80
1. De Vries et ai.
1987). During the increasing eutrophication of the Peel-Harvey estuary,
Australia, the macroalgal species composition suddenly shifted from
Cladophora montagneana Klitz to Chaetomorpha linum Klitz, followed
by gradual shift to Enteromorpha spp., mainly E. intestinalis, and Ulva
rigida (Lavery et al. 1991). Nutrient enrichment of the Baltic Sea coincided with a shift from Furcellaria lumbricalis and Fucus vesiculosus to
Ectocarpaceae (Zmudzinski 1992).
Blooms of macroalgae and phytoplankton represent extreme shifts in
species composition and biomass of marine vegetation. Hypertrophic
conditions in the Venice Lagoon resulted in an extraordinary growth of
Ulva spp. with local biomass maxima up to 20 kg wet weight m -2 (Sfriso
et al. 1989b). Coinciding with eutrophication, the phytoplankton primary
production and biomass almost doubled in the Baltic (Zmudzinski 1992)
and cell numbers increased tenfold in Kastela Bay, Yugoslavia (Kelly and
Naguib 1984). Blooms can have dramatic ecological and economic
consequences if they result in oxygen depletion and the associated mass
mortality of the benthos (Kelly and Naguib 1984; Anonymous 1988;
Sfriso et al. 1989b).
Both macroalgal and phytoplankton dominance can be considered as
final algal succession stages in eutrophic shallow waters. The dominance
may vary in space and time. Macroalgae and phytoplankton blooms
occurred simultaneously in different parts of several lagoons along the
coast of Adriatic Sea, e.g. the Po delta (Pugnetti et al. 1991) and the
Venice Lagoon (Sfriso et al. 1991). In the Peel-Harvey estuary, Australia,
the dense spring bloom of the blue-green alga Nodularia collapsed in
summer, and was followed by a bloom of macro algae (Lavery et al. 1991).
The mechanisms of the decline of seagrasses as the result of eutrophication are extensively reviewed in current literature (Shepherd
et al. 1989) and will not be discussed in this chapter. However, the
mechanism behind the succession from phytoplankton to macro algae is
still unknown. This succession seems contra-indicated from the point of
view that a species with the highest affinity for the limiting resource has
competitive advantage and will thus outcompete the other species. In
general, phytoplankton species have a larger ratio of surface to volume
than macro algae, as do filamentous macroalgae when compared to
foliose species. This ratio affects metabolic rates such as productivity and
nutrient uptake, and hence growth, which is much faster for small
organisms (Littler and Littler 1980). In agreement with these principles,
the production (gCm- 2 year-I) over biomass (gCm- 2 ) ratio (=P/B
ratio year -1) of macroalgae is 10 -100 times lower than that of phytoplankton (Atkinson and Smith 1983; Sfriso et al. 1988). In nutrient-limited
environments, phytoplankton is expected to dominate over macroalgae,
1. De Vries et ai.
1987). During the increasing eutrophication of the Peel-Harvey estuary,
Australia, the macroalgal species composition suddenly shifted from
Cladophora montagneana Klitz to Chaetomorpha linum Klitz, followed
by gradual shift to Enteromorpha spp., mainly E. intestinalis, and Ulva
rigida (Lavery et al. 1991). Nutrient enrichment of the Baltic Sea coincided with a shift from Furcellaria lumbricalis and Fucus vesiculosus to
Ectocarpaceae (Zmudzinski 1992).
Blooms of macroalgae and phytoplankton represent extreme shifts in
species composition and biomass of marine vegetation. Hypertrophic
conditions in the Venice Lagoon resulted in an extraordinary growth of
Ulva spp. with local biomass maxima up to 20 kg wet weight m -2 (Sfriso
et al. 1989b). Coinciding with eutrophication, the phytoplankton primary
production and biomass almost doubled in the Baltic (Zmudzinski 1992)
and cell numbers increased tenfold in Kastela Bay, Yugoslavia (Kelly and
Naguib 1984). Blooms can have dramatic ecological and economic
consequences if they result in oxygen depletion and the associated mass
mortality of the benthos (Kelly and Naguib 1984; Anonymous 1988;
Sfriso et al. 1989b).
Both macroalgal and phytoplankton dominance can be considered as
final algal succession stages in eutrophic shallow waters. The dominance
may vary in space and time. Macroalgae and phytoplankton blooms
occurred simultaneously in different parts of several lagoons along the
coast of Adriatic Sea, e.g. the Po delta (Pugnetti et al. 1991) and the
Venice Lagoon (Sfriso et al. 1991). In the Peel-Harvey estuary, Australia,
the dense spring bloom of the blue-green alga Nodularia collapsed in
summer, and was followed by a bloom of macro algae (Lavery et al. 1991).
The mechanisms of the decline of seagrasses as the result of eutrophication are extensively reviewed in current literature (Shepherd
et al. 1989) and will not be discussed in this chapter. However, the
mechanism behind the succession from phytoplankton to macro algae is
still unknown. This succession seems contra-indicated from the point of
view that a species with the highest affinity for the limiting resource has
competitive advantage and will thus outcompete the other species. In
general, phytoplankton species have a larger ratio of surface to volume
than macro algae, as do filamentous macroalgae when compared to
foliose species. This ratio affects metabolic rates such as productivity and
nutrient uptake, and hence growth, which is much faster for small
organisms (Littler and Littler 1980). In agreement with these principles,
the production (gCm- 2 year-I) over biomass (gCm- 2 ) ratio (=P/B
ratio year -1) of macroalgae is 10 -100 times lower than that of phytoplankton (Atkinson and Smith 1983; Sfriso et al. 1988). In nutrient-limited
environments, phytoplankton is expected to dominate over macroalgae,