Many theories have been formulated to explain the
role of grazers such as zooplankton in controlling the
amount of planktonic algae (phytoplankton) in lakes.
The first theories of such grazer control were merely
based on observations of negative correlations
between algal and zooplankton numbers. A low num-
ber of algal cells in the presence of a high number of
grazers suggested, but did not prove, that the grazers
had removed most of the algae. The converse obser-
vation, of the absence of grazers in areas of high
phytoplankton concentration, led Hardy to propose
his principle of animal exclusion, which hypothe-
sized that phytoplankton produced a repellent that
excluded grazers from regions of high phytoplankton
concentration. This was the first suggestion of algal
defenses against grazing.
Perhaps the fact that many of these first studies
considered only algae of a size that could be collected
in a net (net phytoplankton), a practice that over-
looked the smaller phytoplankton (nannoplankton)
that we now know grazers are most likely to feed on,
led to a de-emphasis of the role of grazers in subse-
quent research. Increasingly, as in the individual
studies of Lund, Round, and Reynolds, researchers
began to stress the importance of environmental
factors such as temperature, light, and water move-
ments in controlling algal numbers. These environ-
mental factors were amenable to field monitoring and
to simulation in the laboratory. Grazing was believed
to have some effect on algal numbers, especially after
phytoplankton growth rates declined at the end of
bloom periods, but grazing was considered a minor
component of models that predicted algal population
dynamics.
The potential magnitude of grazing pressure on
freshwater phytoplankton has only recently been
determined empirically. Studies by Hargrave and
Geen estimated natural community grazing rates by
measuring feeding rates of individual zooplankton
species in the laboratory and then computing com-
munity grazing rates for field conditions using the
known population density of grazers. The high esti-
mates of grazing pressure postulated by these
researchers were not fully accepted, however, until the
grazing rates of zooplankton were determined directly
in the field, by means of new experimental techniques.
Using a specially prepared feeding chamber, Haney
was able to record zooplankton grazing rates in natural
field conditions. In the periods of peak zooplankton
abundance, that is, in the late spring and in the summer,
Haney recorded maximum daily community grazing
rates, for nutrient-poor lakes and bog lakes, respectively,
of 6.6 percent and 114 percent of daily phytoplankton
production. Cladocerans had higher grazing rates than
copepods, usually accounting for 80 percent of the
community grazing rate. These rates varied seasonally,
reaching the lowest point in the winter and early spring.
Haney's thorough research provides convincing field
evidence that grazers can exert significant pressure on
phytoplankton population.