Phytoplankton blooms: a ‘loophole’ in microzooplankton grazing impact?

Phytoplankton size and relations between phytoplankton and microzooplankton (ciliates and heterotrophic dinoflagellates) biomass are analysed in 12 globally distributed areas. In view of the results, a hypothesis is posed where blooming species are those able to escape control by microzooplankton through a combination of predation avoidance mechanisms (e.g. larger size, colonies, spines, and toxic compounds) at the beginning of the bloom. Factors that help to enhance subsequent bloom development include positive feedback from the poor nutritional status of the phototrophic prey which adversely affects predation, inter-microzooplankton grazing and top–down grazing by mesozooplankton on microzooplankton. Blooming conditions are interpreted as physical or chemical perturbations disrupting the predator–prey controls that normally operate at the level of the microbial loop, opening ‘loopholes’ into which some phytoplankton species populations can explode.