Trophic interactions in an aquatic ecosystem: Relationships among predators, zooplankton, and macrophytes and their implications in shallow lake d ynamics
Bombich, Michael Paul (Mike)
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The study of interactions between trophic levels of an aquatic ecosystem are instrumental in determining methods for restoring aquatic communities damaged by eutrophication. Past research suggests that the manipulation of top predators such as bass or pike may support populations of macrophytes and zooplankton while suppressing populations of phytoplankton, thus improving water clarity. The research reported in this paper focuses on how such a manipulation "cascades" through lower trophic levels. A two week manipulation experiment was conducted in 900 L. mesocosms representing shallow lake systems. The purpose of this mesocosm experiment was to understand how fish, invertebrates, and macrophytes structure Daphnia populations, and, in tum, to examine the impacts of these interactions on phytoplankton and attached algae (periphyton, specifically algae on plants and walls). The macrophyte Megalodonta beckii was transplanted into half of each of eight cattle tanks containing lake sediment. D. magna were sampled and observed to determine a spatial and/ or diel response to the treatment: the presence of a green sunfish, Lepomis cyanellus. Chlorophyll a was analyzed from phytoplankton samples to describe primary productivity and from periphyton to determine if a compensatory relationship between phytoplankton and periphyton was caused by the presence of a fish. Samples were also taken of benthic invertebrates and aquatic macroinvertebrates to investigate potential interactions between fish, invertebrates, and Daphnia. Results of the experiment indicate that fish did not have an effect on Daphnia migration, phytoplankton abundance, or periphyton communities. Invertebrate predators and the presence of Megalodonta in high densities may have a confounding effect on interactions among fish, zooplankton and macrophytes. The data indicated that fish did not have an effect on phytoplankton or periphyton.