The effect of macroinvertebrates on bacterial distributions in freshwater microcosms

Bacterial distributions among biofilms and the water column may be altered by a variety of abiotic and biotic factors including feeding and physical a ctivities of macroinvertebrates. The purpose of this study was to examine t he effect of macroinvertebrates on abundances and distributions of three sp ecies of bacteria that differ in their surface properties and niches. Eight invertebrate genera were examined; functional feeding groups included pred ators, shredders, filter feeders, and collector-gatherers. The three bacter ial species responded to the presence of the macroinvertebrates in generall y similar ways, although the species with the most developed ability to att ach to surfaces (Burkholderia cepacia) apparently resisted dislodgment by m acroinvertebrates. Bacillus thuringiensis, a relatively species-specific in secticidal bacterium, achieved higher abundances on invertebrate exteriors than did B. cepacia and Pseudomonas fluorescens. Orconectes reduced abundan ces of introduced bacteria on leaves and shifted distributions; a greater p ercentage of the bacterial populations was found in the sediments of the Or conectes microcosms than in microcosms lacking macroinvertebrates. These al terations were probably the result of consumption of bacteria from epiphyti c biofilms and subsequent deposition in feces. Corbicula shifted distributi ons of two of the bacterial species so that a greater percentage of the pop ulations was in the water column, perhaps as the result of burrowing activi ty. Both of these invertebrate species are common stream residents and may affect the distribution and persistence of natural and introduced bacteria.