348
A. Sfriso and A. Marcomini
and filled the entire water column up to the surface. At this growth stage,
biomass was highly stratified with its productive layer approaching the
water surface. Below the canopy, light was considerably reduced
(PAR = 0-100 IlE m -2 s -1) and decomposition processes prevailed near
the bottom layers. The daily fluctuation of Ulva biomass within the water
column was caused both by tidal variation and oxygen production
during photosynthetic activity. The upper algal layer could remain
exposed to air and water loss for many hours or days during neap tides.
Beer and Eshel (1983) showed that Ulva is neither exceptionally resistant
to desiccation, nor does it benefit photosynthetically from emersion.
Ulva exhibited a negative carbon balance, upon exposure to morning or
noon light intensity, after only 30-90 min. Because of the poor water
renewal by tides due to macro alga masses filling up the entire water
column of the lagoon for many square kilometres, the amount of
dissolved oxygen became insufficient to balance the oxygen consumed by
the total biomass, and this led the macro algae to collapse very rapidly.
The subsequent release of high amounts of nutrients triggered sudden
phytoplankton blooms (especially Skeletonema costatum and Nitzschia
closterium) further contributing to macroalgal decay by shadowing parts
of the seaweed. Water and sediment became completely anoxic, and fish
and benthic animals died. Only Chironomus larvae (Chironomus
salinarius Kieff.), which have an anaerobic metabolim (Ceretti et al.
1985), took advantage of this situation and grew uncontrolledly. As a
result, in August 1985, swarms of such Diptera invaded Venice hindering
railway and airport activities (Sfriso et al. 1987). Algal decomposition
lasted for 2-3 weeks and phytoplankton blooms for 1-2 months, after
which the water became clearer and macro algae recovered. At first,
seasonal species such as Cladophora spp. and Enteromorpha spp. occurred. Later, Ulva again grew reaching a biomass of many kilograms
per square metres; it survived the winter and again exhibited a fast growth
in the succeeding spring. The annual biomass trend of typical Ulva
associations is shown for the three lagoonal stations Lido, S. Giuliano and
Sacca Sessola (Fig. IS.2a).
In contrast to the areas of the lagoon described above, high species
diversity was found near the port entrance (such as Alberoni station),
where tidal currents assure a fast water renewal (Fig. IS.2b). Here, Ulva
represented on average only 15-20% of the total fresh biomass produced
yearly. Other species important for biological production, i.e. each contributing more than 1 % of the total annual biomass, and replacing each
other during the seasons were: Enteromorpha compressa, Enteromorpha
intestinalis, Gracilaria verrucosa, Porphyra leucosticta, Ceramium
rubrum, Dictyota dichotoma, Petalonia fascia, Ectocarpus siliculosus and
A. Sfriso and A. Marcomini
and filled the entire water column up to the surface. At this growth stage,
biomass was highly stratified with its productive layer approaching the
water surface. Below the canopy, light was considerably reduced
(PAR = 0-100 IlE m -2 s -1) and decomposition processes prevailed near
the bottom layers. The daily fluctuation of Ulva biomass within the water
column was caused both by tidal variation and oxygen production
during photosynthetic activity. The upper algal layer could remain
exposed to air and water loss for many hours or days during neap tides.
Beer and Eshel (1983) showed that Ulva is neither exceptionally resistant
to desiccation, nor does it benefit photosynthetically from emersion.
Ulva exhibited a negative carbon balance, upon exposure to morning or
noon light intensity, after only 30-90 min. Because of the poor water
renewal by tides due to macro alga masses filling up the entire water
column of the lagoon for many square kilometres, the amount of
dissolved oxygen became insufficient to balance the oxygen consumed by
the total biomass, and this led the macro algae to collapse very rapidly.
The subsequent release of high amounts of nutrients triggered sudden
phytoplankton blooms (especially Skeletonema costatum and Nitzschia
closterium) further contributing to macroalgal decay by shadowing parts
of the seaweed. Water and sediment became completely anoxic, and fish
and benthic animals died. Only Chironomus larvae (Chironomus
salinarius Kieff.), which have an anaerobic metabolim (Ceretti et al.
1985), took advantage of this situation and grew uncontrolledly. As a
result, in August 1985, swarms of such Diptera invaded Venice hindering
railway and airport activities (Sfriso et al. 1987). Algal decomposition
lasted for 2-3 weeks and phytoplankton blooms for 1-2 months, after
which the water became clearer and macro algae recovered. At first,
seasonal species such as Cladophora spp. and Enteromorpha spp. occurred. Later, Ulva again grew reaching a biomass of many kilograms
per square metres; it survived the winter and again exhibited a fast growth
in the succeeding spring. The annual biomass trend of typical Ulva
associations is shown for the three lagoonal stations Lido, S. Giuliano and
Sacca Sessola (Fig. IS.2a).
In contrast to the areas of the lagoon described above, high species
diversity was found near the port entrance (such as Alberoni station),
where tidal currents assure a fast water renewal (Fig. IS.2b). Here, Ulva
represented on average only 15-20% of the total fresh biomass produced
yearly. Other species important for biological production, i.e. each contributing more than 1 % of the total annual biomass, and replacing each
other during the seasons were: Enteromorpha compressa, Enteromorpha
intestinalis, Gracilaria verrucosa, Porphyra leucosticta, Ceramium
rubrum, Dictyota dichotoma, Petalonia fascia, Ectocarpus siliculosus and