MAP Technical Reports Series No. 106 UNEP

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bloom of Noctiluca miliaris with peaks of 4,800,000 cells/l in June 1977. A bloom of Scrippsiella
trochoidea with 7,000,000 cells/l is reported by Fonda Umani and Honsel (1983) in May 1983 in
a limited zone of the port of Trieste. The same dinoflagellate generated a red tide in September
1987 (Cabrini et al., 1990).
While cases of anoxia in the strictly coastal band are reported only for the Bay of
Muggia (Trieste) (Orel, 1990), more widespread and persistent anoxic crises are recorded in the
deep waters off the Gulf of Trieste and the coasts of Istria. Unlike events on the western shores
of the northern Adriatic, these cases are generally not preceded by algal blooms; therefore, the
most likely cause is hydrodynamic stasis during the summer, accompanied by marked
persistent stratification of the waters (Faganelli et al., 1985; Ghirardelli and Fonda Umani, 1989).
As a consequence, bottom fauna kills have been observed both in the Gulf of Trieste and in the
eastern area of Istria in 1974, 1983, 1988 (Stachowitsch, 1991) and more recently in 1989, 1990
(Hrs Brenko et al., 1992) and 1991 (Aleffi et al., 1992). The benthic ecosystem in areas of anoxia
have shown a general reduction in numbers of species and of individuals.
5.2.4 Slovenia, Croatia, Montenegro. Eastern Adriatic
Justic et al. (1987) emphasize that oxygen supersaturation in the surface waters and
oxygen depletion in bottom waters in the northern Adriatic occur in all seasons except winter.
They attribute this to the increase in phosphorus, since this element is able to govern the
development of the phytoplankton biomass (Justic, 1987; 1991a). The ecological consequences
of states of hypoxia presented by Justic (1991b) agree with those of other authors
(Stachowitsch, 1991, 1992; Rinaldi et al., 1993). Drastic reduction in the density of populations
of Turritella communis after repeated hypoxic crises is significant here.
In the Slovenian coastal area periodical near-anoxia or even anoxia (eg in 1983;
Faganeli et al., 1985) events were related to the regional trophic state combined with the
establishment of unusual oceanographic conditions (marked deep pycnocline, absence of "bura"
wind), rather than to local sewage disposal influence (eg Malej, 1993).
Eutrophication impact on the ecosystem of the open northern Adriatic results from the
combined influence, and changing relative importance, of the external nutrient input cycle (mainly
from the Po River), the degree of water column stratification, and the horizontal water advection,
as they control and/or moderate biological assimilation and regeneration processes in the water
column and at the sediment-water interface (Gilmartin et al., 1990; Stirn, 1993).
Periodically (eg. in 1977), unusually high Po river nutrient discharges greatly increase
organic matter production and decomposition rates (Degobbis et al., 1979). In other cases,
vertical and horizontal water mixing can be reduced significantly because of long periods of calm
weather (eg. in 1988 and 1989), increasing the nutrient residence time and its effect on the
ecosystem of this region (Degobbis, 1989; Degobbis et al., 19991; Ivanæiæ, 1995). As a
consequence, near-anoxic or anoxic conditions occurred in the bottom layers with a mass
mortality of benthic organisms (eg Degobbis et al., 1993).
Interestingly, in 1989 the most critical conditions occurred in the eastern part of the
open northern Adriatic, which is generally considered oligotrophic. Furthermore, in the late
eighties a significant decrease of the potential primary production rate (estimated with 14 C) was
related to a reduction of orthophosphate concentration in the sea and Po waters, despite a total
inorganic nitrogen increase (Precali, 1995). This is not surprising, since phosphorus was
recognized long time ago as the primary limiting element in the open northern Adriatic