Ingeneral, eutrophication can be classified into two categories, naturaleutrophication and manmade eutrophication. Natural eutrophication occurs over aperiod of centuries while manmade eutrophication takes place over a decade, inbetween 8-10 years. Natural eutrophication is a slow process and theeutrophication that occurs is temporary. However, manmade eutrophication occursvery quickly. Other than that, natural eutrophication occurs due to theaddition of nutrients by nature itself whereas manmade eutrophication is due tointerference with the ecosystem.Whatare the sources of eutrophication? The sources can also be divided into tworespective categories, point sources and nonpoint sources. Point sources aresources that can contribute nutrients directly to the water bodies whilenonpoint sources are sources that provide nutrients in a ‘diffuse’ manner.Point sources are easy to regulate but nonpoint sources are the opposite.
It isdifficult to regulate and it varies spatially. At times, it is also said thatnonpoint sources can be very troublesome. Some examples of point sources areuntreated sewage from sewage treatmentplants, leaching of inorganic fertilisers especially phosphates and nitratesfrom agricultural lands, run-offs of animal wastes from farms and run-off frommines as well as oil field. Next, examples of nonpoint sources includes run-offsfrom agriculture, pastures, construction sites and also atmospheric depositionover a water surface. For a detailed difference of sources, please refer toFigure 2.0.1 in the Appendices section.
Asstated earlier, eutrophication occurs due to the presence of nutrients in thewater bodies. Phosphorus is often seen as the main reason of eutrophication inlakes although there are many other nutrients found dispelled into the water. Asthe concentration of nutrients in water sources increases, the concentration ofalgae in the same water sources also increases. Researches carried out in theExperimental Lakes Area in Ontario have shown a relationship between theaddition of phosphorus into water sources and the rate of eutrophication. Ithas also been learnt that phosphorus is the limiting factor for the growth of plantsin freshwater bodies.
Besides that, phosphorus anchors tightly to the soil.This leads us to consider that phosphorus is mainly transported via soilerosion. Once phosphorus has been translocated into water sources, theextraction of phosphate is slow. Therefore, it will be much more difficult toreverse the effect of eutrophication.
Saying this, the usage of detergents canbe further explained. Some detergents contain too much of phosphate. However,this phosphate is not removed by sewage treatment and so, it is discharged intorivers. This increases the concentration of phosphate in the water, thus,increases the growth of microscopic algae.Besidesthe usage of detergents, farming also somehow contributes to the rise ineutrophication.
Ever since the Second World War, more and more land has been ploughedto grow arable crops such as wheat, barley and maize. As the soil is beingexposed this way, the bacteria produces soluble nitrates with the aid of oxygenand water. These nitrates are then disposed into water where they encourage thegrowth of algae.
If, by any chance, the nitrates reach the underground stores,they might increase the concentration of nitrates in drinking water. This is consideredunsafe for babies, infants and individuals who have a very sensitive immunesystems.Movingon, the usage of fertilizers in agriculture will also promote eutrophication.As we know, chemical fertilizers contains various minerals and nutrients thatwill help in the crop yield. However, once these nutrients reach highconcentration levels and the ground is no longer able to assimilate them, theywill flow into the water source and cause algae to be produced at a fasterrate.
Fertilizers are not only used for crops but are also used in aquaculture.For instance, aquaculture scientists and pond managers will usually eutrophywater bodies intentionally, that is, by adding fertilizers directly to the watersource. This is to enhance the primary productivity and increase the densityand biomass of economically important fishes. However, theylater realised the result of their actions, when drinking water becameundrinkable at one point.
The estimated damage cost caused by eutrophication inthe U.S. is said to be around $2.2 billion yearly. Besidesthat, factory farming is also one of the leading cause of eutrophication. Pigs,chickens and calves are usually reared in large sheds. Their wastes are washedout by water and this forms ‘slurry’.
If the ‘slurry’ gets in contact withrivers or any water source, it will provide an excess of nitrates andphosphates for the growth and development of microscopic algae.