What causes eutrophication?

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Sources of eutrophication

Discharge of Denver sewage treatment plant in the Platte River (USA). Photo credit: Jeffrey Beall (2008)

Eutrophication is the (mostly undesired) increase in the concentration of nutrients to an ecosystem. Increased nutrient enrichment can arise from both point and non-point sources (also called 'diffusive pollution sources'):

  • Point source pollution: Pollution that comes from contaminants that enter a waterway from a single identifiable source such as stationary locations or fixed facilities. Examples are discharges from a sewage treatment plant (Fig. 1) or industrial plants and fish farms.
  • Non-point source pollution: Pollution from widespread including human activities with no specific point of discharge or entry into receiving watercourses. Examples are leaching out of nitrogen compounds from fertilized agricultural lands (Fig. 2) and losses from atmospheric deposition.
Manure run-off, an example of non-point source pollution (Photo credit: Chris Court)

The enrichment of water by nutrients can be of a natural origin (natural eutrophication) but is often dramatically increased by human activities (cultural or anthropogenic eutrophication). Natural eutrophication has been occurring for millennia[1]. It is the process of addition, flow and accumulation of nutrients to water bodies resulting in changes to the primary production and species composition of the community. Cultural eutrophication is the process that speeds up natural eutrophication because of human activity. There are three main sources of anthropogenic nutrient input: erosion and leaching from fertilized agricultural areas, and sewage from cities and industrial waste water. Atmospheric deposition of nitrogen (from animal breeding and combustion gases) can also be important[2].

The most common nutrients causing eutrophication are nitrogen N and phosphorus P. The main source of nitrogen pollutants is run-off from agricultural land, whereas most phosphorus pollution comes from households and industry, including phosphorus-based detergents. These nutrients enter aquatic ecosystems via the air, surface water or groundwater. Most of the commercially fixed nitrogen and mined phosphorus goes into production of fertilizer. The rising demand for fertilizer has come from the need to meet the nutritional demands of our rapidly expanding human population. The rise in intensive fertilizer use has serious implications for coastal habitats because greater application results in greater runoff, and the fraction of fertilizer lost from fields will increase with intensity of application. Increased global production of nitrogenous fertilizers have largely been linked to concerns over the relationship between water quality and eutrophication. Nutrient removal in sewage treatment plants and promotion of phosphorus-free detergents are vital to minimize the impact of nitrogen and phosphorus pollution in Europe's waters[3].


Some quantitative estimates

The emissions of N and P to the coastal ocean have doubled during the past century. More than half of these emissions are related to agriculture[4]. In 2020, the global annual production of anthropogenic reactive N (mainly ammonia, urea, nitrate) was in the order of 260 Tg N (=260 million tons N), of which 160 Tg is produced synthetically (120 Tg used as fertilizer), 25-40 Tg comes from fossil fuels, 30-35 Tg from N fixation by cultivated crops and rice, 15-20 Tg from animal manure, 10 Tg from sewage and 2-3 Tg from mariculture[5][6][7][8]. About one quarter of this production finds its way (mainly through river runoff and atmospheric deposition) to the coastal ocean[5], which receives an annual load of 60-70 Tg N[4]. China is by far the greatest user of N fertilizer, taking 31 % of the total share. Other important users are India, USA, Europe, Brazil and Pakistan (see Fig. 3).

The annual global anthropogenic production of mineral phosphorus P in 2020 is estimated at about 20 Tg P [8], almost entirely based on mining of phosphate rock. When the use of livestock slurry and manure is included, the total phosphorus application in agriculture amounts to 25–29 Tg P per year[6]. It is estimated that about 10 Tg P is flushed to the coastal ocean[9]. Part of this emission is the gradual release of the historical phosphate enrichment of soil and sediments from human activities over more than a century[4]. Because phosphorus is hardly transported by air (only as aerosol), anthropogenic P is more strongly concentrated near the land-sea interface than anthropogenic N.


Fig. 3. Estimated net anthropogenic nitrogen inputs (NANI) according to the world’s main river catchments. Source: Billen et al., 2013[10].


Possible measures for eutrophication reduction

Public awareness and political priority are overarching requirements for tackling the eutrophication problem[11]. Eutrophication is largely related to agriculture; point sources of nutrients play a minor role and can more easily be tackled[4][5]. Dedicated fertilization strategies taking into account the N:P balance and agricultural practices such as crop rotation, drip irrigation, etc. can avoid depletion of soil nutrients[12]. Nutrient leaching from agriculture can be reduced by recycling animal manure to cropland within watersheds. Individual behavior with regard to nutrition plays an important role; in large parts of the world, there is overconsumption of protein-rich food. Of the 120 Tg N used for crop and grass production, about 100 Tg is consumed by livestock while only 20 Tg is available for direct human consumption[13]. Preservation and restoration of landscapes, especially at the land-water interfaces, promotes denitrification processes and contributes to reducing the nutrient discharge to coastal waters[5]. Eutrophication from mariculture can be reduced by fish farming in land-based recirculating systems along with limited use of animal feed[11].


Related articles

Possible consequences of eutrophication
Harmful algal bloom
Continental Nutrient Sources and Nutrient Transformation
Algal bloom dynamics
Threats to the coastal zone
European policy on eutrophication: introduction

See also other articles in the Category:Eutrophication.


Further reading

Our Nutrient World: the Challenge to Produce More Food and Energy with Less Pollution (2013) M.A. Sutton, A. Bleeker, C.M. Howard (eds.) https://www.pbl.nl/en/publications/our-nutrient-world-0


References

  1. De Jonge, V.N., Elliot, M. and Orive, E. 2002. Causes, historical development, effects and future challenges of a common environmental problem: eutrophication. Hydrobiologia 475-476: 1-19
  2. EEA, 2005. Source apportionment of nitrogen and phosphorus inputs into the aquatic environment. European Environment Agency Report 7, Office for Official Publications of the European Communities: Luxembourg. ISBN 92-9167-777-9. 48 pp
  3. EC, 2002. Eutrophication and health. European Commission, Office for Official Publications of the European Communities: Luxembourg. ISBN 92-894-4413-4. 28 pp
  4. 4.0 4.1 4.2 4.3 Le Moal, M., Gascuel-Odoux, C., Menesguen, A., Souchon, Y., Etrillard, C., Levain, A., Moatar, F., Pannard, A., Souchu, P., Lefebvre, A. and Pinay, G. 2019. Eutrophication: A new wine in an old bottle? Science of the Total Environment 651: 1-11
  5. 5.0 5.1 5.2 5.3 Malone, T.C. and Newton, A. 2020. The Globalization of Cultural Eutrophication in the Coastal Ocean: Causes and Consequences. Front. Mar. Sci. 7:670
  6. 6.0 6.1 Penuelas, J., Poulter, B., Sardans, J., Ciais, P., van der Velde, M., Bopp, L., et al. 2013. Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe. Nat. Commun. 4: 2934
  7. Lu, C. and Tian, H. 2017. Global nitrogen and phosphorus fertilizer use for agriculture production in the past half century: shifted hot spots and nutrient imbalance. Earth Syst. Sci. Data 9, 181–192
  8. 8.0 8.1 FAO. 2019. World fertilizer trends and outlook to 2022. Rome
  9. Seitzinger, S.P., Mayorga, E., Bouwman, A.F., Kroez,e C., Beusen, A.H.W., Billen, G., Van Drecht, G., Dumont, E., Fekete, B.M., Garnier, J. and Harrison, J.A. 2010. Global river nutrient export: A scenario analysis of past and future trends. Global Biogeochemical Cycles 24, Gb0a08
  10. Billen, G., Garnier, J. and Lassaletta, L. 2013 The nitrogen cascade from agricultural soils to the sea: modelling nitrogen transfers at regional watershed and global scales. Phil. Trans. R. Soc. B 368: 20130123
  11. 11.0 11.1 Grimvall, A., Sundblad, E-L. and Sonesten, L. 2017. Mitigating marine eutrophication in the presence of strong societal driving forces. Report No. 2017:3. Swedish Institute for the Marine Environment
  12. IFA, 2007. IFA International Workshop on Fertilizer Best Management Practices, Brussels, Belgium, 7-9 March 2007. Imprint: Paris : International Fertilizer Industry Association, 2007
  13. Sutton, M.A.B., Howard, C.M., Bekunda, M., Grizzetti, B., de Vries, W., van Grinsven, H.J.M., Abrol, Y.P., Adhya, T.K., Billen, G., Davidson, E.A., Datta, A., Diaz, R., Erisman, J.W., Liu, X.J., Oenema, O., Palm, C., Raghuram, N., Reis, S., Scholz, R.W., Sims, T., Westhoek, H. and Zhang, F.S. 2013. Our Nutrient World: the Challenge to Produce More Food and Energy with Less Pollution. Centre for Ecology and Hydrology, (Edinburgh) UK, pp. 1-128


The main author of this article is Knockaert, Carolien
Please note that others may also have edited the contents of this article.

Citation: Knockaert, Carolien (2020): What causes eutrophication?. Available from http://www.coastalwiki.org/wiki/What_causes_eutrophication%3F [accessed on 5-12-2020]