MAP Technical Reports Series No. 106 UNEP

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and oxygen consumption due to allochthonous organic material. In practice, the discrimination
is not easy, and perhaps even not necessary. The main point here is the implication that
untreated, or insufficiently treated sewage is not only of sanitary concern, but is also one of the
causes that contributes to local situations of hypoxia and anoxia.
3.3.5 Loading criteria
Nutrient supply from external sources is the driving factor of aquatic eutrophication. From
a practical point of view of management the question arises, therefore, whether it is possible to
establish a quantitative relationship between nutrient supply and the degree of eutrophication.
To this end, Vollenweider (1968) introduced the concept of loading tolerance, which was further
developed in several papers by the same author with particular reference to phosphorus. This
concept defines the general relationship between nutrient supply and the trophic reaction of an
aquatic system, with emphasis placed on levels of nutrient supply that determine the transition
between oligotrophic and eutrophic conditions in any given situation.
Vollenweider's concept has been quantified for lake systems using a few systems
parameters, such as mean depth, hydraulic load and water residence time. In this specific form,
the model conception is not immediately transferable to marine systems, yet the concept as
such remains of general applicability regardless of the specificity of bodies of water. Chapra and
Reckhow in their treatise on modelling (Chapra and Reckhow, 1983) consider also the principles
that apply to coastal areas. Lateral coastal currents and inshore-offshore exchange are of
particular relevance for the build-up, dissipation of nutrients and maintenance of nutrient
gradients along shores. Giovanardi and Tromellini (1992a; 1992b), instead, considered the
situation south of the river Po, using a simple dispersion model with salinity as a tracer, and
redefined the OECD trophic classification scheme for that area. The combination of both would
provide a basis to define acceptable loadings, which, however has not been done yet.
A variety of this approach, more along the above conception has been pursued by Wallin
and Håkanson (cf. Wallin, 1991). In addition to parameters considered by Vollenweider, the
authors introduced also a coastal form factor, Vd, and a statistically defined sensitivity array, S,
which are used to construct a kind of loading tolerance diagram for coastal marine areas
corresponding to that developed by Vollenweider for lakes.
Clearly, more complex models and alternative approaches have been pursued in the
appropriate scientific literature, all with the implied aim to define in some way nutrient load
tolerance conditions of aquatic systems. Although substantial progress has been made thus far,
non of these various approaches will be fully applicable to all marine situations; each situation,
and accordingly the method taken to characterize the particular circumstances that determine
the local or regional marine trophic conditions, must be evaluated in its proper context.
3.3.6 Nutrient limitation in the Mediterranean
Of particular interest in the context of the present report is the question of nutrient
limitation in the Mediterranean Sea. Studies of this sort are not plentiful, however. Experimental
studies in the coastal waters of Emilia-Romagna have shown that phosphorus rather than
nitrogen limits production (Chiaudani et al., 1980; Marchetti, 1985; Mingazzini et al., 1992), as
has also been found for the brackish waters of the Baltic (Fonselius, 1978). On the other hand,
it must also be noted that marine waters adjacent to major river outfalls are not nutrient limited
at all after periods of high water discharge and/or discharges of nutrient