Dreissenid mussels (Dreissena polymorpha and Dreissena bugensis) reduce microzooplankton and macrozooplankton biomass in thermally stratified lakes

We conducted a survey of 50 thermally stratified lakes with similar nutrient concentrations and morphometries in Michigan to examine the direct and indirect effects of dreissenid mussels on the biomass and community composition of microzooplankton and macrozooplankton. Twenty-five lakes were infested with dreissenid mussels (invaded), while 25 lakes were dreissenid free (uninvaded). In invaded lakes, phytoplankton biomass was 24% lower, and water clarity was 21% greater. Total microzooplankton biomass was 44% lower, with ciliate and rotifer biomass 39% and 45% lower, respectively, in invaded lakes. Total macrozooplankton biomass was 33% lower, largely driven by a 40% lower biomass of Daphnia spp. in invaded lakes. In contrast, dreissenid status had no significant influence on total copepod biomass, calanoid biomass, or cyclopoid biomass. Our microzooplankton results are similar to those of previous studies conducted in shallow, well-mixed systems, although the magnitude of the dreissenid influence in our study was smaller, as might be expected in thermally stratified systems. On the other hand, ours is the first study to document lower biomass of Daphnia spp. and reduced rotifer diversity in invaded lakes. In contrast, we found no difference in macrozooplankton community structure. Dreissenids likely affected zooplankton directly through predation (microzooplankton) and indirectly through resource competition (micro-and macrozooplankton). Understanding how dreissenid mussels affect both microand macrozooplankton will help us to identify the potential mechanisms by which higher trophic levels (e.g., fish) are influenced by these invaders.