4 Herbivores and Algae: Food Utilization,
Growth and Reproduction in Generalist Filter
Feeders
The pelagic zone of the ocean or a large lake
represents an essentially limitless food
supply for the individual zooplanktonic
animal. but. at the same time. this source of
energy may be so thinly dispersed that the
problem of getting enough to eat is still
formidable.
Conover (1968).
In a biogeographic perspective. almost all lakes can be considered ephemeral environments. The constraints of using resting stages for dispersal and
risk avoidance in such an environment is probably a major reason why
only the Rotifera and a few orders of Arthropoda (mainly Cladocera.
Calanoida. and Cyclopoida) have been successful in exploiting the pelagic
zone of lakes. while marine zooplankton also have representatives from at
least six other phyla (Lehman 1988). When compared to the multitude of
species commonly found in the phytoplankton of fresh waters. the zooplankton of lakes generally appear to be species-poor communities
(Pennak 1957).
Freshwater zooplankton community structure is closely linked to the
selective feeding activity of vertebrate predators (Brooks and Dodson
1965). Prey selection in planktivorous fish is generally determined by prey
visibility. such that large herbivore species like Daphnia will be selectively
removed when planktivore biomass is high. On the other hand. large filterfeeders like Daphnia seem to be competitively superior and tend to dominate the zooplankton community in the absence of strong vertebrate
predation pressure. The key competitive advantage of Daphnia has not
been unambiguously identified. though several alternative explanations
have been proposed (see. for example. the recent review by DeMott 1989).
The presence or absence of large Daphnia species has also been shown to
have greater impact on the phytoplankton community than total zooplankton abundance (Pace 1984; McQueen et al. 1986). emphasizing the
key role of the genus Daphnia in the pelagic food web.
Growth and Reproduction in Generalist Filter
Feeders
The pelagic zone of the ocean or a large lake
represents an essentially limitless food
supply for the individual zooplanktonic
animal. but. at the same time. this source of
energy may be so thinly dispersed that the
problem of getting enough to eat is still
formidable.
Conover (1968).
In a biogeographic perspective. almost all lakes can be considered ephemeral environments. The constraints of using resting stages for dispersal and
risk avoidance in such an environment is probably a major reason why
only the Rotifera and a few orders of Arthropoda (mainly Cladocera.
Calanoida. and Cyclopoida) have been successful in exploiting the pelagic
zone of lakes. while marine zooplankton also have representatives from at
least six other phyla (Lehman 1988). When compared to the multitude of
species commonly found in the phytoplankton of fresh waters. the zooplankton of lakes generally appear to be species-poor communities
(Pennak 1957).
Freshwater zooplankton community structure is closely linked to the
selective feeding activity of vertebrate predators (Brooks and Dodson
1965). Prey selection in planktivorous fish is generally determined by prey
visibility. such that large herbivore species like Daphnia will be selectively
removed when planktivore biomass is high. On the other hand. large filterfeeders like Daphnia seem to be competitively superior and tend to dominate the zooplankton community in the absence of strong vertebrate
predation pressure. The key competitive advantage of Daphnia has not
been unambiguously identified. though several alternative explanations
have been proposed (see. for example. the recent review by DeMott 1989).
The presence or absence of large Daphnia species has also been shown to
have greater impact on the phytoplankton community than total zooplankton abundance (Pace 1984; McQueen et al. 1986). emphasizing the
key role of the genus Daphnia in the pelagic food web.