; we may assume that the minimum total phosphorus values are the typical 'natural levels' of Po Valley basins. Dissolved phosphorus is extremely variable with similar discharges; the whole data base, however, evidences a general inverse proportionality between these two parameters, with a significant difference between winter and summer samples which is due to differences in biological consumptions. The analysis of the liquid/solid phosphorus ratio highlights the importance of the dissolved fraction in medium-low river flows, whereas 90-98% of this element is associated with sediments on high water or flood events.