Direct and Indirect Effects of Herbivorous Zooplankton
5
can be made to persist throughout the growing season, and why species with
a refuge from grazing losses often seem less favored by high grazing pressure
than might be expected. As many species usually classified as inedible to zooplankton, like colony-forming blue green algae, also have very undesirable
effects on water quality, the success of biomanipulation in improving water
quality must be closely linked to mechanisms that might reduce the competitive ability of inedible algae. It is likely that one key to the suppression of
blue green algae in successful biomanipulations lies in the changes in the
flows of nutrients accompanying the restructuring of the food web.
1.2 Direct and Indirect Effects of Herbivorous
Zooplankton
The Chemostat Analogy of Grazer-Controlled Systems. No animals have
100% efficient utilization of their food, so that grazers will always recycle
some unutilized fraction of the nutrients contained in their food and will
thus function as both source and sink of phosphorus and other nutrients.
From this point of view, an increased grazing pressure will imply an
increased turnover of nutrients in the prey compartments in a way that
shares some likeness with increasing the flow rate in a chemostat.
Decreasing the turnover time or increasing the dilution rate in a population
of nutrient-limited algae generally has the effect of decreasing the yield of
algal biomass per unit of available nutrient. Reinertsen et al. (1989) found
such a reduction of algal phosphorus growth yield driven by increased
turnover from grazing to be quite important in improving the water quality
after the elimination of fish in Lake Haugatj0nna; but from the trophic
state concepts of Hutchinson (1967), this would be only an apparent oligotrophication of the system, as the carrying capacity in terms of total phosphorus for potential production of algal biomass would remain the same.
Since the relative abilities of species of plankton algae to succeed in competition for nutrients have been shown to change with both the dilution
rate and the supply mode of nutrient (Sommer 1985, 1986; Olsen et al.
1989), changes in the turnover rate resulting from zooplankton grazing also
could affect the resource competition between prey species. Selective grazing could either increase or reverse the advantage of the superior competitor for nutrients, depending on whether this species is selected or rejected
by grazers (Kilham 1987; Sterner 1989). The chemostat analogy of grazercontrolled systems thus seems to have several interesting aspects that
might contribute to explaining why many Daphnia-dominated communities apparently are uninvadable to inedible phytoplankton species, but the
complex interplay between differential loss rates and competitive abilities
cannot be easily analyzed without the help of some modeling effort.
5
can be made to persist throughout the growing season, and why species with
a refuge from grazing losses often seem less favored by high grazing pressure
than might be expected. As many species usually classified as inedible to zooplankton, like colony-forming blue green algae, also have very undesirable
effects on water quality, the success of biomanipulation in improving water
quality must be closely linked to mechanisms that might reduce the competitive ability of inedible algae. It is likely that one key to the suppression of
blue green algae in successful biomanipulations lies in the changes in the
flows of nutrients accompanying the restructuring of the food web.
1.2 Direct and Indirect Effects of Herbivorous
Zooplankton
The Chemostat Analogy of Grazer-Controlled Systems. No animals have
100% efficient utilization of their food, so that grazers will always recycle
some unutilized fraction of the nutrients contained in their food and will
thus function as both source and sink of phosphorus and other nutrients.
From this point of view, an increased grazing pressure will imply an
increased turnover of nutrients in the prey compartments in a way that
shares some likeness with increasing the flow rate in a chemostat.
Decreasing the turnover time or increasing the dilution rate in a population
of nutrient-limited algae generally has the effect of decreasing the yield of
algal biomass per unit of available nutrient. Reinertsen et al. (1989) found
such a reduction of algal phosphorus growth yield driven by increased
turnover from grazing to be quite important in improving the water quality
after the elimination of fish in Lake Haugatj0nna; but from the trophic
state concepts of Hutchinson (1967), this would be only an apparent oligotrophication of the system, as the carrying capacity in terms of total phosphorus for potential production of algal biomass would remain the same.
Since the relative abilities of species of plankton algae to succeed in competition for nutrients have been shown to change with both the dilution
rate and the supply mode of nutrient (Sommer 1985, 1986; Olsen et al.
1989), changes in the turnover rate resulting from zooplankton grazing also
could affect the resource competition between prey species. Selective grazing could either increase or reverse the advantage of the superior competitor for nutrients, depending on whether this species is selected or rejected
by grazers (Kilham 1987; Sterner 1989). The chemostat analogy of grazercontrolled systems thus seems to have several interesting aspects that
might contribute to explaining why many Daphnia-dominated communities apparently are uninvadable to inedible phytoplankton species, but the
complex interplay between differential loss rates and competitive abilities
cannot be easily analyzed without the help of some modeling effort.