Cyanobacterial blooms occur when one or a few species of cyanobacteria dominate an ecosystem. These blooms can be detrimental to human well being and ecosystems as many bloom-forming cyanobacterial species produce toxins. At a regional scale, cyanobacterial blooms are more likely in eutrophic lakes. In mesotrophic lakes, however, it is difficult to predict cyanobacterial blooms. This may be due in part to a lack of working models to describe cyanobacteria-zooplankton interactions as well as a lack of field studies at appropriate scales. This thesis investigates how, at different temporal scales, toxin-producing, bloom-forming cyanobacteria interact with zooplankton and the importance of biotic and abiotic factors in explaining both microcystin content and cyanobacterial population dynamics. To accomplish this objective, a range of methods was employed: meta-analysis of literature data, experimentation and time-series analysis of a shallow mesotrophic lake (Constance Lake, Ontario) based on high frequency sampling over 2--3 years. The meta-analysis revealed high variability both within and across zooplankton species in their response to cyanobacteria. However, in most cases, zooplankton maintained positive growth rates when fed a diet containing cyanobacteria. A laboratory experiment with Daphnia showed that zooplankton can adapt to avoid toxic strains of cyanobacteria. Both these results suggest that zooplankton may play a role in controlling cyanobacterial biomass and potentially shifting cyanobacterial strains towards toxic genotypes. Field observations found that abiotic conditions, mainly temperature and pH, were more important than zooplankton when explaining the variability of microcystins. This lack of relationship between zooplankton and microcystins may have been due to the low cladoceran abundances in Constance Lake. Negative correlations, however, were detected between Anabaena and Daphnia and with cyclopoid copepods. Generally across cyanobacterial taxa, correlations with biotic variables such as the growth rates of rotifers and other cyanobacterial taxa, were more important than abiotic variables in explaining variability in cyanobacterial population growth rates. Surprisingly, these biotic relationships were predominantly positive suggesting that facilitation may play a larger role than previously thought in regulating cyanobacterial populations.

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