Chemical elements at or near the earth's surface and in the air are in a continuing state of movement or subject to forces which can cause movement. The outstanding characteristic is that outgo of inorganic elements (including plant nutrients) from land to water greatly exceeds input over most land surfaces of the earth. Nutrients have been accumulating under water along the shorelines of the world for ages of time.
When man entered the geological picture, this movement (or water-enrichment process) became more active as the soil was worked to grow food. This caused more soil particles, nutrients, and organic matter to move into water bodies.
Eutrophication is nature's age-old geological process of utilizing these nutrients and the sun's energy for the accumulation of organic matter in aqueous environments. Without eutrophication, there would be no marine life or coal, oil, or muck deposits on our planet.
All bodies of water have four stages of life: birth, existence, old age, and disappearance. Examples of each stage are found in the USA, Canada, Siberia, Brazil, and Africa, in surroundings practically untouched by man and certainly untouched by agriculture.
Only very small quantities of nitrates and mineral nutrients are necessary to sustain luxuriant growth of aquatic plants. Most water bodies contain ample amounts for this purpose. Few situations likely can ever be found where stopping agricultural's contribution would stop eutrophication. Slowing the process of eutrophication will be difficult because it will require control over C inputs. Organic matter from sewage, manure runoff, industrial waste, or wildlife is utilized by bacteria, which evolve CO2 for promoting the growth of aquatic plants. Once the process is in operation, it is difficult to control because photosynthesis releases oxygen to stimulate bacteria. When they and other plants die, their C and nutrients are consumed by more bacteria, which in turn, evolve more CO2 to grow more algae, thus continuing the cycle. Although details of this symbiosis are not fully understood, enough is known to realize that removing P or N from city effluents or from farm fertilizers will not control growth of aquatic vegetation in natural or man-made bodies of water.