THE LARGE-SCALE FLUX OF NUTRIENTS FROM LAND TO WATER AND THE EUTROPHICATION OF LAKES AND MARINE WATERS

The large-scale flux of nitrogen and phosphorus in modern society is c oupled to the exponentially growing world population. During the last three decades there has been a several-fold growth in use of nitrogen (N) and phosphorus (P), for example, for agriculture production, in ch emical products such as in detergents, and as food additives. Feeding a growing population means increased human nutrient excretion. The mob ility of N differs from that of P. For the easily mobile nitrate ion, river exports are positively correlated to the size of local human pop ulations. P on the other hand is fixed in different systems, at least temporarily. Modern agro-ecosystems accumulate about 60% of the annual input of P. The Stockholm region representing an urban ecosystem serv ed by dephosphatation in sewage, accumulated about 80% of imported P d uring 1990. With increasing accumulation there is an apparent risk for increasing non-point source pollution. In some shallow recipient lake s the sediments have been saturated with P. Even after the reduction o f external loading these lakes are exporting more P than they are impo rting. Lakes normally are traps for P. In order to prevent damage to n atural resources and to stop the large-scale flux of nutrients from la nd to water, the world population growth rate must be decreased and a number of measures taken: agricultural structures must be changed incl uding new concepts for the use of fertilizers; new municipal sewage tr eatment processes must be developed permitting high-degree recycling o f nutrients; unnecessary use of nutrients must be stopped, for example , in detergents and by reducing nutrient additives in food and drinks. Cola drinks, for example, contain 180 mg P l(-1), a concentration abo ut 350 times higher than that of the effluent (0.5 mg P l(-1)) from mo dern sewage treatment plants operating with dephosphatation. Financial resources and effort are required to stop large-scale eutrophication of marine waters.