Italy - The Lagoon of Venice
359
weight basis, the total GPP between the 1970s and the 1990s was
estimated at'" 200 (i.e. 20kgm- Z x 1.5 x 6.7) kgm- Z wet weight (Sfriso
et al. 1993).
Macroalgae strongly interfere with the lagoon's nutrient cycles,
especially in the areas covered by massive Ulva rigida populations. The
seasonal variations of phosphorus and nitrogen in the water and surface
sediments (0-5 cm) were strictly related to the biomass densities.
During the spring-summer algal growth, total inorganic nitrogen
(TIN = sum of NH: -N, NOZ1-N and N0 3 1 -N) in water rapidly reduced
from 30-90 to 2-7Ilmoldm-3. In contrast, reactive phosphorus (RP)
exhibited during this period the highest concentrations detected over the
year (from <0.1 to 16Ilmoldm-3; Fig. 15.7). Phosphorus, similarly to
nitrogen, was rapidly taken up by macro algae during their luxuriant
growth. The increasing layer of decaying seaweeds close to the sediment
surface caused reducing redox potentials (- 100 to - 250 m V),
which favoured the release of phosphorus together with reduced forms
from the sediments (Golterman 1973; Nixon et al. 1980; Cossu et al.
1983). Concurrently, active bacterial denitrifying processes took place
(Nixon et al. 1979; Henriksen et al. 1981; Jenkins and Kemp 1984; Rodnner
1985).
In the sediments, nitrogen and phosphorus as well as organic carbon
concentrations exhibited similar behaviour (Fig. 15.8). Macroalgae take
up nutrients from waters while growing, and release them onto the
surface sediments when biomass collapses. In 1985-86, in a station near
the Lido watershed covered by Ulva, surface sediment concentrations of
phosphorus, nitrogen and organic carbon increased by 75, 160 and 70%,
respectively (Fig. 15.8). Sediment nutrient concentrations decreased to
pre-spring values during winter because nutrients were slowly released
into the overlying water as a result of diagenetic processes.
The opposite behaviour of nutrients that occurs at the water-sediment interface under oxic or anoxic conditions also explains the different
trends of water and sediment N:P atomic ratios (Fig. 15.9). The nitrogen
to phosphorus ratio in water was high in the cold season (100-300),
when macroalgal growth was negligible and decreased nearly to zero in
spring-summer, during the highest macroalgal changes. In the upper
5 cm of the surface sediment, the N:P ratio increased temporarily from
7 -8 to 11-12, because of the nutrients released from the continuously
decaying biomass (N:P atomic ratio in Ulva was on average 25 ± 5;
Table 15.7).
The total amount of nutrients yearly recycled by macroalgae was
estimated from the nutrient concentrations in Ulva (accounting for
about 90% of the macroalgal biomass produced in the central lagoon
359
weight basis, the total GPP between the 1970s and the 1990s was
estimated at'" 200 (i.e. 20kgm- Z x 1.5 x 6.7) kgm- Z wet weight (Sfriso
et al. 1993).
Macroalgae strongly interfere with the lagoon's nutrient cycles,
especially in the areas covered by massive Ulva rigida populations. The
seasonal variations of phosphorus and nitrogen in the water and surface
sediments (0-5 cm) were strictly related to the biomass densities.
During the spring-summer algal growth, total inorganic nitrogen
(TIN = sum of NH: -N, NOZ1-N and N0 3 1 -N) in water rapidly reduced
from 30-90 to 2-7Ilmoldm-3. In contrast, reactive phosphorus (RP)
exhibited during this period the highest concentrations detected over the
year (from <0.1 to 16Ilmoldm-3; Fig. 15.7). Phosphorus, similarly to
nitrogen, was rapidly taken up by macro algae during their luxuriant
growth. The increasing layer of decaying seaweeds close to the sediment
surface caused reducing redox potentials (- 100 to - 250 m V),
which favoured the release of phosphorus together with reduced forms
from the sediments (Golterman 1973; Nixon et al. 1980; Cossu et al.
1983). Concurrently, active bacterial denitrifying processes took place
(Nixon et al. 1979; Henriksen et al. 1981; Jenkins and Kemp 1984; Rodnner
1985).
In the sediments, nitrogen and phosphorus as well as organic carbon
concentrations exhibited similar behaviour (Fig. 15.8). Macroalgae take
up nutrients from waters while growing, and release them onto the
surface sediments when biomass collapses. In 1985-86, in a station near
the Lido watershed covered by Ulva, surface sediment concentrations of
phosphorus, nitrogen and organic carbon increased by 75, 160 and 70%,
respectively (Fig. 15.8). Sediment nutrient concentrations decreased to
pre-spring values during winter because nutrients were slowly released
into the overlying water as a result of diagenetic processes.
The opposite behaviour of nutrients that occurs at the water-sediment interface under oxic or anoxic conditions also explains the different
trends of water and sediment N:P atomic ratios (Fig. 15.9). The nitrogen
to phosphorus ratio in water was high in the cold season (100-300),
when macroalgal growth was negligible and decreased nearly to zero in
spring-summer, during the highest macroalgal changes. In the upper
5 cm of the surface sediment, the N:P ratio increased temporarily from
7 -8 to 11-12, because of the nutrients released from the continuously
decaying biomass (N:P atomic ratio in Ulva was on average 25 ± 5;
Table 15.7).
The total amount of nutrients yearly recycled by macroalgae was
estimated from the nutrient concentrations in Ulva (accounting for
about 90% of the macroalgal biomass produced in the central lagoon