108
1. De Vries et al.
flushing time of the central area is also in the order of day (Sects. 3.2.1
and 3.4.3). The calculated annual average time constant for phytoplankton growth is 5-6 days.
Thus, control of phytoplankton biomass by the combined action of
flushing and grazing is probably very effective in the Venice Lagoon,
especially in the central part.
3.4.5 Conclusions
Eutrophication of marine coastal areas generally results in a decline of
seagrasses and in an increasing dominance of macro algae or phytoplankton. These shifts in species composition and biomass towards
macroalgal and phytoplankton dominance as final algal succession stages
under eutrophic conditions are also apparent in the three (semi-) enclosed polyhaline lagoons considered in this comparative study. The
carbon and nitrogen mass balances of the three water systems indicate,
however, that the causes of eutrophication, viz. high nutrient inputs and
resulting concentrations, do not determine the specific symptoms that
will emerge. The mass bloom and accumulation of Ulva spp. in the
Venice Lagoon arises from similar nitrogen loadings and resulting DIN
concentrations as in Lake Veere, where phytoplankton is dominant.
Thus, factors other than nutrient loading must play a decisive role in
determining the specific eutrophication symptoms which emerge under
generally eutrophic conditions.
Four factors which increase the competitive capabilities of foliose
macro algae over phytoplankton are identified. Although the evidence is
by no means unambiguous, all four factors appear capable of explaining
the dominance hierarchy among the primary producers at comparable
high levels of nitrogen input. However, it is difficult to determine which
factor plays the most important role in Lake Veere and in the Venice
Lagoon, as the factors involved operate simultaneously and may also
interact with each other.
Climatic conditions, especially low winter temperatures which prevent
the survival of macro algal biomass in temperate regions, are suggested as
causal factors for the absence of new macroalgal production in eutrophic Lake Veere. With no initial biomass present, phytoplankton and
microphytobenthos take advantage of their high P/B ratio and outcompete macroalgae in the initial stage of the growing season. The
sustained dominance of phytoplankton over macro algae in Lake Veere in
the regeneration production phase is consistent with the light climate,
determined by the bathymetry of the lake and light extinction. Due to the
1. De Vries et al.
flushing time of the central area is also in the order of day (Sects. 3.2.1
and 3.4.3). The calculated annual average time constant for phytoplankton growth is 5-6 days.
Thus, control of phytoplankton biomass by the combined action of
flushing and grazing is probably very effective in the Venice Lagoon,
especially in the central part.
3.4.5 Conclusions
Eutrophication of marine coastal areas generally results in a decline of
seagrasses and in an increasing dominance of macro algae or phytoplankton. These shifts in species composition and biomass towards
macroalgal and phytoplankton dominance as final algal succession stages
under eutrophic conditions are also apparent in the three (semi-) enclosed polyhaline lagoons considered in this comparative study. The
carbon and nitrogen mass balances of the three water systems indicate,
however, that the causes of eutrophication, viz. high nutrient inputs and
resulting concentrations, do not determine the specific symptoms that
will emerge. The mass bloom and accumulation of Ulva spp. in the
Venice Lagoon arises from similar nitrogen loadings and resulting DIN
concentrations as in Lake Veere, where phytoplankton is dominant.
Thus, factors other than nutrient loading must play a decisive role in
determining the specific eutrophication symptoms which emerge under
generally eutrophic conditions.
Four factors which increase the competitive capabilities of foliose
macro algae over phytoplankton are identified. Although the evidence is
by no means unambiguous, all four factors appear capable of explaining
the dominance hierarchy among the primary producers at comparable
high levels of nitrogen input. However, it is difficult to determine which
factor plays the most important role in Lake Veere and in the Venice
Lagoon, as the factors involved operate simultaneously and may also
interact with each other.
Climatic conditions, especially low winter temperatures which prevent
the survival of macro algal biomass in temperate regions, are suggested as
causal factors for the absence of new macroalgal production in eutrophic Lake Veere. With no initial biomass present, phytoplankton and
microphytobenthos take advantage of their high P/B ratio and outcompete macroalgae in the initial stage of the growing season. The
sustained dominance of phytoplankton over macro algae in Lake Veere in
the regeneration production phase is consistent with the light climate,
determined by the bathymetry of the lake and light extinction. Due to the