340
A. Sfriso and A. Marcomini
Schiffner and Vatova 1938). Unfortunately, taxonomic identification was
a difficult task in the past because of both inadequate technical means
and lack of reliable classification keys. Different authors often identified
macroalgae differently, and the same species were frequently classified
under different names. This situation led in the past to the description of
hundreds of different species, subspecies, varieties and forms. Only the
most recent classifications (Pignatti 1962; Sfriso 1987), performed according to well-established taxonomic revisions of some genera (Bliding
1963, 1968; Van Den Hoek 1963; Feldmann 1981) allow comparison of
the changes in the Venice macro algal typology. Pignatti (1962) studied
macro algae in a phytosociological context also and characterized the
main associations of benthic algae, determining their vertical distribution as vertical bands or "vegetational zones", which corresponded to the
"etages" of Feldmann (1938, 1959).
As far as our research work is concerned, after comparing the flora
and the vegetational association changes that have occurred in the last
50 years (Sfriso 1987), our attention was addressed to the study of the
dynamics of algal biomass production and decomposition, with particular reference to the lagoon nutrient cycles and balances (Sfriso et al.
1987, 1988b, 1989, 1990). In this chapter, we present a comprehensive
account on the most recently available results.
15.2 The Lagoon Morphology and Hydrography
The Venice lagoon (Fig. 15.1) has a water surface exposed to the tidal
regime of 432 km 2 , and a mean depth of about 1 m. Tidal difference is
about ± 31 cm (Pirazzoli 1974). The water exchange between the lagoon
and the sea, through the Lido, Malamocco and Chioggia mouths was
estimated to be 3.1-5.4 x 10 8 m 3 , during one tidal cycle (12 h), which
approximately corresponds to the total water volume of the lagoon.
Complete water renewal by tidal exchange with the sea occurs in the
areas near the lagoon inlets. Water renewal in the inner parts is much
less efficient (Battiston et al. 1983).
Therefore pollutants which enter the lagoon through the small rivers
and canals at an average flow rate of 30-40 m 3 S-1 (Vazzoler et al. 1987)
may have a quite long residence time (Battiston et al. 1983), and will
probably be enriched in the sediments of the lagoon. From a drainage
basin of approximately 1840 km 2 , mostly treated or untreated municipal
effluents from Venice, Mestre, Chioggia, and from the populated islands
A. Sfriso and A. Marcomini
Schiffner and Vatova 1938). Unfortunately, taxonomic identification was
a difficult task in the past because of both inadequate technical means
and lack of reliable classification keys. Different authors often identified
macroalgae differently, and the same species were frequently classified
under different names. This situation led in the past to the description of
hundreds of different species, subspecies, varieties and forms. Only the
most recent classifications (Pignatti 1962; Sfriso 1987), performed according to well-established taxonomic revisions of some genera (Bliding
1963, 1968; Van Den Hoek 1963; Feldmann 1981) allow comparison of
the changes in the Venice macro algal typology. Pignatti (1962) studied
macro algae in a phytosociological context also and characterized the
main associations of benthic algae, determining their vertical distribution as vertical bands or "vegetational zones", which corresponded to the
"etages" of Feldmann (1938, 1959).
As far as our research work is concerned, after comparing the flora
and the vegetational association changes that have occurred in the last
50 years (Sfriso 1987), our attention was addressed to the study of the
dynamics of algal biomass production and decomposition, with particular reference to the lagoon nutrient cycles and balances (Sfriso et al.
1987, 1988b, 1989, 1990). In this chapter, we present a comprehensive
account on the most recently available results.
15.2 The Lagoon Morphology and Hydrography
The Venice lagoon (Fig. 15.1) has a water surface exposed to the tidal
regime of 432 km 2 , and a mean depth of about 1 m. Tidal difference is
about ± 31 cm (Pirazzoli 1974). The water exchange between the lagoon
and the sea, through the Lido, Malamocco and Chioggia mouths was
estimated to be 3.1-5.4 x 10 8 m 3 , during one tidal cycle (12 h), which
approximately corresponds to the total water volume of the lagoon.
Complete water renewal by tidal exchange with the sea occurs in the
areas near the lagoon inlets. Water renewal in the inner parts is much
less efficient (Battiston et al. 1983).
Therefore pollutants which enter the lagoon through the small rivers
and canals at an average flow rate of 30-40 m 3 S-1 (Vazzoler et al. 1987)
may have a quite long residence time (Battiston et al. 1983), and will
probably be enriched in the sediments of the lagoon. From a drainage
basin of approximately 1840 km 2 , mostly treated or untreated municipal
effluents from Venice, Mestre, Chioggia, and from the populated islands