Relative grazing impact of microzooplankton and mesozooplankton on a bloom of the toxic dinoflagellate Alexandrium minutum

Grazing can be a substantial loss factor for harmful algal blooms (HABs) and a possible cause of their termination. There is, however, a considerable gap in our knowledge of the relative grazing impact of the different components of the planktonic community on a red tide. In this study, we examine the importance of grazing control on a naturally occurring bloom of the toxic dinoflagellate Alexandrium minutum. We estimated the grazing impact of microzooplankton (by the dilution method) and copepods (by 24 h incubations with cultured populations of Acartia grani and Oithona davisae) on a bloom of A. minutum. Grazing by microzooplankton (0.84 d(-1)) was equal to and even exceeded A. minutum growth rates (0.79 d(-1)). On the other hand, copepod impact on the A. minutum population was not significant, although they were actively feeding on the dinoflagellate, which accounted for 78 and 86% of the daily carbon intake of A. grani and O. davisae, respectively. Even if these results were extended to the highest realistic copepod abundance attainable in the area, the grazing impact on highly concentrated HABs would not be sufficient to significantly reduce the algal concentrations. In summary, grazing by microzooplankton can represent an important factor for the regulation of HABs.