4
The Eutrophication Problem in Temperate Lakes: Practical Aspects and Theoratical ...
Pace (1984) found that total zooplankton biomass explained less of the
residual variance in the relationship between phosphorus and chlorophyll a
than the relative abundance of large cladoceran grazers, emphasizing the
importance of zooplankton community structure on the actual impact of
grazing on the phytoplankton community.
Large cladoceran grazers are also preferred prey of most planktivorous
fish species, so that their dominance will be possible only at low planktivore abundance (Hrbacek et al. 1961; Brooks and Dodson 1965). This led
Shapiro et al. (1975) to introduce the concept oflake restoration by biomanipuiation, where reduction of the predation pressure on large cladocerans
is attempted through either complete removal of fish by, for example, rotenone poisoning, or control of planktivorous fish populations by stocking
with piscivorous fish. Successful treatments of this kind have been found to
give a plankton community with a high zooplankton biomass dominated
by large Daphnia, and a reduced phytoplankton biomass dominated by
small, fast-growing species that are considered easily edible and assimilable
to zooplankton (e.g., Gulati et al. 1990, and references therein).
Since the initial enthusiasm created by the introduction of the biomanipulation concept, several cases have also been reported where food chain
manipulations have failed to improve water quality. In some cases with
very shallow lakes, the initial problem of excessive phytoplankton growth
has simply been replaced by a new problem of excessive growth of benthic
algae and macrophytes (e.g., Van Donk et al. 1989). In deeper lakes, failure
is usually associated with the invasion of the phytoplankton community by
large, inedible species instead of algal species that can be efficiently processed by the planktonic food chains.
Most reported biomanipulation experiments have been performed in
isolated lakes of the seepage type, where good phosphorus loading measurements are difficult to obtain. Benndorf (1987) observed that in all biomanipulations that were classified as successful according to his criteria the
phosphorus loading was either moderate or unknown, and that biomanipulation should therefore have the greatest likelihood of failure in lakes
receiving a phosphorus loading above some critical level. Based on this observation and on experiences from several biomanipulation experiments,
Benndorf (1987) suggested that this critical loading might be around 0.2 to
0.4 ~g P I-I day-I, although this must be considered as highly tentative until
more data from biomanipulations in lakes with different loading conditions have become available.
The immediate effect of an increased grazing pressure will necessarily be
reductions in the size of the prey populations of edible algae. Many species
have morphological adaptations that reduce ingestibility through a large
cell or colony size or digestibility through development of a resistant cell
wall or gelatinous sheath, so that different groups of prey species will be
captured or utilized with unequal efficiency by grazers. It is therefore not
intuitively obvious how a short, efficient food chain from algae to grazers
The Eutrophication Problem in Temperate Lakes: Practical Aspects and Theoratical ...
Pace (1984) found that total zooplankton biomass explained less of the
residual variance in the relationship between phosphorus and chlorophyll a
than the relative abundance of large cladoceran grazers, emphasizing the
importance of zooplankton community structure on the actual impact of
grazing on the phytoplankton community.
Large cladoceran grazers are also preferred prey of most planktivorous
fish species, so that their dominance will be possible only at low planktivore abundance (Hrbacek et al. 1961; Brooks and Dodson 1965). This led
Shapiro et al. (1975) to introduce the concept oflake restoration by biomanipuiation, where reduction of the predation pressure on large cladocerans
is attempted through either complete removal of fish by, for example, rotenone poisoning, or control of planktivorous fish populations by stocking
with piscivorous fish. Successful treatments of this kind have been found to
give a plankton community with a high zooplankton biomass dominated
by large Daphnia, and a reduced phytoplankton biomass dominated by
small, fast-growing species that are considered easily edible and assimilable
to zooplankton (e.g., Gulati et al. 1990, and references therein).
Since the initial enthusiasm created by the introduction of the biomanipulation concept, several cases have also been reported where food chain
manipulations have failed to improve water quality. In some cases with
very shallow lakes, the initial problem of excessive phytoplankton growth
has simply been replaced by a new problem of excessive growth of benthic
algae and macrophytes (e.g., Van Donk et al. 1989). In deeper lakes, failure
is usually associated with the invasion of the phytoplankton community by
large, inedible species instead of algal species that can be efficiently processed by the planktonic food chains.
Most reported biomanipulation experiments have been performed in
isolated lakes of the seepage type, where good phosphorus loading measurements are difficult to obtain. Benndorf (1987) observed that in all biomanipulations that were classified as successful according to his criteria the
phosphorus loading was either moderate or unknown, and that biomanipulation should therefore have the greatest likelihood of failure in lakes
receiving a phosphorus loading above some critical level. Based on this observation and on experiences from several biomanipulation experiments,
Benndorf (1987) suggested that this critical loading might be around 0.2 to
0.4 ~g P I-I day-I, although this must be considered as highly tentative until
more data from biomanipulations in lakes with different loading conditions have become available.
The immediate effect of an increased grazing pressure will necessarily be
reductions in the size of the prey populations of edible algae. Many species
have morphological adaptations that reduce ingestibility through a large
cell or colony size or digestibility through development of a resistant cell
wall or gelatinous sheath, so that different groups of prey species will be
captured or utilized with unequal efficiency by grazers. It is therefore not
intuitively obvious how a short, efficient food chain from algae to grazers