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Harmful cyanobacterial blooms represent one of the most serious ecological stressors in lakes, rivers, estuaries and marine environments. When there are persistent or frequent blooms with high biomass of cyanobacterial cells, colonies or filaments in the water, a wide range of impacts on the ecosystem may occur. These are well established in the scientific literature and are summarized in Paerl et al. (2001). Blooms may shade the water and thereby inhibit growth of other primary producers including phytoplankton, benthic algae and vascular plants and may elevate pH, particularly in poorly buffered waters. High population densities of large cyanobacteria interfere with food collection by filter–feeding zooplankton. The senescence and subsequent microbial decomposition of blooms may impact benthic macro–invertebrate community structure, as well as fish and other biota, due to increased organic loading and resulting anoxia of sediments, accumulation of NH4 in the water and accompanying increases in pH. Blooms of toxic cyanobacteria have been implicated in mass mortalities of birds and fish (e.g., Matsunaga et al. 1999; Rodger et al. 1994), but the importance of cyanotoxins relative to the other stressors that accompany blooms remains unknown. With persistent blooms, there are substantial declines in biodiversity at all levels ranging from phytoplankton and zooplankton to birds. Changes in nutrient cycling and disruptions of carbon and energy flow in pelagic and benthic food webs are observed (Paerl et al 1998). Where blooms become severe in shallow lakes, a positive feedback loop develops through various biological mechanisms related to the presence of cyanobacteria and fish that maintains a turbid water state (Scheffer and Carpenter 2003).