Effects of zebra mussels (Dreissena polymorpha) on bacterioplankton: Evidence for both size-selective consumption and growth stimulation

Zebra mussels had significant direct and possible indirect effects on heterotrophic bacteria in two contrasting sites in Saginaw Bay. At a eutrophic site in the inner portion of Saginaw Bay, mussels fed directly on bacterial-sized particles and had a negative impact on bacterial abundances. Mussels removed large bacteria (> 0.9 mu m) more effectively than small bacteria at this site. individual mussels cleared from 37-89 ml per day. Results using different sizes of fluorescent microspheres suggest that zebra mussels have a lower limit for particle size removal that is less than 0.4 mu m. Contradictory to inner bay results, mussels at an outer bay oligotrophic site had a positive impact on heterotrophic bacterial abundance, perhaps as a result of indirect effects, such as nutrient or organic carbon excretion by the mussels. Differences in the impact of mussels on the bacterial communities of the inner bay and outer bay probably result from differences in trophic state and bacterial community structure. A hypothesized smaller size of bacteria at outer bay sites may enable them to escape heavy predation pressure from mussels and the high rates of mussel nutrient excretion may facilitate their growth in these nutrient depleted conditions.