Italy - The Lagoon of Venice
345
-0.3- -1.2 to - 0.2- - 0.8 m (Table 15.1). A similar trend was found in
the Trieste Gulf, where a general increase in water turbidity remarkably
reduced the beds of rhizophytes of the sea infralittoral zone (Bressan
1991).
From the 1960s to the 1980s, a change in the flora toward species
more tolerant to hypertrophic/dystrophic conditions was recorded. The
number of Chlorophyceae species increased from 20 to 34% at
the expense of the Rhodophyceae, which decreased from 65 to 51 % of
the total number of algal species (Table 15.2). Nitrophilic species such as
Ulva rigida C. Ag., Enteromorpha spp. and Cladophora spp. progressively
replaced both the species more sensitive to human activities and rhizophytes, which previously had covered great bottom areas around the
city of Venice. Many species previously very abundant, such as Cystoseira fimbriata, Cystoseira barbata, Dictyopteris membranacea, Cladostephus verticillatus, or Taonia atomaria, disappeared rapidly during the
1960s and 1970s, and the seagrasses Zostera marina, Cymodocea
nodosa, and Zostera noltii, excellent indicators of areas never affected by
anoxia (Sfriso 1987; Sfriso et al. 1988a,b), were confined to the northern
and sourthern basins. Since 1990, rhizophytes (mostly Zostera marina)
have reappeared in some areas of the central lagoon, because of the
decline of Ulva overgrowth and anoxic situations. Moreover, new
macroalgal species such as Grateloupia doryphora are spreading over
large areas of the lagoon. A re-population of Dictyopteris membranacea
and Cystoseira fimbriata was recorded in the sourthern lagoon near
Chioggia, together with abundant Sargassum muticum, which has spread
from Japan to Pacific North America and the British southern coasts
since the 1970s (Chapman and Chapman 1980). Even some specimens of
Undaria pinnatifida were found in the Giudecca Canal near the historical
centre of Venice. The presence of Sargassum and Undaria is ascribed
mainly to the development of commercial exchanges with the Japanese
and Chinese sea areas, where these Phaeophyceae are largely cultivated.
This is confirmed by the diffusion of these species into the Mediterranean
Sea, particularly in the Languedoc region.
The presence of eutrophic species and the poor water exchange in the
wide shallow areas of the lagoon split the macro algal associations of the
soft lagoon bottoms into two main typologies: one characterized by the
succession of different species, the other by the prevalance of almost
mono specific Ulva rigida populations (over 95% of the total biomass)
responsible for frequent distrophic conditions. From the 1970s, Ulva
spread in the areas little flushed by tides and caused the disappearance of
the other species, which were shaded out by floating, I-2m wide Ulva
plants. They easily reached standing crops of 15-20 kg m -2 (wet weight)
345
-0.3- -1.2 to - 0.2- - 0.8 m (Table 15.1). A similar trend was found in
the Trieste Gulf, where a general increase in water turbidity remarkably
reduced the beds of rhizophytes of the sea infralittoral zone (Bressan
1991).
From the 1960s to the 1980s, a change in the flora toward species
more tolerant to hypertrophic/dystrophic conditions was recorded. The
number of Chlorophyceae species increased from 20 to 34% at
the expense of the Rhodophyceae, which decreased from 65 to 51 % of
the total number of algal species (Table 15.2). Nitrophilic species such as
Ulva rigida C. Ag., Enteromorpha spp. and Cladophora spp. progressively
replaced both the species more sensitive to human activities and rhizophytes, which previously had covered great bottom areas around the
city of Venice. Many species previously very abundant, such as Cystoseira fimbriata, Cystoseira barbata, Dictyopteris membranacea, Cladostephus verticillatus, or Taonia atomaria, disappeared rapidly during the
1960s and 1970s, and the seagrasses Zostera marina, Cymodocea
nodosa, and Zostera noltii, excellent indicators of areas never affected by
anoxia (Sfriso 1987; Sfriso et al. 1988a,b), were confined to the northern
and sourthern basins. Since 1990, rhizophytes (mostly Zostera marina)
have reappeared in some areas of the central lagoon, because of the
decline of Ulva overgrowth and anoxic situations. Moreover, new
macroalgal species such as Grateloupia doryphora are spreading over
large areas of the lagoon. A re-population of Dictyopteris membranacea
and Cystoseira fimbriata was recorded in the sourthern lagoon near
Chioggia, together with abundant Sargassum muticum, which has spread
from Japan to Pacific North America and the British southern coasts
since the 1970s (Chapman and Chapman 1980). Even some specimens of
Undaria pinnatifida were found in the Giudecca Canal near the historical
centre of Venice. The presence of Sargassum and Undaria is ascribed
mainly to the development of commercial exchanges with the Japanese
and Chinese sea areas, where these Phaeophyceae are largely cultivated.
This is confirmed by the diffusion of these species into the Mediterranean
Sea, particularly in the Languedoc region.
The presence of eutrophic species and the poor water exchange in the
wide shallow areas of the lagoon split the macro algal associations of the
soft lagoon bottoms into two main typologies: one characterized by the
succession of different species, the other by the prevalance of almost
mono specific Ulva rigida populations (over 95% of the total biomass)
responsible for frequent distrophic conditions. From the 1970s, Ulva
spread in the areas little flushed by tides and caused the disappearance of
the other species, which were shaded out by floating, I-2m wide Ulva
plants. They easily reached standing crops of 15-20 kg m -2 (wet weight)