Large variation in vulnerability to grazing within a population of the colonial phytoplankter, Microcystis aeruginosa

We asked if intraspecific variation within a population of the toxic, bloom-forming phytoplankter, Microcystisaeruginosa, leads to differential vulnerability to grazing by the invasive, filter-feeding zebra mussel (Dreissenapolymorpha). We performed two series of laboratory feeding experiments in which D. polymorpha were presentedwith several different sympatric M. aeruginosa clones in a two-species mixture with the high-quality, nutritiousalga, Ankistrodesmus falcatus. Mean selectivity across M. aeruginosa clones ranged from near zero (i.e., no feedingon M. aeruginosa) to near one (equal feeding on the two algal species), evidence of maximal variation in grazingvulnerability across twenty clones of M. aeruginosa-a species assumed to be largely 'inedible' to grazers-fromthe same population. This range of vulnerability is essentially equal to that typically measured across allphytoplankton species. Large intraspecific variation in grazing vulnerability, and its ecological consequences,could influence the promotion or control of noxious blooms of toxigenic cyanobacteria.