The literature search strictly concerns the pollution of marine waters through the two nutrients nitrogen (N) and phosphorus (P) from agricultural sources, whether from non-point or point sources. It also shows the pathways of nitrogen (N) and phosphorus (P) losses from agriculture to watersheds, surface waters, rivers and estuaries and to the air. Recognizing that there are also other sources of pollution with nitrogen (N) and phosphorus (P), it also includes to some extent publications covering losses from agriculture as well as from such other sources.

Man and animals depend on plants to live and to reproduce. For normal growth and development plants need air, water and nutrients. Nutrients are indispensable for all plant growth. There are six plant nutrients which plants use in relatively large amounts: nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg), the so-called macronutrients. Necessary nutrients, which are however, only required in small or trace amounts are: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl) and boron (B), the so-called micronutrients.
Furthermore, there are some beneficial nutrients for some plants such as silicon (Si), sodium (Na) and cobalt (Co).

In addition to carbon as CO2 from the air and hydrogen (H) and oxygen (O) as water (H2O), plants receive the nutrients from the soil, organic substances, plant residues, animal manure and mineral fertilizers. In addition to the depletion of nutrients through the removal of the growing plants, soil-borne nutrients and those applied to the soil may be lost through leaching, soil fixation, erosion and volatilisation. Losses caused by de-nitrification (N2, N2O), volatilisation (NH3/NH4) and naturally occurring leaching (NO3), are unavoidable, even with best agricultural practices.

Losses of nutrients may have negative consequences for the environment: losses of N in form of nitrate (NO3) may pollute ground- and drinking water. Phosphorus, which is mainly lost through erosion, may cause eutrophication in surface waters and lakes. Both these nutrients may further be transported by rivers and streams to estuaries, marine waters, seas and oceans.

Nitrous oxide (N2O), which is formed in a side reaction - though in relatively small quantities - of the de-nitrification of nitrate (NO3), and the nitrification of ammonia, is as a source of NOX an important greenhouse gas, and may contribute to the global greenhouse potential (236, 298, 306).

Oceans, seas and continental shelves are the most productive extensive areas in the world. For normal development the phytoplankton also needs nutrients (72). These are mainly nitrogen (N), phosphorus (P) and carbon (C), which are needed in a given proportion (Redfield ratio: C : N : P = 100 : 16 : 1) (25, 68, 92). Of some further importance are silicon (Si), and trace elements such as cobalt (Co), molybdenum (Mo) and iron (Fe). Whereas in general the limiting nutrient for normal phytoplankton/algae growth in inland waters and lakes is phosphorus (P) (109, 343, 356, 368), it is nitrogen (N) (162, 320) in oceans and seas (85, 106, 107, 108, 109, 131, 132, 142, 144, 156, 162, 176, 179, 343, 368).

The nutrients nitrogen (N) and phosphorus (P) which are lost from agriculture (but also from other human activities such as urbanization, traffic and industries) to rivers discharging to seas and oceans (estuaries, bights and shelves) may affect the quality of the marine waters, i.e. the aquatic ecosystems. However, nitrogen (N) - fixation (227), phosphorus (P) - sedimentation (350), the sediment water interface, tidal effects, floods (173, 351), the water residence time (228), as well as climatic and seasonal variations (213, 324, 329, 356) all also play a complex and important role (23, 62, 83, 117, 134, 233). Hence, where continental shelves are dominated by cross-shelf advection, the increased inputs from rivers are relatively minor (229), a similar effect refers to water exchange processes (currents) (274).

A negative effect may also be caused by the volatilisation of nitrogen, i.e. by evaporation of ammonia (NH3), which reacts relatively fast to ammonium (NH4), which contributes to nitrogen (N) - build-ups in marine sediments and to the acidulation of soil and waters.