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OECD Probability Distribution Diagrams-
Vollenweider's dictum

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: April 12, 2019                                        

Contents:

• Shortcomings of the Fixed Boundary approach

• Vollenweider's dictum

• Probability Distribution Diagrams, Figures 3 to 6 incl.

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

Shortcomings of the Fixed Boundary approach

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 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:

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

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

Figure-6: 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

(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)

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