MAP Technical Reports Series No. 106 UNEP

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Forcing functions such as inflows of nutrient, light, temperature and wind, drive the
model. Those forcing functions which are subject to alteration by man are called control
variables or functions.
Some of the following control variables will always be present in a eutrophication model:
- mass discharge rate of nutrients from point and diffuse sources;
- location of the discharge points;
- harvesting or biomass;
- dredging of nutrient-rich sediments;
- burial of nutrient-rich sediments with inorganic material;
- biocide inputs, etc.
"What-if" experiments can be made with the model in order to support the decisions
of the manager. If the chosen control strategy achieves the predicted response, the hypotheses
or the model stand. Any disagreement between predicted and observed response of the trophic
system necessitates a revision of the model. Clearly, the approach presented here demands
the co-operation of many different specialists and provides a focus for it (Fedra, 1988).
10. POSSIBLE PREVENTIVE AND REMEDIAL ACTIONS
There is evidence that in some coastal areas of the Mediterranean Sea the inputs of
eutrophying substances, particularly phosphorus and nitrogen, exceed the capacity of the
receiving environment.
Since the Mediterranean is generally an oligotrophic sea, small sewage discharges on
open coastline shores, if properly spaced, can normally be disposed into the sea without
extensive treatment, via submarine pipelines with diffuser outlets at an appropriate depth and
distance from shore.
For larger discharges or the concentration of several small ones in an area, especially
when situated within bays, additional treatment or other measures for reduction of nutrient loads,
e.g. reuse of waste waters, recycling in aqua culture, is required.
The type of treatment and disposal design depends on the overall inputs and
environmental receiving capacity (for pertinent Guidelines etc. see UNEP/WHO, 1982. This
should be decided on a case-by-case basis, taking into consideration all the load of existing and
planned discharges versus the receiving capacities.
When important loads, carried by rivers and originating from point and distributed
sources, overwhelm the point discharges along the shores, the simple control of the latter is not
sufficient. In these more complex situations, such as in the North Adriatic, it is essential to add
appropriate interventions in order to reduce the inland nutrient loads.
The fundamental steps in the decision-making processes at the strategic planning and
policy level can only be performed through the improvement of knowledge, as obtained by
research, assessment and monitoring.