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Eutrophication of Waters (OECD)

Monitoring, Assessment and Control

Research of the Organization for Economic Co-Operation and Development (OECD)

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Modified: October 11, 2017                           Limnology

(cf. Janus and Vollenweider, 1981;  Kerekes, 1983;  Mandaville, 2000;  Vollenweider, 1976; and  Vollenweider and Kerekes, 1982)



Man-made accelerated eutrophication of inland waters in OECD Member countries can generally be viewed as an undesirable degradation of the environment resulting in a deterioration of water quality which interferes with most of the beneficial uses of waters; it is causing, in many cases, significant economic losses.

The impact of eutrophication on recreation and tourism is probably the most sensitive area for the public. It may severely alter the recreational value of many water bodies and impair related activities (swimming, fishing, etc.) as a result of the objectionable aspect of the waters, such as reduced transparency, odour, and increased incidence of stinging insects, swimmer's itch, etc. Both social impacts and economic losses may be important and make eutrophication control necessary.

The impact of eutrophication on drinking water supply may be serious. For a number of reasons it generally reduces its final quality and may diminish its safety. Furthermore, it makes its preparation more difficult and costly.

The problems encountered include: rapid clogging of filters by diatoms and other algae; disturbance of floculation treatment by organic substances; persistent and unpleasant taste and odour (e.g. geosmine); abnormal concentration of substances such as manganese, iron and ammonia giving rise to colour or other disturbances; risk of increased bacterial growth in drinking water due to the fouling of the distribution networks and the nutrient content.

Furthermore, because of the high content of organic substances in eutrophied waters and some of the problems listed above (taste, ammonia, regrowth of organisms, etc.) these waters are often extensively chlorinated during treatment as well in the initial transportation and final distribution networks. High levels of both chlorine and organic substances lead to significant concentrations of organochlorinated compounds in drinking water, and these substances are now considered to be potentially hazardous for human health (carcinogenic risk). Waters for potable use should thus be protected from eutrophication.


The OECD lakes ranged from "pond-size" lakes to the Great North American Lakes. The momentum initiated by the International Biological Programme in 1964 was maintained. The information available was broad enough to establish the general statistical behaviour of lakes with respect to nutrient load and trophic response. It should be noted, however, that subtropical (in USA) and Arctic lakes (including high Alpine) were poorly represented, and saline, closed basin lakes were not represented at all in the programme. The OECD study was restricted mainly to lakes of the temperate zone.

The final report is a synthesis of the main results of the OECD Cooperative Programme on Eutrophication under the Chairman of the Technical Bureau, Dr. Richard Vollenweider. It is the outcome of several years' concerted effort by 18 Member countries, at 50 institutes. The objectives were to establish, through international cooperation, a basis for eutrophication control of inland waters (lakes and reservoirs in particular), and to develop better guidelines for fixing nutrient load criteria compatible with water use objectives.

This report is both complementary and supplementary to the four Regional Project Reports published as follows. A fifth report (the Canadian contribution) has been developed which tests the OECD results on bodies of waters not included in the analytical part.

  1. Title: Regional Project Alpine Lakes
  2. In the Alpine regions are the headwaters of a large number of European waters. 30 lakes, consisting of 38 lake basins, are included in this project.

  3. Title: The Nordic Project: OECD Eutrophication Programme
  4. This project includes the lakes resulting from the retreat of the great quaternary glaciers. 10 lakes, consisting of 15 lake basins, are included in this project.

  5. Title: Project, Shallow Lakes and Reservoirs, Final Report
  6. This project includes man-made lakes and reservoirs and other relatively shallow lakes, lagoons and estuarine waters covering a wide variety of geographical situations. 32 waterbodies are included in this project.

  7. Summary Analysis of the North American (U.S. Portion) OECD Eutrophication Project: Nutrient Loading-Lake Response Relationship and Trophic State Indices. 455p.
  8. This project includes waterbodies covering a wide spectrum of trophic and morphometric conditions and ranging from ultra-oligotrophic pristine lakes to highly eutrophic ones, and from small, shallow, highly flushed lakes to the Laurentian Great Lakes and three sections of the Potomac Estuary. 34 waterbodies were considered.

  9. Title: Summary Report of the North American (Canadian Contribution) OECD Eutrophication Project
  10. The Canadian lakes selected are used as a test-case to study the feasibility of applying the OECD results to a large variety of limnological situations.

The results of the OECD study and approach have already been successfully applied in several instances in North America, Europe and elsewhere.

Statute of the report: The conclusions of this report have been successively agreed by the Water Management Policy Group, the Environment Committee and finally the Council. The technical part of this synthesis report has also been approved by the Water Management Policy Group.

Summary and Conclusions:

The objectives of the programme have been largely achieved, and applicaion of the results to practical control of eutrophication is possible. However, it is recommended that the results be handled with caution and not applied to cases which lie outside the ranges and situations covered by the programme.

The main control strategy is reduction of the external load. In cases where such a reduction to the required tolerance level is impracticable, or impossible (as e.g. in situations of intensive use of the catchment system), management measures other than nutrient reduction have to be employed. Such alternative measures, however, can only be defined using advanced modelling techniques. Notwithstanding the positive achievements of the programme, eutrophication remains a very complex problem and numerous questions are still unanswered.

Although nitrogen or some other factor may be in some particular instances outweigh the role of phosphorus as the limiting factor, most attention here is based on phosphorus control; however, where the term "nutrients" appears, both phosphorous and nitrogen may be considered in order to include potential cases of nitrogen limitation.

It is safer and generally more economic to take early preventive measures to control eutrophication than to develop curative strategies later when water quality has already deteriorated.

Water Resource Planning

The results of the OECD programme have shown that:
  1. in most cases, phosphorus is the factor which determines the development of eutrophication;
  2. even when another nutrient such as nitrogen is (occasionally or normally) the limiting factor, phosphorus may still be made to play the role of limiting factor through appropriate control.
Control of point sources of pollution from municipalities and industries is usually given priority as it is generally the most cost-effective measure.

After reduction of phosphorus from point sources, the relative role of phosphorus from diffuse sources will increase. This means that measures against diffuse sources may become necessary if improvement of water quality cannot be achieved by further elimination of phosphorus point sources.

Diffuse source control is more difficult to achieve. Yet, in many cases, effective prevention of eutrophication, or restoration of eutrophied waters cannot be achieved without such control.

Therefore, the improvement of all aspects of agricultural practices which contribute nutrients to water bodies should be encouraged, with special reference to:
- control of waste from intensive animal husbandry;
- control of the dose, period and methods of fertiliser application in order to achieve minimal loss and optimum up-take by crops;
- control of erosion and run-off from tillage land and from forestry operations;
- control of over-irrigation (fertiliser leaching to ground waters).
Attention should also be given to urban run-off and storm overflows, which normally by-pass conventional treatment plants. A more integrated approach may be necessary, and should include modifications to system design standards, source controls, sewer separation, flow detention, and overflow treatment.

In certain situations nutrient contributions from septic tanks may be important. Potential contributions from such sources should be carefully assessed, although no generally applicable methodology exists to control septic tank leaching. However, when septic tank effluent is dispersed by tiled drains in a field, a high degree of phosphorus removal can be achieved for a considerable time, especially with certain types of soils.

Eutrophication cannot really be controlled by herbicides and algicides. As the use of these products presents a hazard, they should only be considered as an interim measure to alleviate symptoms in exceptional cases and strictly limited environments.

Although the OECD Programme was designed to control the mainly negative effects of eutrophication, there may be exceptional cases where an increase in the trophic level can have limited beneficial aspects (e.g. for increasing the population of certain fish species). The potential benefits are, however, rapidly outweighed by the negative effects on all uses as trophic levels increase.

Greater attention should be given to the economic use and the promotion of by-products from water pollution and eutrophication control measures. Instances include: low cost energy gains from sludge digestion (bio-gas); potential use of sludge as fertiliser products; irrigation with waste water ("fertirrigation"); harvesting of primary products from controlled lagooning, etc.

Table-1: Trophic characterisation of lakes impairment of various uses:

Limnological characterisation Oligotrophic Mesotrophic Eutrophic
General level of production ........ low medium high
Biomass ............ low medium high
Green and/or blue-green algae fractions low variable high
Hypolimnetic oxygen content ..... high variable low
Impairment of multi-purpose use of lake little variable great

Nutrient Load Assessment

For practical purposes, the following break-down scheme is recommended for estimating the total load.
  1. external:
  2. - the phosphorus and nitrogen load via the tributaries (including "point" sources along the tributaries, and "diffuse" or "non-point" sources on the drainage basin);
    - the point and diffuse sources load which directly enter the lake through the shores;
    - the phosphorus and nitrogen load which falls on to the surface of a lake as wet or dry precipitation;
  3. internal:
  4. - phosphorus and nitrogen which re-enter from sediments. The net result of the interchange of nutrients between water and bottom sediments can be estimated by making nutrient balances covering relevant periods of time.

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