The role of zooplankton grazing on harmful cyanobacteria blooms in Vancouver Lake, WA
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Harmful algal blooms in urban aquatic systems are an increasing problem, both regionally and worldwide. Since 2007 we have been investigating the factors that influence seasonal cyanobacteria blooms in Vancouver Lake – a large, shallow, urban lake in the lower Columbia River flood plain. Over two years (2008-09) we conducted bi-weekly experiments during summer/fall to measure cyanobacteria and algal growth rates, and the grazing rates of both small (<200 μm “microzooplankton”) and large (>200 μm “mesozooplankton”) pelagic consumers, to assess how zooplankton grazing may have influenced the magnitude and timing of cyanobacteria blooms. In spring of both 2008 and 2009 algal growth rates were maximal and microzooplankton grazing rates were relatively low. By contrast, from mid-June to mid-July of both years (immediately preceding the cyanobacteria blooms), algal growth rates were strongly negative, suggesting conditions for algal growth had substantially degraded. Algal growth rates rapidly increased to maximal rates at the beginning of the cyanobacteria bloom, and remained high during the bloom from late July to early September of both 2008 and 2009. However zooplankton grazing rates also increased markedly as the bloom progressed, such that by the end of the blooms grazing rates were comparable to algal growth rates. This suggests grazers may have contributed to the rapid decline in cyanobacteria abundance by September/October. These experimental results demonstrate that zooplankton grazing may play an important role in the development and decline of cyanobacteria blooms in large, shallow turbid lakes.