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 e utrophic site in the inner portion of Saginaw Bay, mussels fed directl y on bacterial-sized particles and had a negative impact on bacterial abundances. Mussels removed large bacteria (> 0.9 mu m) more effective ly than small bacteria at this site. individual mussels cleared from 3 7-89 ml per day. Results using different sizes of fluorescent microsph eres suggest that zebra mussels have a lower limit for particle size r emoval 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 he terotrophic bacterial abundance, perhaps as a result of indirect effec ts, such as nutrient or organic carbon excretion by the mussels. Diffe rences in the impact of mussels on the bacterial communities of the in ner bay and outer bay probably result from differences in trophic stat e and bacterial community structure. A hypothesized smaller size of ba cteria at outer bay sites may enable them to escape heavy predation pr essure from mussels and the high rates of mussel nutrient excretion ma y facilitate their growth in these nutrient depleted conditions.