Toxic Cyanobacteria and Plant Defenses Against a Generalist Herbivore
Type of Degreethesis
Fisheries and Allied Aquacultures
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Toxin-producing cyanobacteria have frequently been hypothesized to limit the ability of herbivorous zooplankton (such as Daphnia) to control phytoplankton biomass by inhibiting Daphnia feeding, population growth, and in extreme cases, causing Daphnia mortality. Using two limnocorral experiments in eutrophic ponds, we tested the hypothesis that high levels of cyanobacterial toxins prevent Daphnia from strongly reducing phytoplankton abundance. At the start of the first experiment, phytoplankton communities were dominated by toxic Microcystis and Anabaena (~96% of total phytoplankton biomass), and concentrations of the toxin microcystin were ~3 µg/L. Daphnia pulicaria were collected from a eutrophic, fishless lake, cultured in the laboratory, and stocked at a low density (~0.3/L) into half (n = 4) of the enclosures. Over the first two weeks after adding Daphnia, toxin levels increased to ~6.5 µg/L, yet Daphnia populations increased. By the third week, Daphnia had suppressed phytoplankton biomass by ~74% relative to “no Daphnia” controls, and reduced phytoplankton biomass persisted until the conclusion of the experiment (6 August 2007). In the second experiment, phytoplankton communities were dominated by Anabaena and Microcystis, and our four treatments included a no Daphnia control and each of three genetically-distinct D. pulicaria clones that varied in tolerance to toxic Microcystis in a previously published laboratory experiment. We found no differences among the three D. pulicaria genotypes in their population growth rates and carrying capacities, and all three D. pulicaria genotypes reduced phytoplankton biomass similarly relative to controls. By the conclusion of the experiment (19 May 2008), D. pulicaria had completely eliminated cyanobacteria, regardless of their genotype. The ability of Daphnia to increase and suppress phytoplankton biomass, despite high levels of cyanobacteria and associated toxins, suggests that toxin-producing cyanobacteria do not prevent strong Daphnia control of phytoplankton biomass.