Coastal Eutrophication and Marine Benthic Vegetation
105
years, sufficiently high to enhance the initially low competitive ability
caused by the low P/B ratio, and subsequently supports a rapid spring
growth. The growth of Ulva spp. occurs through vegetative increase of
pre-existing frond fragments, or by sexual reproduction on solid
supports, e.g. shells and crab carapaces (Sfriso et al. 1991). The high new
production of macroalgae depletes the DIN accumulated in the water
column during winter as well as the considerable nutrient inputs which
occurred in winter and early spring. The collapse of the macro algae
bloom after nutrient depletion is followed by partial decomposition of
the produced material. The resulting sudden availability of nutrients thus
released into the water triggers a subsequent low regenerated production
of phytoplankton (Sfriso et al. 1988). The biomass of the macro algae
fluctuates throughout the summer and autumn, until the climatic
conditions again become limiting but not lethal to the macro algae.
The principal difference between the three water systems is that new
production consists of phytoplankton in Lake Grevelingen and Lake
Veere, whereas the new production in the Venice Lagoon consists of
macro algae (Fig. 3.13). We suggest that the causal factor for this difference is climatic. Low winter temperatures prevent the survival of macroalgae in temperate regions. Mild winters at lower latitudes allow the
survival of macro algae, representing initial storage capacity for nutrients,
and enabling them to out-compete phytoplankton at the onset of the
growing season.
3.4.3 Nutrient Supply to Macroalgae by Flushing
A poor water exchange is generally considered to be beneficial for
macroalgae development and accumulation. Stagnancy of the water is
even suggested as the main factor responsible for the mass blooming of
Ulva spp. in the Venice Lagoon under the present conditions of nutrient
availability (Sfriso et al. 1989b).
However, if a distinction is made between the partly overlapping
phases in the seasonal cycle of (1) nutrient uptake and initial biomass
production, and (2) biomass accumulation and decay, it may be argued
that a certain amount of water exchange or flushing is beneficial or even
necessary for macro algae growth during the initial production phase.
It has been found for Porphyra tenera (Rhodophyta), an algal species
morphologically similar to Ulva. that increasing water exchange enhances growth at sites with comparatively low nutrient concentrations (Matsumoto 1959). In general, a current velocity of at least 15 cm s -1 was
needed for optimal growth, while in nutrient-deficient waters 30 cm S-1
105
years, sufficiently high to enhance the initially low competitive ability
caused by the low P/B ratio, and subsequently supports a rapid spring
growth. The growth of Ulva spp. occurs through vegetative increase of
pre-existing frond fragments, or by sexual reproduction on solid
supports, e.g. shells and crab carapaces (Sfriso et al. 1991). The high new
production of macroalgae depletes the DIN accumulated in the water
column during winter as well as the considerable nutrient inputs which
occurred in winter and early spring. The collapse of the macro algae
bloom after nutrient depletion is followed by partial decomposition of
the produced material. The resulting sudden availability of nutrients thus
released into the water triggers a subsequent low regenerated production
of phytoplankton (Sfriso et al. 1988). The biomass of the macro algae
fluctuates throughout the summer and autumn, until the climatic
conditions again become limiting but not lethal to the macro algae.
The principal difference between the three water systems is that new
production consists of phytoplankton in Lake Grevelingen and Lake
Veere, whereas the new production in the Venice Lagoon consists of
macro algae (Fig. 3.13). We suggest that the causal factor for this difference is climatic. Low winter temperatures prevent the survival of macroalgae in temperate regions. Mild winters at lower latitudes allow the
survival of macro algae, representing initial storage capacity for nutrients,
and enabling them to out-compete phytoplankton at the onset of the
growing season.
3.4.3 Nutrient Supply to Macroalgae by Flushing
A poor water exchange is generally considered to be beneficial for
macroalgae development and accumulation. Stagnancy of the water is
even suggested as the main factor responsible for the mass blooming of
Ulva spp. in the Venice Lagoon under the present conditions of nutrient
availability (Sfriso et al. 1989b).
However, if a distinction is made between the partly overlapping
phases in the seasonal cycle of (1) nutrient uptake and initial biomass
production, and (2) biomass accumulation and decay, it may be argued
that a certain amount of water exchange or flushing is beneficial or even
necessary for macro algae growth during the initial production phase.
It has been found for Porphyra tenera (Rhodophyta), an algal species
morphologically similar to Ulva. that increasing water exchange enhances growth at sites with comparatively low nutrient concentrations (Matsumoto 1959). In general, a current velocity of at least 15 cm s -1 was
needed for optimal growth, while in nutrient-deficient waters 30 cm S-1