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Eutrophication: better control through improved understanding

Eutrophication in lakes, rivers, and coastal waters manifests itself via blooms of algae, some of them toxic. At the request of the French Ministries of Agriculture and of the Environment, scientists at CNRS, IFREMER, INRA, and IRSTEA wrote a collective scientific report on eutrophication. The results were presented at a colloquium that took place on September 19, 2017.

Prolifération et accumulation d’algues vertes du genre Ulva appelées communément « laitue de mer » sur la plage de Saint-Michel en Grève (côte d'Armor) liées à l'eutrophisation du milieu marin.. © INRA, LE BASTARD Rémi
By Nicole Ladet, translated by Jessica Pearce
Updated on 07/12/2018
Published on 09/19/2017

Eutrophication causes major disturbances in aquatic environments, affecting ecosystem resources and services, environmental and human health, and regional economic activity. It is therefore responsible for both serious environmental problems as well as conflicts among stakeholders, whose priorities may differ. The government has sought to limit eutrophication by regulatory means, which has provoked discussion and debate regarding the phenomenon's underlying causes. Recently, around 40 scientists from France and other countries came together to produce a joint scientific report assessing the known manifestations, causes, consequences, and predictability of eutrophication events.

Anthropogenic eutrophication

Eutrophication occurs when nitrogen or phosphorus levels in aquatic systems increase or become imbalanced due to anthropogenic factors. Plant growth ramps up, and the swell of vegetation gradually obstructs the penetration of light into the water column. Soon, growth is limited by light rather than by nutrients, which are present in excess. The system's original plant species are then replaced by opportunists—like water ferns (Azolla species) or water chestnut—that are better adapted to the new environmental conditions. Bacteria decompose the abundant plant matter and, in the process, exhaust (or nearly exhaust) available oxygen. They may also release poisonous gases (i.e., carbon dioxide, methane, or hydrogen sulfide). Additionally, cyanobacteria, which commonly thrive in eutrophied lakes in France, can produce toxins. In coastal areas, eutrophication results in blooms of green algae (Ulva species), which can kill off seabed fauna, contribute to fishery collapse, and threaten human health (e.g., release of hydrogen sulfide).

Doubling of nitrogen and phosphorus flows

Over the last century, environmental concentrations of nitrogen and phosphorus have soared, especially in aquatic systems, where the nutrients ultimately end up. This trend is the result of many factors: the world's growing population, urbanisation and industrialisation, increasingly specialised agricultural systems (i.e., the decoupling of crop farming from livestock farming), the availability of mined phosphorus, and the production of mineral nitrogen fertilisers (via the Haber-Bosch process). Indeed, recent models indicate that the levels of nitrogen and phosphorus flowing into the ocean have doubled over the last 100 years. Agriculture's contribution has increased significantly for both elements: from 20 to 50% for nitrogen and from 35 to 55% for phosphorus.

Between 1970 and 1990, governments in industrialised countries focused their regulatory attention on household and industrial waste. Wastewater treatment and a mandated reduction in phosphate levels in laundry detergent (which ultimately became a ban) dramatically reduced phosphorus point-source pollution. These actions gradually cleared up certain cases of eutrophication, like those seen in Lake Erie and Lake Geneva. Since then, however, a new type of eutrophication has spread. It is observed in lakes, reservoirs, rivers, and coastal areas across the globe. Consequently, governments are now focused on dealing with non-point pollution from agricultural sources. One challenge is that nitrogen and phosphorus may take decades to make their way through natural systems. The environment, including the soil and the groundwater table, contains traces of pollution past. Nitrogen and phosphorus leaching impacts terrestrial habitats, marine habitats, and all the ecotones in between. While control measures implemented in industrialised countries have had positive effects on freshwater systems, the frequency of eutrophication events has remained unchanged in marine systems since the 2000s. It is important to note that climate change is likely to amplify eutrophication by acting on its underlying causes; certain effects are already apparent.

Defining a course of action

Models are being used to characterise eutrophication’s mechanisms and risks with a view to developing control strategies. Predictive modelling is a common tool for exploring potential outcomes. However, its utility is often limited by a lack of data for zones of interest, and model uncertainty is rarely addressed. For example, few models account for climatic variability and the ecological sensitivity of aquatic systems. Furthermore, there are almost no bioeconomic models, whose results could help inform remediation methods. Consequently, research in these areas should be pursued. These issues aside, modelling has certainly helped guide control strategies.

For example, alleviating the symptoms of eutrophication (e.g., by infusing oxygen into the water column, employing algicides) is clearly a short-term solution. To truly fight eutrophication over the longer term, it is necessary to limit nitrogen and phosphorus inputs from urban, industrial, and agricultural sources. In agricultural systems, the key is to recycle livestock waste, employ effective fertilisation regimes, and both preserve and restore landscapes. However, this three-tiered approach remains insufficient for certain watersheds harbouring particularly sensitive aquatic ecosystems. In these areas, land use and agricultural systems will need to be gradually modified. The objective is to adopt economically feasible, socially acceptable, and regionally appropriate strategies for reducing nitrogen and phosphorus flows as much as possible. Synergies among food production, biodiversity, climatic conditions, and effective resource use and recycling may be important.

The role of governmental regulation

Although different eutrophication-related legislation has been passed, issues remained. For example, nitrate concentrations in drinking water cannot exceed 50 mg/L, which is too high a threshold to protect aquatic systems from eutrophication risks. Indeed, according to some research, in areas of low anthropogenic pressure, where nitrate concentrations equal 1 to 3 mg/L, the composition of biological communities shows early signs of tipping towards eutrophication at only slightly higher concentrations.

Experience strongly suggests that region-specific incentives or regulations will be more effective than broad-scale measures. The most effective strategy of all is adopting a flexible management approach, in which objectives and methods are re-evaluated based on the results they produce. The field of environmental sociology is poorly developed in France, except when it comes to the study of algal blooms. A case study that included the impact of sociological factors indicates that eutrophication control strategies should be socio-ecosystem specific and account for the concerns of different stakeholders. Future research should adopt a systems perspective that incorporates biophysical, economic, and sociological data and should promote knowledge diffusion among stakeholders.

Scientific contact(s):

  • Gilles Pinay (CNRS)
  • Chantal Gascuel (INRA)
  • Alain Ménesguen (Ifremer)
  • Yves Souchon (Irstea)

to learn more

  • Special reports (available for download):

Report summary (8 pages; PDF, 4 MB)

Report synthesis (148 pgs; PDF, 4 MB)

  • See the CNRS website for more information about the report's contents (French only)

key points

  • 500 marine areas were oxygen depleted or anoxic in 2010
  • This was 3x as many as in 1960
  • These areas account for 245,000 km2 of habitat
  • Certain bodies of water experience recurrent eutrophication: the Baltic Sea, the Great Lakes, Chesapeake Bay, the Gulf of Mexico, various lakes and coastal areas in China, Lake Victoria, the coast of Brittany, and Mediterranean coastal lagoons, among others

a collective scientific report

The collective scientific report on eutrophication is the work of around 40 scientists from CNRS, IFREMER, INRA, and IRSTEA. They are French or international researchers specialising in the fields of ecology, hydrology, biogeochemistry, the biotechnical sciences, the social sciences, law, and economics. Different aquatic ecosystems were studied: lotic systems (e.g., streams and rivers), lentic systems (e.g., lakes and reservoirs), estuaries, coastal zones, the open sea, and all the ecotones in between. The report cites around 4,000 references, mostly peer-reviewed scientific articles (in certain fields), scientific or technical reports, and legal texts.