MAP Technical Reports Series No. 106 UNEP

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coverage of the sources of eutrophicants is extremely important, as is the choice of stations.
These should cover the full range of environmental conditions from near-shore eutrophic waters
to offshore, more oligotrophic waters.
Direct measurement by moving ship of many of the variables previously mentioned,
when used in conjunction with computerized date acquisition, allows for a practically real time
display of the conditions encountered in the area.
In order to estimate the variation of potential eutrophication, a monthly frequency is
recommended. The frequency should be increased during critical periods, which may be
identified during sanitary monitoring programmes.
Because of the great variability of the pelagic system, strongly correlated to
meteorological changes, bursts of intensive sampling and/or measurements (round the clock)
during one day periods, may be preferable to more sparse sampling campaigns.
Monitoring of long-term changes over at least 5 to 10 years is necessary, and must
concentrate of selected variables which are easy to estimate. On a long-term scale it is most
useful to measure changes in the area in which the surface chlorophyll concentration is above
a certain value, and the oxygen concentration in the lower layers is below a certain value.
9.5 Policy analysis
Environmental managers seek advice on which policies to pursue in managing the
problem of eutrophication. Modelling, field data collection, and laboratory and field
experimentation have an important role to play in the evaluation of these policies. The results of
policy analyses presented to the manager can be regarded, in their simplest form, as a table or
score care on a single page which facilitates the complex trade-off which must be made in the
making of a decision.
The score card consists of an array of squares. Each square contains a number or
qualitative index, which measures, or scores, the performance, impact, cost or benefit of each
policy option under chosen political, economic, social, legal and environmental headings.
Models, whether they are socio-economic or ecological mathematical models solved on a
computer, or mesocosms in the laboratory are the tools which provide the entries in the score
card. Hence, modelling should always be governed by a question to be answered, in this case:
what value should be entered in a given square of the score card and what is its uncertainty ?
When there are significant aspects of the eutrophication phenomenon which are not
understood (for example, the response of plankton species to a new nutrient control technique)
a combined application of mathematical modelling, field data collection, laboratory and field
experimentation can be recommended. In this case also, modelling is governed by a question
to be answered.
Since models are always a simplification of reality, the question posed guides the
simplification. The resulting model is a limited set of working hypotheses to be confronted with
laboratory and field experiments which are designed to test them. The qualitative and quantitative
comparison of model predictions with field and laboratory data may force a revision of the model,
and the emergence of new hypotheses.