Grazing on colonial and filamentous, toxic and non-toxic cyanobacteria by the zebra mussel Dreissena polymorpha

Colony forming and toxic cyanobacteria form a problem in surface waters of shallow lakes, both for recreation and wildlife. Zebra mussels, Dreissena polymorpha, have been employed to help to restore shallow lakes in the Netherlands, dominated by cyanobacteria, to their former clear state. Zebra mussels have been present in these lakes since they were created in the 19th century by the excavation of peat and are usually not considered to be an invasive species. Most grazing experiments using Dreissena have been performed with uni-cellular phytoplankton laboratory strains and information on grazing of larger phytoplankton taxa hardly exists. To gain more insight in to whether D. polymorpha is indeed able to decrease cyanobacteria in the phytoplankton, we therefore performed grazing experiments with zebra mussels and two species of cyanobacteria, that greatly differ in shape: colony forming strains of Microcystis aeruginosa and the filamentous species Planktothrix agardhii. For both species a toxic and a non-toxic strain was selected. We found that zebra mussels cleared toxic Planktothrix at a higher rate than non-toxic Planktothrix, toxic or non-toxic Microcystis. Clearance rates between the other strains were not significantly different. Both phytoplankton species, regardless of toxicity, size and shape, were found in equal amounts (based on chlorophyll concentrations) in the excreted products of the mussels (pseudofaeces). The results show that zebra mussels are capable of removing colonial and filamentous cyanobacteria from the water, regardless of whether the cyanobacteria are toxic or not. This implies that the mussels may be used as a biofilter for the removal of harmful cyanobacterial blooms in shallow (Dutch) lakes where the mussels are already present and not a nuisance. Providing more suitable substrate for zebra mussel attachment may lead to appropriate mussel densities capable of filtering large quantities of cyanobacteria.