Effects of Nutrients and Top Predators on the Structure and Dynamics of a Freshwater Plankton Community: Experimental Studies
Vanni, Michael James
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Permalink
https://hdl.handle.net/2142/77623
Description
Title
Effects of Nutrients and Top Predators on the Structure and Dynamics of a Freshwater Plankton Community: Experimental Studies
Author(s)
Vanni, Michael James
Issue Date
1985
Department of Study
Ecology, Evolution, and Conservation Biology
Discipline
Ecology, Evolution, and Conservation Biology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Ecology
Language
eng
Abstract
Field experiments were conducted during two summers (1980 and 1981) in Dynamite Lake, Illinois, to determine the roles of planktivorous fish and nutrients in regulating the dynamics of the plankton community. Dynamite Lake contains high levels of bluegill sunfish and low nutrient levels. In both years nutrient and fish levels were manipulated in replicated, factorial-design experiments.
Nutrient addition greatly increased phytoplankton levels, resulting in elevated food for zooplankton. Population densities of zooplankton were affected more by manipulations of nutrients than by fish. Cladocerans increased more than other zooplankton in response to elevated food, but several rotifers and copepods also increased. Fish predation removed the largest size classes of the cladocerans Bosmina, Ceriodaphnia and Diaphanosoma, but these species responded by initiating reproduction at a smaller size and producing smaller offspring in the presence of fish. Flexibility in these life history traits allows the cladocerans to withstand what appears to be intense fish predation.
Nutrient enrichment increased total phytoplankton, in the presence and absence of fish; green algae increased more than other divisions. In 1980 the reduction in mean zooplankton size due to fish predation led to greater phytoplankton abundance, especially small, naked algae, with a corresponding decline in the proportion of biovolume comprised of species with gelatinous sheaths or hard coverings. In 1981 fish predation increased phytoplankton abundance only toward the end of the experiment, but resulted in an increased proportion of small phytoplankton species, with a concomitant decrease in the proportion of larger species, throughout the experiment.
To determine if the increase in edible (small) phytoplankton arising from fish predation results in elevated survivorship and/or fecundity of remaining cladocerans, life table experiments were run on cladocerans using, as media, natural phytoplankton assemblages from enclosures with and without fish. Fecundity and survivorship of Diaphanosoma, and fecundity of Ceriodaphnia, were higher on the water from the fish enclosure, indicating that predation on large size classes has positive effects on remaining individuals. Through use of projection matrices, it was shown that these indirect, positive effects of predation can greatly influence the age and size structure of populations.
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