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OECD Probability Distribution Diagrams-
Vollenweider's dictum

Excerpts from the peer consensus, 16-year, 18-country Organization for Economic Co-Operation and Development (OECD) research which also forms the primary backbone of the Canadian Council of Ministers of Environment- CCME (2004) guidelines on trophic states

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: June 04, 2017                                         Eutrophication of Waters (OECD)


Contents:



Shortcomings of the Fixed Boundary approach

cf. Janus and Vollenweider, 1981; Vollenweider and Kerekes, 1982

What emerged from the assessment of all information available, however, led to the conclusion that there is no possibility of defining strict boundary values between trophic categories. Whilst the progression from oligo- to eutrophy is a gliding one- as has been stressed many times in literature- any one combination of trophic factors, in terms of trophic category allocation, can only be used in a probabilistic sense. Objective reasons exist for the uncertainty of classifying a given lake in different categories by two or more investigators, depending on the management of that body of water.

Average conditions, expressed by "average nutrient concentrations", "average biomass values", "average transparency", etc., do not necessarily express the degree of variability, particularly with regard to peak levels, frequency of their occurence, and their qualitative nature (type of phytoplankton).

From the management viewpoint, such situations and their frequency are as important as average conditions.

For this reason, prediction uncertainties must be accounted for. This can be achieved by reinterpreting the summary values listed in Table-3 in terms of classification probabilities. The resulting probability distribution is given in Figures 3 to 6 for the main components: average lake phosphorus, average and peak chlorophyll concentrations and average yearly Secchi disk transparency.



Vollenweider's dictum

click the picture to listen to the 4-minute mp3 sound file Click on the mp3 sound file (4-minute duration) to listen to the rationale behind the OECD Probability Distribution Diagrams relating to the scientifically credible methodology of ascertaining trophic states which can be achieved only in a `probabilistic sense' as described above.

Environment Canada's Dr. Richard Vollenweider has been the first Canadian (1986/7) to have ever received the top international medal in limnology, the Naumann-Thienemann medal.



Probability Distribution Diagrams

Figure-3: Probability distribution curve for the average lake phosphorus:

Probability distribution curve for the average lake phosphorus


Figure-4: Probability distribution curve for the average chlorophyll a:

Probability distribution curve for the average chlorophyll a


Figure-5: Probability distribution curve for the peak chlorophyll a:

Probability distribution curve for the peak chlorophyll a


Figure-6: Probability distribution curve for the average yearly Secchi disk transparency:

Probability distribution curve for the average yearly Secchi disk transparency




Example of an application of the OECD Probability Distribution Diagrams for lakes in Halifax, Nova Scotia

Citation: Soil & Water Conservation Society of Metro Halifax. 1991. Limnological study of twenty seven Halifax Metro lakes. 136p.: ill., maps.
(Mandaville, S. M., and Shacklock, P. Based on our Stage-I protocol. Albro, Banook, Beaverbank, Bell, Bissett, Chocolate, First, Hubley Big, Kearney, Kinsac, Loon, Maynard, MicMac, Miller, Morris, Nicholson, Oathill, Papermill, Portuguese Cove, Rocky, Sandy [Bedford], Second, Settle, Springfield, Third, Tucker, and Williams [Halifax] lakes)
Since, the data of the 27 lakes below was only seasonal, i.e., three samples per year in most cases, peak chlorophyll a was not considered. One needs more extensive yearly data to confidently ascertain peak chlorophyll a.

[Img-probability-example-1]


Note: We continue to follow this methodology in addition to our extensive biomonitoring. We are indeed pleased that many of the lake problems all over Nova Scotia were easily explained on a scientific basis, since these events spanning several decades could not be rationalized utilizing simpler metrics.




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