Microzooplankton grazing before, during and after a cyanobacterial bloom in Vancouver Lake, Washington, USA

Abstract

We conducted 16 dilution experiments from April 2008 to January 2009 to estimate microzooplankton grazing and intrinsic phytoplankton growth rates before, during and after a bloom of filamentous cyanobacteria in Vancouver Lake, Washington, USA. Intrinsic phytoplankton growth rates were low in April (similar to 0.4 d(-1)), increased to a maximum (1.2 d(-1)) in May, and then declined to zero and became negative in June and early July, prior to a phytoplankton bloom dominated by cyano bacteria (Aphanizomenon flos-aquae). Phytoplankton growth rates rose as the bloom progressed, reaching rates > 1.0 d(-1) in August and September, then declined through autumn. Spring microzooplankton grazing rates were low (-0.3 to 0.3 d(-1)), then became substantially negative (-1.1 to -1.5 d(-1)) preceding the chlorophyll a bloom. During the bloom, grazing rates quickly increased to a maximum of 0.8 d(-1) and remained high as the bloom declined. Microzooplankton grazing specifically on cyanobacteria was high in spring (1.0 d(-1)), negative just before the bloom (-0.7 d(-1) to -1.0 d(-1)), and low in autumn (0.3 to 0.7 d(-1)). Negative grazing on cyanobacteria immediately before the bloom may have been due to preferential grazing on other co-occurring prey, thus enabling the bloom to form, while higher grazing rates on cyanobacteria, especially on Aphanizomenon flos-aquae, in autumn is likely to have contributed to the bloom's decline. These findings show that microzooplankton can potentially influence cyanobacterial blooms directly and indirectly through grazing.