alone, even under conditions of nutrient enrichment.
In controls, where fish
predation has eliminated the most effective algal grazers, leaving a system
dominated by rotifers and small. Cladocera, algal blooms and low water trans-
Similar effects have occurred in macrophyte-dominated lakes to which
grass carp have been introduced.
When macrophytes are eliminated, the zoo-
plankton community may shift from one dominated by large-bodied Cladocera, or
Copepoda-Cladocera, to one dominated by Rotifera and the small-bodied
Cladocera, Bosmina and
The algal blooms that sometimes accom-
pany the elimination of macrophytes (whether through grass carp or through
mechanical or chemical methods) are often attributed to increased nutrient
They may in fact be due to the elimination of cover for
plankton, followed by intense predation by fish and the elimination of effec-
tive algal grazers.
Or, algal blooms may in part be explained not by the low
grazing rate of small-bodied zooplankton, but by the high rates of nutrient
cycling associated with small-bodied zooplankton (Henry 1985).
A reduction in zooplankton grazing on algae may also occur when dis-
solved oxygen in deep water is low or absent.
Tooler, darker waters of the metalimnion and hypolimnion during the day to
escape predation by sight-feeding fish, and then return to surface waters to
graze at night when sight-feeding by fish is less
Low dissolved oxygen will restrict this daily migration and per-
As discussed in
mit effective fish feeding on zooplankton during daylight.
another section, one of the benefits of artificial circulation and
aeration may be to provide an oxygenated refuge for zooplankton from the
sight-feeding of fish.
Any factor that produces significant and prolonged zooplankton mortality
may bring about persistent algal blooms, low transparency, and associated
problems with dissolved oxygen and quality of potable water.
points out that agricultural runoff can have significant concentrations of
He lists 10 com-
pesticides that are lethal to zooplankton in trace amounts.
monly used Insecticides,
including malathion, diazinon, and Baytex, for which
LC 50 concentration to zooplankton is less than 1.0
sulfate, the most commonly used algicide, is very toxic to zooplankton
Taylor, and Taylor
and Farrell (1976) and Winner
al. (1977) found significant mortality to species of Daphnia, an effective