CLEARANCE AND PROCESSING OF ALGAL PARTICLES BY ZEBRA MUSSELS (DREISSENA-POLYMORPHA)

The exotic zebra mussel, Dreissena polymorpha Pallas, has become a dom inant member of nearshore benthic communities in the Laurentian Great Lakes. Suspension-feeding bivalves such as the zebra mussel filter alg al particles from the water column and either reject them as pseudofec es, digest them, or egest them as feces. We used laboratory experiment s to compare clearance and particle processing of two green algal spec ies by zebra mussels. The effect of algal concentration on clearance r ate of Chlamydomonas reinhardtii varied between large and small mussel s. When mussels were fed Pandorina morum, clearance rate declined with increasing algal concentration. Mussel size affected clearance of C. reinhardtii but not P. morum. On a diet of P. morum, pseudofeces produ ction was constant across algal concentrations. When fed C. reinhardti i, mussels increased pseudofeces production as algal concentration inc reased once a threshold was crossed. Below this threshold, no pseudofe ces were produced. Measured clearance rates tended to be as high or hi gher than those previously reported indicationg that incipient limitin g concentrations vary with the types of particle processed. Absorption efficiencies were similar for both algal species. Our results show th at particle processing by zebra mussels depends on the types of partic les present in the water column and the size structure of the mussel p opulation. To accurately determine the impacts of zebra mussels on the trophic structure of ecosystems and the cycling of contaminants, inve stigators must use realistic algal assemblages and account for the siz e structure of mussel populations.