364
A. Sfriso and A. Marcomini
The increasing eutrophication observed over the 1950-1980s, in
particular, moved the lower limits of the vegetation upwards, decreased
the total number of species liVing in the Venice littorals and changed
the species characteristic of the main associations sampled in the
past.
Some species, such as Ulva rigida, prevailing in the soft bottoms with
weak tidal currents, spread over most of the centraNagoon. This species
easily reaches biomasses over 20 kg m -2 wet weight, forming thick layers
in which respiration processes may exceed photosynthetic oxygen
production, resulting in anoxic conditions.
Until 1990, net macroalgal production was 1 to 2 orders of magnitude
higher than that of phytoplankton, accounting for about 1.5 million t wet
weight, in the central lagoon alone.
The estimated total gross primary production which would balance
the phosphorus in SPM, surface sediment and algal biomass was six to
seven times higher than net macroalgal production.
Under natural conditions, daily macro algal production yield per metre
square strictly depends on the initial biomass: low initial biomass
(0.1-0.5 kg m- 2 , wet weight) can increase as much as 100-200%, while
high initial biomass (10-15 kg m- 2 , wet weight) usually shows low
increase (0-5%). The optimal starting biomass to obtain the highest
yields was 4-7 kg m- 2 •
Macroalgae strongly interfere with nutrient cycles. During spring and
summer, nitrogen and phosphorus concentrations in the water and
sediments are strictly related to biomass changes and, after total decay of
the macro algae, can temporarily increase two to three times in surface
sediments. Due to the different behaviour of these elements (release of
phosphorus into the water and atmospheric loss of nitrogen from
reduced sediments), nitrogen can temporarily become the limiting factor
for algal growth.
The amout of nutrients recycled by macroalgae exceeded considerably
the nutrient loads entering the lagoon annually. By evaluating the amout
of P and N, temporarily stored in standing crop and surface sediments
during the warm season, and estimating the loss of N due
to denitrification occuring extensively in the anoxic sediments, it
follows that the sea must be a significant source of nutrients for algal
growth.
A. Sfriso and A. Marcomini
The increasing eutrophication observed over the 1950-1980s, in
particular, moved the lower limits of the vegetation upwards, decreased
the total number of species liVing in the Venice littorals and changed
the species characteristic of the main associations sampled in the
past.
Some species, such as Ulva rigida, prevailing in the soft bottoms with
weak tidal currents, spread over most of the centraNagoon. This species
easily reaches biomasses over 20 kg m -2 wet weight, forming thick layers
in which respiration processes may exceed photosynthetic oxygen
production, resulting in anoxic conditions.
Until 1990, net macroalgal production was 1 to 2 orders of magnitude
higher than that of phytoplankton, accounting for about 1.5 million t wet
weight, in the central lagoon alone.
The estimated total gross primary production which would balance
the phosphorus in SPM, surface sediment and algal biomass was six to
seven times higher than net macroalgal production.
Under natural conditions, daily macro algal production yield per metre
square strictly depends on the initial biomass: low initial biomass
(0.1-0.5 kg m- 2 , wet weight) can increase as much as 100-200%, while
high initial biomass (10-15 kg m- 2 , wet weight) usually shows low
increase (0-5%). The optimal starting biomass to obtain the highest
yields was 4-7 kg m- 2 •
Macroalgae strongly interfere with nutrient cycles. During spring and
summer, nitrogen and phosphorus concentrations in the water and
sediments are strictly related to biomass changes and, after total decay of
the macro algae, can temporarily increase two to three times in surface
sediments. Due to the different behaviour of these elements (release of
phosphorus into the water and atmospheric loss of nitrogen from
reduced sediments), nitrogen can temporarily become the limiting factor
for algal growth.
The amout of nutrients recycled by macroalgae exceeded considerably
the nutrient loads entering the lagoon annually. By evaluating the amout
of P and N, temporarily stored in standing crop and surface sediments
during the warm season, and estimating the loss of N due
to denitrification occuring extensively in the anoxic sediments, it
follows that the sea must be a significant source of nutrients for algal
growth.