The amount of nutrients (N, P, Si) carried to the coastal zone by large river systems, as well as the balance between these elements, are the major determinants of coastal marine eutrophication problems. Many of these problems are the consequence of the new production of non-siliceous algae, sustained by nitrogen and phosphorus delivered by rivers in excess over dissolved silica, in regard to the requirements for diatom growth.
Besides their N:P:Si ratios, an indicator of coastal eutrophication potential (ICEP) of riverine nutrient inputs is proposed. It represents the carbon biomass potentially produced in the receiving coastal water body through new production sustained by the flux of nitrogen or phosphorus (according to which one is limiting with respect to the other) delivered in excess over silica. For the purpose of a river to river comparison, it is expressed by unit of watershed area, in kg C km- 2 day- 1.
This indicator reaches high, positive values in many temperate European and North American river systems known to cause severe eutrophication problems in their receiving coastal zones, while it remains at negative values for pristine Nordic rivers as well as for most (sub-)tropical river systems.
The analysis of reconstructed past scenarios of the biogeochemical functioning of the Seine and Scheldt river systems, as well as a number of retrospective studies carried out for other river systems under temperate conditions and strong human influence, shows that both nitrogen and phosphorus were delivered to the coastal zone in excess over silica as soon as the beginning of the 19th century. The period 1950–1990 was the worst in terms of unbalanced nutrient delivery to the coastal zone. Since the 1990's, phosphorus fluxes have decreased very rapidly, largely as a result of improved urban wastewater treatment and concomitant banishment of polyphosphates in the washing powders, while nitrogen delivery still remains at high levels as a consequence of intensive agricultural practices, creating an unprecedented biogeochemical situation in the coastal zone, with low phosphorus but high nitrogen contamination.