Assessing the role of zooplankton grazing on the development and decline of cyanobacteria blooms in Vancouver Lake, WA, USA

Background/Question/Methods

Since 2007 we have been investigating the factors that influence the development of intense seasonal cyanobacteria blooms in Vancouver Lake – a large, tidally-influenced shallow lake in the lower Columbia River flood plain. Over two complete bloom cycles (May – October) in 2008 and 2009, we conducted bi-weekly dilution experiments and grazer incubation experiments to concurrently measure cyanobacteria/algal growth rates, microzooplankton (ciliates, dinoflagellates) community grazing rates, and mesozooplankton (copepod and cladoceran) clearance and ingestion rates. 

Results/Conclusions

From April to June of both years, algal/cyanobacterial growth rates were maximal and microzooplankton grazing rates were relatively low.  By contrast, from mid-June to mid-July (immediately preceding each year’s cyanobacteria bloom), both algal growth rates and microzooplankton grazing rates were often negative, suggesting a “trophic cascading” effect may have led to conditions which promoted rapid cyanobacteria growth.  Algal growth rates rapidly increased back to maximal rates after the cyanobacteria bloom began, and remained high during the bloom from late July to early September.  However grazing rates of both microzooplankton and copepods also increased markedly as the bloom progressed, such that by September/October of 2008 and 2009 grazing rates were approximately equal to algal growth rates.  This suggests grazers may have contributed to the rapid decline in cyanobacteria abundance, and demonstrate that zooplankton grazing may play an important role in the overall development and decline of cyanobacteria blooms.