MAP Technical Reports Series No. 106 UNEP

- 150 -
The definition of most appropriate strategies of intervention requires a preliminary costeffectiveness
evaluation, in an overall policy analysis framework, of the role of the different
factors which relate to the origin, transport and dispersion of nutrients. The analysis of the point
sources (urban and industrial effluents), of the distributed sources (diffuse and linear erosion,
fertiliser run-off, etc.), of the transport and diffusion mechanisms, as well as the biological and
ecological processes which are driven by the meteorological, hydrological, hydrochemical
conditions, is of increasing complexity.
If eutrophication is to be contained, an initial evaluation is needed to establish which
nutritional substances provide the main stimuli for algal growth, and which tend to limit it.
Research must also identify any concurrent factor (limited circulation of water, imbalances in
the food chain, etc.) and the ways in which some of these might be corrected.
These elements of information must come from an exhaustive investigation of the
system. A reliable monitoring program and an expert analysis of the data collected will provide
the ultimate basis for decision on any actions to be taken.
In the case of coastal marine areas, where the organisms involved in eutrophication are
primarily phytoplankton, the most effective remedial action will be reducing the growth limiting
factor. Around the Mediterranean situations of eutrophication dependent, in terms of growth
limitation, on either or both, nitrogen and phosphorus. Nitrogen limitation is likely the prevailing
condition in open offshore waters, prevailing phosphorus limitation has been identified in the
Northwest Adriatic. Both factors are supplied by sewage discharges, which reach the sea either
directly, or via rivers.
The situation in lagoons and deltas differs from open coastal areas. Such systems are
characterised in most instances by brackish, shallow waters of limited communication with the
open sea; they will often be affected by invasive accumulations of macroalgae (mainly ulvaceae).
Here, eutrophication is generally the result of an excessive influx of nutrients (nitrogen
compounds in particular) coming mainly from the drainage of farmlands. In addition to the trophic
component, due consideration must be given to man-made structures (harbour walls, docks,
breakwaters, etc.) which in many instances isolate stretches of water from the sea and thus give
rise to stagnation.
In respect of nutrients, the scope for preventive and remedial action is relatively wide.
The possibilities include:
a) Elimination of nutrients at source. Preventive measures can be taken to rationalize the
methods of cultivation used in farming; control the distribution of livestock husbandry
activities over the territory; reduce or replace tripolyphosphates in detergents;
encourage the adoption of manufacturing technologies with a low trophic impact.
b) Reduction of nutrients in effluent. Implementation of advanced treatment in sewage
purification plants (tertiary treatment) to reduce phosphorus and nitrogen levels in
sewage effluents is recommended for treatment systems on coastal sites, or
anywhere near to eutrophic bodies of water.
c) Isolation of nutrients from coastal water systems. Effluent discharged from treatment
plants may be recycled back into agriculture (fertirrigation) or carried by submarine
pipelines to outflow points removed from the immediate coastline.