Genetically-based trait variation and size variation within a single population (Gull Lake, Michigan) of the toxic, bloom-forming phytoplanker, Microcystis aeruginosa, are explored as drivers of grazing vulnerability to the invasive zebra mussel (Dreissena polymorpha). Laboratory feeding experiments were used in which preference for M. aeruginosa was assessed relative to a high-quality alga (Ankistrodesmus falcatus). Chapter 1 demonstrates that some M. aeruginosa genotypes from the same population are maximally edible, whereas others are not ingested by mussels. Thus, the range in vulnerability of sympatric M. aeruginosa clones to mussels is equal to that reported across all phytoplankton species. Mussel selectivity was not related to microcystin toxin quota and is most likely driven by variation in expression of a genetically-based factor. In Chapter 2, the mortality rate of M. aeruginosa to 16-21 mm mussels is quantified as a function of colony size. Mussels were fed a single, highly palatable clone in size fractions. Colonies ≥ 111 μm were not consumed, representing a critical size threshold of invulnerability. Smaller colonies and single cells were consumed non-selectively. Field surveys of the Gull Lake M. aeruginosa population revealed that colony size consistently decreases during the summer, from above to less than or equal to the vulnerability threshold, which could allow for major shifts in the overall susceptibility of the population to mussel grazing. Intraspecific variation (genetic and phenotypic) in ecological traits could thus influence the promotion or control of noxious blooms of toxigenic cyanobacteria.
Read
