Eutrophication

The eutrophication of an aquatic environment, such as pond or river, indicates originally and simply its eutrophic character (of the Greek have : “well, truth” and trophein : “to nourish”), i.e. wealth of nutritive elements, without negative character. Starting from the Years 1970, the term was employed to qualify the degradation of the big lakes like the Lac of Annecy, the Lac of Le Bourget or the Lac Léman by excess of nutrients. It has today a direction close to dystrophy and often comes like qualifier from negative direction for aquatic environments from fresh water or sailors.

An aquatic environment low in nutritive elements is known as Oligotrophe; in the intermediate case, one qualifies the medium of Mésotrophe. Since the natural factors produce mediums more or less charged in nutrients apart from any human intervention, the state of eutrophication of an aquatic environment must be appreciated according to its nature and cannot base itself on absolute indicators.

Eutrophication is also one of the stages of the natural processes which slowly transform the Lac S not very major into Marais, then in meadow or Mégaphorbiaie S and finally in Forêt. The filling of a pond or a marsh is very accelerated by the presence of artificial nutrients, by the proximity of trees (cf dead sheets), but also by the absence of fauna nourishing itself in water while exporting the nutrients (Ex: Amphibian S, Duck S or dash eating watery algae, invertebrates, and plants, by tens of kg per day in the case of the dash). The alluvial deposit of a small pond in underwood can be done in a few decades, whereas the natural lakes even fill them of tens of thousands in million years.

Causes of eutrophication

Eutrophication usually indicates the imbalance which results from an excessive contribution of Nutriment S: Nitrogen (of nitrates for example), Carbon (carbonates, hydrogénocarbonates, organic matters…) and Phosphorus in particular. Phosphorus being generally the factor limiting in the natural aquatic environments (law of Liebig), they are its compounds, in particular the Phosphate S (orthophosphates, polyphosphates) which allow the racing of the process. This unbalanced medium, dystrophe , become hypertrophe then.

This process can result from agricultural spreadings (manures rich in nitrogen and phosphorus) as well as use of detergent products rich in polyphosphates, on the Bassin pouring lake or river.

The increase in the rejections industrial or urban or agricultural excessively rich in Manure (nitrates, ammonium), the presence of Polyphosphate S in the Lessive S make eutrophication a frequent process, reaching even the oceanic zones, being able to cause the extension of dead zones), or the development of toxic algae, such Dynophysis , on the littorals, for example in Brittany (France). In the current meaning, eutrophication is thus often synonymous with Pollution, although this one can cover many other aspects: biological contamination (Bacterium S, parasite S…), chemical (Pesticide S, metals, Solvent S…) or physics (Heat, Radionuclide S…).

Fishing is a cause of eutrophication when réempoissonnement are excessive and that balls of starter are thrown in closed ponds, channels or river to slow current.

Mediums touched by eutrophication

Eutrophication can reach fresh water, brackish and salted, the seamen circle like the continental environments, deep water like surface waters, and in particular:

  • stagnant waters (Mare S rich person in dead sheets or collecting waste water, the water polluted by Engrais, pond S, Lac S, Lagune S.)

  • the rivers having a low flow or which accommodate effluents too rich or in too great quantities resulting for example, farms, human or industrial;
  • the Estuary S, Gulf S, bay S and others half-closed extents are particularly touched, because located downstream from the basins slopes. Thus UNO alerted in 2003 in his report/ratio GEO 3 on the fact that in 1998, more than 60% of the estuaries and bays of the USA were “ moderately or seriously degraded by the contamination caused by the nutritive elements ”, in particular because of the nitrogen contributions mainly. A hundred dead zones appeared at sea, downstream from the estuaries. Greatest measurement more than 20.000 km2, downstream from the the Mississippi.

The process

Eutrophication can break up into some stages:

  1. of the nutrients, in particular the Phosphate S and the Nitrate S resulting from agriculture, are poured in great quantity in the aquatic environment;
  2. water thus enriched allows the fast multiplication of the watery plants, in particular the proliferation of Algue S, (Efflorescence algale, or bloom );
  3. the stock of oxygen being very limited in water (approximately 30 times less than in the same volume of air), this one is quickly exhausted during the periods during which the Respiration of the organizations and the decomposition of the produced matters exceed the production by photosynthesis and the exchanges possible with atmospheric oxygen.
The possible development of floating plants - the such duckweeds (Lemna sp.) , thus prevents the passage of the light photosynthesis in the lower layers of water, and also obstructs the exchanges with the atmosphere;
  1. the medium becomes easily then hypoxic then anoxic, favorable to the appearance of made up Réducteur S and foul air (Thiol S, Méthane);
  2. it can result death from it from watery organizations Aérobie S - insects, shellfish, fish, but so vegetable -, of which decomposition, consuming oxygen, imbalance amplifies.

Effects of eutrophication

The principal disadvantages of eutrophication are the reduction in the Biodiversité and water quality as a resource. It has negative effects on tourism (with often like visible consequences the loss of transparency, development of odors and Envasement), which are indication of problems:

  • increase in the biomass algale;

  • increase in the biomass of the gelatinous zooplancton;
  • deterioration of the organoleptic qualities of water (aspect, color, odor, savor);
  • faster silting, and appearance of putrid, dark and malodorous vase.
  • toxic development of phytoplankton;
  • development the pathogenic ones by reduction in the penetration of UV which has a disinfecting capacity.
  • reduction in the biotic index;
  • reduction in the biodiversity (animal and vegetable);
  • reduction in the output of fishing (though the effect can be contrary);
  • and if a stage of dystrophy is reached, died of the higher organizations (Macrophyte S, Insecte S, Cnidaire S, Crustacé S, Mollusque S, Poisson S, etc).

Sometimes the algae can stop the hydrants, the filters, block the operation of locks even engine of small boats for the filamentous algae

The case of the big lakes

In the years 1950 with 1970, the big lakes had become the natural outfalls of sewers of the surrounding cities. Rich person out of nitrogen and phosphorus, the urine of the inhabitants sufficed for strongly degrading the quality of the aquatic environment. To that other pollution like those of the many machines was added to engine of the time, very pollutants, who could contaminate water by the scrubbing of their smoke and their oil rejections.

The remedy

Eutrophication is revealing testifying to the limit of the capacities of self-purification of the aquatic environments. Means of fight are necessary and exist:

  • to decrease the use of pollutants eutrophisants as of the upstream of the catchment area;

  • to decrease the use of pesticides and their arrival in the rivers where, while killing out of many organizations, those can contribute to eutrophication;
  • to use manures in agriculture rationally (to analyze the agronomic value grounds and to privilege natural manures);
  • to arrange basins slopes reconstituting of the networks of Scrap-metal, slope, hedges, and enherbées bands, sufficient cuts some and coherent with the relief and the Pédologie; the streaming of rain water can support the drive of nutrients as phosphorus which will be better retained if the capacities of infiltration of the ground are restored;
  • to replace phosphates of the Detergent S by antiliming agents without impact on the Environment, the such Zeolite S;
  • to better eliminate nitrogen and phosphorus in purification plants (which can be equipped with processes of Dénitrification and Déphosphatation).

See too

External bonds

  • Exposé on the eutrophication of the basin of the Seine
  • Carte of the French forests touched by eutrophication (INRA)
  • Lacs in danger
  • Let us act while it is time!
  • banner page atlas paneuropéen on the eutrophisants (in English)

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