106
1. De Vries et al.
was required. Menesguen (1992) explains the excessive proliferation of
Ulva spp. on very open sites along the French Atlantic coast by the
dynamic trapping of water masses in bays where the tidal residual drift
vanishes.
The following calculation illustrates the amount of flushing necessary
for the initial Ulva production in the Venice Lagoon. A typical maximum
Ulva biomass, averaged over the area occupied by Ulva, is 2.5 kg m- 2
wet weight. This biomass develops in approximately 1.5 months (MarchApril) from the winter wet weight biomass of 0.5 kg m -2. For such an
increase in biomass 7.2 g Nm -2 would be needed. The supply of nitrogen
released from the sediment may be assumed to be negligible, since the
lowest concentration of nitrogen in the sediment was found in this
period (February-April; Sfriso et al. 1988). The typical winter concentration of DIN in the inner lagoon is 0.7 gN m -3. Given a mean water depth
of 1 m in the central area, the water volume of the area occupied by Ulva
would have to be exchanged ten times in a period of 1.5 months to
supply the needed amount of nitrogen. It therefore seems probable that
the effective flushing time of the central area of the lagoon of Venice
occupied by VIva during March-April is less than 1 week. The flushing
may be over estimated in this manner, since storage of nitrogen in Ulva
tissue during winter may sustain part of the growth in the initial months
of the growing season (Rosenberg and Ramus 1982). On the other hand,
the calculation is based on the average maximum biomass in the central
area of the lagoon. Maximum standing crop can locally exceed 20 kg m- 2
wet weight, developing from a very low winter biomass (Sfriso et al.
1988, 1989b).
3.4.4 Control of Phytoplankton by grazing and Flushing
Grazing, particularly by benthic suspension feeders, can effectively reduce
phytoplankton biomass, thus acting as a natural control on effects induced
by eutrophication. Grazing by suspension feeders is more effective than
grazing by zooplankton, since the overwintering standing stock of
suspension feeders represents a grazing capacity already present at the
onset of phytoplankton development. Phytoplankton biomass is controlled
by grazing when the water recycling time for the suspension feeding
community is equal to the time constant of phytoplankton growth (Cloern
1982; Officer et al. 1982). This comparison of time constants is
appropriate, since it takes account of the increase of the production and
PI B ratio of phytoplankton caused by the acceleration of nutrient cycling
by suspension feeding activity (DeVries and Hopstaken 1984).
1. De Vries et al.
was required. Menesguen (1992) explains the excessive proliferation of
Ulva spp. on very open sites along the French Atlantic coast by the
dynamic trapping of water masses in bays where the tidal residual drift
vanishes.
The following calculation illustrates the amount of flushing necessary
for the initial Ulva production in the Venice Lagoon. A typical maximum
Ulva biomass, averaged over the area occupied by Ulva, is 2.5 kg m- 2
wet weight. This biomass develops in approximately 1.5 months (MarchApril) from the winter wet weight biomass of 0.5 kg m -2. For such an
increase in biomass 7.2 g Nm -2 would be needed. The supply of nitrogen
released from the sediment may be assumed to be negligible, since the
lowest concentration of nitrogen in the sediment was found in this
period (February-April; Sfriso et al. 1988). The typical winter concentration of DIN in the inner lagoon is 0.7 gN m -3. Given a mean water depth
of 1 m in the central area, the water volume of the area occupied by Ulva
would have to be exchanged ten times in a period of 1.5 months to
supply the needed amount of nitrogen. It therefore seems probable that
the effective flushing time of the central area of the lagoon of Venice
occupied by VIva during March-April is less than 1 week. The flushing
may be over estimated in this manner, since storage of nitrogen in Ulva
tissue during winter may sustain part of the growth in the initial months
of the growing season (Rosenberg and Ramus 1982). On the other hand,
the calculation is based on the average maximum biomass in the central
area of the lagoon. Maximum standing crop can locally exceed 20 kg m- 2
wet weight, developing from a very low winter biomass (Sfriso et al.
1988, 1989b).
3.4.4 Control of Phytoplankton by grazing and Flushing
Grazing, particularly by benthic suspension feeders, can effectively reduce
phytoplankton biomass, thus acting as a natural control on effects induced
by eutrophication. Grazing by suspension feeders is more effective than
grazing by zooplankton, since the overwintering standing stock of
suspension feeders represents a grazing capacity already present at the
onset of phytoplankton development. Phytoplankton biomass is controlled
by grazing when the water recycling time for the suspension feeding
community is equal to the time constant of phytoplankton growth (Cloern
1982; Officer et al. 1982). This comparison of time constants is
appropriate, since it takes account of the increase of the production and
PI B ratio of phytoplankton caused by the acceleration of nutrient cycling
by suspension feeding activity (DeVries and Hopstaken 1984).