Size-structured vulnerability of the colonial cyanobacterium, Microcystis aeruginosa, to grazing by zebra mussels (Dreissena polymorpha)

aeruginosa, to grazing by the invasive filter-feeding zebra mussel (Dreissena polymorpha) as a function of size in both organisms with laboratory feeding experiments. In one experiment, size-selectivity of 16- to 21-mm mussels was assessed for a single M.aeruginosa clone across a wide size range (similar to 5-88m median equivalent diameter, ED). Consumption of colonies 80m median ED (109m median maximum linear dimension) was undetectable, indicating a size threshold of grazing invulnerability. Smaller colonies and single cells were consumed at rates similar to a highly palatable alga (Ankistrodesmus). In a second experiment, the size-selectivity of three size classes of mussels (8-11, 17-20 and 25-28mm shell length) was assessed across three size classes of M.aeruginosa (similar to 32-75m median ED). There were no systematic differences in the abilities of the different mussel size classes to consume the largest colonies within this size range. An 8-year field survey of the M.aeruginosa population in Gull Lake, MI (U.S.A.), the source of the experimental organisms, revealed that median colony size consistently decreased during each summer, from above to below the size threshold of effective mussel feeding we identified, which suggests major within-season shifts in the overall vulnerability of the M.aeruginosa population to mussel grazing. Variation in the size structure of M.aeruginosa may help explain highly variable effects of D.polymorpha on the dynamics of this harmful phytoplankter within and across systems.