6
The Eutrophication Problem in Temperate Lakes: Practical Aspects and Theoratical ...
Competition, Stoichiometry, and Resource Supply Ratios. Even with a relatively unselective grazer like Daphnia feeding on similarly sized, edible
algae, grazing could affect the outcome of resource competition. The resource competition theory of Tilman (1982) predicts that two species of
plankton algae can coexist under certain supply ratios of two limiting
nutrients, while other supply ratios can lead to the competitive exclusion of
one of the species.
Blue-green algae seem to have lower optimal N:P ratios than other phytoplankton algae (Tilman et al. 1982; Smith 1983), so that the supply ratio
of nitrogen to phosphorus might have an important effect on the competitive success of blue green algae. From measured elemental ratios in crustacean zooplankton species, Andersen and Hessen (1991) proposed that one
should expect a general tendency for higher N:P ratio in recycled nutrients
from zooplankton communities dominated by Daphnia than from copepod-dominated plankton, so that, in addition to the effects of selective
grazing, the zooplankton community structure could have an indirect
selection force on the species composition of the phytoplankton community through the N:P ratio of the recycled nutrients.
Resource Partitioning in Food Webs. While much emphasis has been
placed on the relationship between algae and phosphorus in water quality
management, all living organisms need phosphorus to build vital biomolecules like nucleic acids and membrane lipids. Recent results indicate that
the heterotrophic components of pelagic community (bacteria, zooplankton, and fish) have in fact higher requirements for phosphorus than typical
plankton algae (Vadstein et al. 1988; Brabrand et al. 1990; Andersen and
Hessen 1991).
For example, from the data on zooplankton phosphorus contents in
Andersen and Hessen (1991), we can calculate that in situations of heavy
Daphnia grazing like the spring clear-water phase in Lake Constance
(Lampert et al. 1986), up to 40% of total phosphorus can be located in
Daphnia biomass. Mazumder et al. (1988, 1989, 1990) found that planktivorous fish had a strong influence on the size distribution of particulate
phosphorus in both enclosure experiments and natural plankton communities, with more phosphorus being located in zooplankton fractions in the
absence of vertebrate predation. This skewing of the phosphorus size distribution toward large organisms like zooplankton was also found to increase
the sedimentation of phosphorus (Mazumder et al. 1989) and decrease the
hypolimnetic oxygen consumption (Mazumder et al. 1990).
These two observations are consistent if we take into account the comparatively high P:C ratio found in crustacean zooplankton (Andersen and
Hessen 1991), although reduced decomposition in the water column of
large, fast-sinking organisms, as suggested by Mazumder et al. (1990),
might also playa role in reducing the 02 consumption in the water column
per unit of sedimented phosphorus. The results of Mazumder et al. (1988,
The Eutrophication Problem in Temperate Lakes: Practical Aspects and Theoratical ...
Competition, Stoichiometry, and Resource Supply Ratios. Even with a relatively unselective grazer like Daphnia feeding on similarly sized, edible
algae, grazing could affect the outcome of resource competition. The resource competition theory of Tilman (1982) predicts that two species of
plankton algae can coexist under certain supply ratios of two limiting
nutrients, while other supply ratios can lead to the competitive exclusion of
one of the species.
Blue-green algae seem to have lower optimal N:P ratios than other phytoplankton algae (Tilman et al. 1982; Smith 1983), so that the supply ratio
of nitrogen to phosphorus might have an important effect on the competitive success of blue green algae. From measured elemental ratios in crustacean zooplankton species, Andersen and Hessen (1991) proposed that one
should expect a general tendency for higher N:P ratio in recycled nutrients
from zooplankton communities dominated by Daphnia than from copepod-dominated plankton, so that, in addition to the effects of selective
grazing, the zooplankton community structure could have an indirect
selection force on the species composition of the phytoplankton community through the N:P ratio of the recycled nutrients.
Resource Partitioning in Food Webs. While much emphasis has been
placed on the relationship between algae and phosphorus in water quality
management, all living organisms need phosphorus to build vital biomolecules like nucleic acids and membrane lipids. Recent results indicate that
the heterotrophic components of pelagic community (bacteria, zooplankton, and fish) have in fact higher requirements for phosphorus than typical
plankton algae (Vadstein et al. 1988; Brabrand et al. 1990; Andersen and
Hessen 1991).
For example, from the data on zooplankton phosphorus contents in
Andersen and Hessen (1991), we can calculate that in situations of heavy
Daphnia grazing like the spring clear-water phase in Lake Constance
(Lampert et al. 1986), up to 40% of total phosphorus can be located in
Daphnia biomass. Mazumder et al. (1988, 1989, 1990) found that planktivorous fish had a strong influence on the size distribution of particulate
phosphorus in both enclosure experiments and natural plankton communities, with more phosphorus being located in zooplankton fractions in the
absence of vertebrate predation. This skewing of the phosphorus size distribution toward large organisms like zooplankton was also found to increase
the sedimentation of phosphorus (Mazumder et al. 1989) and decrease the
hypolimnetic oxygen consumption (Mazumder et al. 1990).
These two observations are consistent if we take into account the comparatively high P:C ratio found in crustacean zooplankton (Andersen and
Hessen 1991), although reduced decomposition in the water column of
large, fast-sinking organisms, as suggested by Mazumder et al. (1990),
might also playa role in reducing the 02 consumption in the water column
per unit of sedimented phosphorus. The results of Mazumder et al. (1988,