Coastal Eutrophication and Marine Benthic Vegetation .
109
comparatively large depth and episodic high turbidity, the maximum
depth and bottom area where macro algae can successfully compete with
phytoplankton for light is restricted, and are estimated at 1.3-2.3 m
and 25-40% of the total area, respectively. A further restriction for
macroalgal development in the shallow areas is formed by the shearing
force of wind-induced waves in the western part of the lake. It is
suggested that export of Ulva biomass into the gullies by wave action is
the cause of macroalgal production not being increased by the grazing
control of phytoplankton by suspension feeders.
Milder climatic conditions at lower latitudes enable the survival of
macro algal biomass. The coherent storage capacity for nutrients compensating the low P/B ratio may explain the dominance of Ulva spp.
over phytoplankton in the new production phase in the Venice Lagoon.
The dominance of Ulva is further facilitated by the shallowness of the
Venice Lagoon. Notwithstanding the high turbidity, the maximum depth
and area where macroalgae can successfully compete for light with
phytoplankton are estimated at 0.8-1.2 m and 40-50% of the total area,
respectively. Moreover, the irradiance at larger depths is sufficiently high
to allow colonization of deeper bottom areas when phytoplankton
growth is impeded by factors other than light.
It is argued that the effective flushing of the central area of the lagoon
is in two ways beneficial for macro algae development. We estimate that
the effective flushing time of the central area of the lagoon must be less
than 1 week to supply sufficient nutrients allowing the observed initial
biomass production during March-April. The same amount of flushing
may effectively reduce the phytoplankton concentration by exporting
phytoplankton out of the lagoon at a rate equal to or higher than the
production rate. Grazing by suspension feeders probably intensifies the
control of the phytoplankton biomass. With regard to the hydrodynamic
aspects, flushing or water exchange (m 3 S-I) which is beneficial or even
necessary for nutrient supply in the production phase has to be distinguished from the absence of high shearing forces resulting from high
flow velocities (m S-I) and wave action in the biomass accumulation
phase.
In summary, the mass blooming of Ulva in the Venice Lagoon and its
dominance over phytoplankton at the present level of nutrient input can
be explained by the combined influence of mild winters, shallowness and
effective flushing of the lagoon, and grazing of phytoplankton by
suspension feeders.
109
comparatively large depth and episodic high turbidity, the maximum
depth and bottom area where macro algae can successfully compete with
phytoplankton for light is restricted, and are estimated at 1.3-2.3 m
and 25-40% of the total area, respectively. A further restriction for
macroalgal development in the shallow areas is formed by the shearing
force of wind-induced waves in the western part of the lake. It is
suggested that export of Ulva biomass into the gullies by wave action is
the cause of macroalgal production not being increased by the grazing
control of phytoplankton by suspension feeders.
Milder climatic conditions at lower latitudes enable the survival of
macro algal biomass. The coherent storage capacity for nutrients compensating the low P/B ratio may explain the dominance of Ulva spp.
over phytoplankton in the new production phase in the Venice Lagoon.
The dominance of Ulva is further facilitated by the shallowness of the
Venice Lagoon. Notwithstanding the high turbidity, the maximum depth
and area where macroalgae can successfully compete for light with
phytoplankton are estimated at 0.8-1.2 m and 40-50% of the total area,
respectively. Moreover, the irradiance at larger depths is sufficiently high
to allow colonization of deeper bottom areas when phytoplankton
growth is impeded by factors other than light.
It is argued that the effective flushing of the central area of the lagoon
is in two ways beneficial for macro algae development. We estimate that
the effective flushing time of the central area of the lagoon must be less
than 1 week to supply sufficient nutrients allowing the observed initial
biomass production during March-April. The same amount of flushing
may effectively reduce the phytoplankton concentration by exporting
phytoplankton out of the lagoon at a rate equal to or higher than the
production rate. Grazing by suspension feeders probably intensifies the
control of the phytoplankton biomass. With regard to the hydrodynamic
aspects, flushing or water exchange (m 3 S-I) which is beneficial or even
necessary for nutrient supply in the production phase has to be distinguished from the absence of high shearing forces resulting from high
flow velocities (m S-I) and wave action in the biomass accumulation
phase.
In summary, the mass blooming of Ulva in the Venice Lagoon and its
dominance over phytoplankton at the present level of nutrient input can
be explained by the combined influence of mild winters, shallowness and
effective flushing of the lagoon, and grazing of phytoplankton by
suspension feeders.