MAP Technical Reports Series No. 106 UNEP

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In contrast, it seems that the seabirds (gulls in particular) seem to be benefiting from
the situation: when the benthic fish come to the surface gasping for oxygen before dying, flocks
of gulls (Larus ridibundus and Larus argentatus) gather in the area. It is well known that the
numbers of these species are growing because of their ability to adapt in an opportunist way to
various forms of human activity (fishing, agriculture and landfills), causing damage to more
"delicate" species, particularly during the breeding and migration seasons. The way in which they
interfere with and even prey on the eggs and nestlings of species such as oyster catchers
(Haematopus ostralegus), avocets (Recurvirostra avosetta) etc. which nest in areas adjoining
or overlapping with those of the herring gull (L. argentatus) is in fact well known.
6.2.3 Immature macrozoobenthic community along the coast of the Emilia- Romagna region
The macrozoobenthic community in the northern Adriatic Sea, south of the Po river,
along the coast of the Emilia-Romagna region, was sampled in 1985 (Crema et al., 1991). The
sampling site was in the centre of a highly eutrophied area with greatly increased intensity and
frequency of dystrophic events over recent decades. The sampled community differs from all
those described in the same area in a period (1934-1936) (Vatova, 1949) previous to the current
degree of eutrophication. Large abundances of species indicative of unstable bottoms, such as
the bivalve Corbula gibba and the polychaete Lumbrineris latreilli were recorded.
The recent biocenosis is characterized by a large abundance of Corbula gibba, a
species typical of the transition zone between detrital and muddy bottoms. Its dominance is of
particular interest. In fact, Corbula gibba is known to be a pioneer species in the recolonization
of defaunated bottoms (Bonvicini Pagliai et al., 1985; Curini Galletti, 1987; Crema, 1989), and
is prominent in subnormal zones in areas polluted or enriched by organic material (Ghirardelli
and Pignatti, 1968; Pearson and Rosenberg, 1978; Bourcier et al., 1979; Russo, 1982).
This bivalve was also dominant, in association with Lumbrineris latreilli, in the Gulf of
Fos, subjected to extensive dredging, and it has been included in a stock of species typical of
unstable bottoms (Salen Picard, 1981).
Moreover, the community structural features indicate a state of immaturity, such as in
early successional stage communities. The increased frequency of acute dystrophic events and
consequent shortening of the time between consecutive disturbances is proposed as the cause
of the modification of biocenosis, and its current structure and composition in the Northern
Adriatic Sea, and in eutrophicated coastal areas of the Mediterranean.
6.2.4 Research in the laboratory and in the field on the resistance of bivalves to oxygen
deficiency
De Zwaan et al. (1992) carried out laboratory survival tests to determine the resistance
of benthic organisms to oxygen deficiency. In sea water where oxygen levels were reduced by
bubbling through nitrogen, he performed tests on a number of bivalves endemic to the North-
West Adriatic such as: Chamelea gallina, Tapes philippinarum, Mytilus galloprovincialis and
Schapharca inaequivalvis. The 50% survival times of the individuals used demonstrated that the
most resistant is S. inaequivalvis with 19 days, followed in decreasing order by M.
galloprovincialis with about 16 days, T. philippinarum with 12 and finally C. gallina with about 6
days.