AME 64:163-174 (2011)  -  DOI: https://doi.org/10.3354/ame01514

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 (~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 cyano­bacteria (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.

KEY WORDS: Microzooplankton · Grazing · Aphanizomenon flos-aquae · Harmful algal bloom · Eutrophication · Cyanobacteria