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OECD Probability Distribution Diagrams-
Excerpts from the peer consensus, 16-year, 18-country Organization for Economic Co-Operation and Development (OECD) research
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Updated: April 12, 2019
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
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.
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
Citation: Soil & Water Conservation
Society of Metro Halifax. 1991. Limnological study of twenty seven
Halifax Metro lakes. 136p.: ill., maps.
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.
- (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)
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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