Zebra mussel (Dreissena polymorpha) selective filtration promoted toxic Microcystis blooms in Saginaw Bay (Lake Huron) and Lake Erie

Microcystis aeruginosa, a planktonic colonial cyanobacterium, was not abund ant in the 2-year period before zebra mussel (Dreissena polymorpha) establi shment in Saginaw Bay (Lake Huron) but became abundant in three of five sum mers subsequent of mussel establishment. Using novel methods, we determined clearance, capture, and assimilation rates for zebra mussels feeding on na tural and laboratory M. aeruginosa strains offered alone or in combination with other algae. Results were consistent with the hypothesis that zebra mu ssels promoted blooms of toxic M. aeruginosa in Saginaw Bay, western Lake E rie, and other lakes through selective rejection in pseudofeces. Mussels ex hibited high feeding rates similar to those seen for a highly desirable foo d alga (Cryptomonas) with both large ( > 53 mum) and small (< 53 mum) colon ies of a nontoxic and a toxic laboratory strain of M. aeruginosa known to c ause blockage of feeding in zooplankton. In experiments with naturally occu rring toxic M. aeruginosa from Saginaw Bay and Lake Erie and a toxic isolat e from Lake Erie, mussels exhibited lowered or normal filtering rates with rejection of M. aeruginosa in pseudofeces. Selective rejection depended on "unpalatable" toxic strains of M. aeruginosa occurring as large colonies th at could be rejected efficiently while small desirable algae were ingested.