Global warming, pollution of waters by micropollutants, health problems linked to the bioaccumulation of toxic chemical substances, or the upsetting of hormonal function by endocrine disruptors are all essentially consequences of human activities. And they are not the only ones.

The eutrophication of aquatic environments is another topic of major concern, as much for human health as for the environment or the economic system. Plagued by green algae-laden tides, the farming region of Brittany also has observers worried.. The same is true of the proliferation of cyanotoxins in bathing waters.

What is eutrophication? What are the causes and consequences? What solutions are there? Here are some answers.

Understanding the eutrophication of aquatic environments

Nutrients, such as phosphorus and nitrogen, are essential to the growth of aquatic plants. However if these arrive in excessive quantities, some algae are “doped” and multiply faster than they should. Light transmission is obstructed and the bacteria necessary to their breakdown proliferate, consuming ever more oxygen, until the ecosystem becomes unbalanced, or even “asphyxiated”. This is what is referred to as the eutrophication of aquatic environments.

While eutrophication is a natural phenomenon, human activities – whether domestic (waste waters), industrial or agricultural – result in “artificial” eutrophication that is prejudicial to aquatic ecosystems by progressively modifying their biological equilibrium.

Eutrophication of aquatic environments: in numbers

The two nutrients essentially responsible for water pollution are nitrogen and phosphorus, found mainly in fertilisers and waste waters. When the amounts that plants can absorb are exceeded, these nutrients end up in waters and promote eutrophication.

With global consumption of synthetic nitrogen fertilisers multiplying by a factor of nine since the 1960s, the nitrate content of waters has increased considerably. In France, this currently exceeds the World Health Organisation’s limit of 50 milligrammes per litre. 66% of nitrate pollution  derives from agriculture, 22% from local authorities and 12% from industry.

What are the causes of eutrophication in aquatic environments?

Nitrogen fertiliser use, agricultural effluents, and domestic and industrial waste waters are responsible for most of the pollution of aquatic environments. Brittany’s green algae tides are a prime example: rainwater run-off carries away nitrogen and phosphorus contained in slurry, manure and fertiliser before flowing back into coastal water courses and lakes.

The amounts of nutrients contained in industrial and domestic waste waters are also a factor, especially if not properly treated.

Other factors contributing to eutrophication: urbanisation, land use, and changes made to water courses. Impermeable surfaces, such as roads and roofs, increase run-off of pollutant-laden waters toward water courses. Deforestation and the removal of vegetation in turn have a doubly negative impact, both accelerating soil erosion and promoting the transport by rainwater of nutrient-laden sediment toward water courses; with the disappearance of hedges and grassed areas, those “buffer zones” can no longer limit the run-off of rainwaters and can no longer store nutrients. Finally, modifications made to rivers (displacement, straightening, dredging, stabilisation etc.) may make banks less stable, also promoting erosion.

What are the consequences of eutrophication?

This form of pollution represents a threat to the environment, public health, and the national economy, particularly in the fishing and tourism sectors:

  • Increased concentration of nitrogen and phosphorus in waters favours certain plant and algae species to the detriment of others, which are then deprived of sufficient light and oxygen. It also results in loss of biodiversity, which can sometimes be dramatic.
  • The quality of drinking water is also affected because strong concentrations of nitrates become toxic, particularly to infants.
  • Excessive presence of nutrients in bathing lakes will result in abnormal amounts of cyanobacteria. These produce powerful toxins called cyanotoxins which, following contact, swallowing or inhalation, can cause digestive issues and may even reach the liver, the nervous system, the reproductive system or the mucous membranes of bathers. Where threshold levels of toxigenic cyanobacteria are exceeded, the authorities are often forced to close recreational waters.

Fish caught while angling in polluted waters may also be unfit for consumption.

De-eutrophication, a remedial solution

The de-eutrophication process is the reverse of eutrophication. This remedial solution aims to de-pollute ecosystems by reducing nutrient inflow. Water purification may be achieved by lagooning (particularly phyto-purification or bioremediation), as well as by mechanical, chemical or biological removal of algae and other unwanted plants, or again by the introduction of aquatic herbivorous species. However, this solution must be closely managed so not to reverse the concentrations of nutrients to outside the normal limits. Preventive solutions are therefore preferable.

Preventive solutions to tackle eutrophication

In France, the decree of 30 January 2023 relating to Nitrate action programmes imposed good agricultural practices in vulnerable areas: balanced fertilisation, periods during which the spreading of nitrogen fertilisers is prohibited, autumnal soil coverage by intermediary crops known as nitrate traps, mandatory grass strips along water courses and minimum storage capacities for livestock effluents. These programmes may be steered using planning tools such as SAGE (French water development and management plan) and SDAGE (French water development and management master plan). Planning can also integrate means for landscaping the area and soils to limit run-off, erosion and transfer of pollutants, for example.

However, these preventive solutions cannot limit themselves only to vulnerable areas and agriculture. The preservation and restoration of soils (grassy banks, hedgerows, etc.) should be applied to every environment. Utilisation of fertilisers, like the usage of household or industrial products, needs to be rational, or at least limited. The treatment of waste waters and livestock manure has to be improved in order to better remove nitrogen and phosphorus before their discharge into the environment.

Combating eutrophication: prevention is better than cure

Essentially the result of human activity and its excessive use of chemical products, eutrophication of aquatic environments may have serious consequences for the wider environment, human health and the global economy.

Even if depollution of waters is possible, it is not a remedial solution and may not be sustainable. It is therefore imperative that the combat of eutrophication be achieved by prevention. Because this affects each and every one of us, it can only be tackled through collective action.


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