Grazing on microcystin-producing and microcystin-free phytoplankters by different filter-feeders: implications for lake restoration

Abstract.

We studied grazing on the green alga Scenedesmus obliquus and the cyanobacterium Planktothrix agardhii (a microcystin-producing and a microcystin- free strain) by three different filter-feeders to compare the efficiency with which these grazers remove Planktothrix from the water. The filterfeeders were two mussel species (the invasive species Dreissena polymorpha and the native Anodonta anatina) and one zooplankton species (Daphnia galeata). On a weight-specific basis, D. galeata had the highest observed clearance rates and A. anatina the lowest. However, D. galeata was only able to feed on Scenedesmus and not on Planktothrix, probably as a consequence of the morphology of Planktothrix (filaments), since neither microcystin-producing nor microcystin-free strains were grazed by Daphnia. Both mussel species were able to graze on Planktothrix, irrespective of microcystin content. Dreissena showed an especially high clearance rate. On an individual basis, however, Anodonta showed the highest clearance rates, which are a consequence of its large biomass. Densities of Anodonta in the field, however, are presently too low to exert a high grazing pressure on the phytoplankton. We suggest therefore that water managers, who may be interested in using filter-feeders as a biomanipulation tool, focus on the improvement of settlement conditions for native bivalves, such as Anodonta, instead of exotic species like Dreissena.