MAP Technical Reports Series No. 106 UNEP

- 85 -
that the dynamics of the phytoplankton populations are governed by the seasonal sequence in
the hydrodynamic and physical characteristics of the mass of water in the basin (Franco and
Michelato, 1992). In their hypothesis, the marked stratification of the water column in the summer
determines a heterogeneous vertical distribution of phytoplankton, with the highest biomass
values in river input diluted surface layers. Vertical instability during late autumn and winter and
a marked frontal system along the western coast separating the diluted waters from the rest of
the basin generate a distribution of phytoplankton biomass where horizontal heterogeneity
predominates.
The causes of the serious deterioration that has occurred in the northern area for over
twenty years are attributed to the a nutrient input in amounts that exceed the basin's natural
assimilation capacity (Warren et al., 1992). The Po, carrying some 100,000 tons/year of
inorganic nitrogen and some 6,000 tons/year of inorganic phosphorus, contributes the most of
the total nutrient load of the northern Adriatic basin (Marchetti, 1990). The next largest of the
rivers flowing into the northern Adriatic, the Adige, contributes 14,002 tons/year of total nitrogen
and 1,202 tons/year of total phosphorus, although its mean nutrient concentrations are lower
than those monitored in the Po (Provini et al., 1980). The total nitrogen and total phosphorus
discharged into the northern Adriatic from Italy alone amounts to some 270,000 and 24,000
tons/year, respectively; cf. Table 21 (data produced by Marchetti, 1987; Regione Emilia-
Romagna, 1991). To these must be added the inputs from Istria estimated at 12,600 and 600
tons/year of total nitrogen and total phosphorus, respectively (Degobbis and Gilmartin, 1990).
5.2.5 Mucilage aggregates in the Adriatic and Tyrrhenian Seas
Adriatic Sea. After a year of apparent absence of visible agglomerations of mucilage
in the surface waters, though present in deeper strata (5 to 10 m) in 1990, the phenomenon
reappeared in 1991, first toward the end of May in Yugoslav waters, from where it spread to the
northern and northeastern part of the Adriatic basin (see Fig. 15).
Mucilaginous material observed with underwater telecameras in regular weekly surveys
presented itself initially as loose "marine snow" and ribbon like aggregates with highest
concentrations in the thermocline. Later, after conditions of continuing calm weather and high
insolation, batches of mucilage of various spatial extension and duration in time, surfaced over
the whole northern Adriatic Sea. The phenomenon was most pronounced during the warmer
hours of the day, but locally the appearance of mucilaginous batches was influenced by
prevailing winds and currents.
In comparison to the situation found in 1988 and 1989, the 1991 mucilage events were
of less severity, however, but of considerable longer persistency over time, i.e. from June 5 to
the end of August. While strong sea-storms and surface mixing, as well as strong north-south
currents were absent for the whole period, frequent light winds from land (Libeccio) and modest
mixing of waters prevented the buildup of more serious mucilage accrual in coastal areas. On
the other hand, further offshore and in deeper waters and near the sediments, considerable
amounts of mucilage continued to be present, creating - besides difficulties to fisheries - a
situation of hypoxia and modest bottom fauna kills.
Chemical and microscopical analyses confirmed that the prevailing components of the
mucilaginous material are mucous polysaccharide agglomerates in which inorganic and organic
particulate matter such as mineral fragments, organic detritus, zooplankton (mostly