EFFECT OF MACROINVERTEBRATES ON DETACHMENT OF BACTERIA FROM BIOFILMS IN STREAM MICROCOSMS

Transport patterns of bacteria in streams depend largely on processes of detachment from and attachment to surfaces. Stream substrata are co ated with various microorganisms, including bacteria, embedded in a ma trix of extracellular material. These complex communities, called biof ilms, may be disrupted by movement and feeding activities of macroinve rtebrates causing bacterial cells to detach. Laboratory experiments we re conducted to assess the role of stream macroinvertebrates in detach ment of bacteria from biofilms. Marked bacteria that had a rare combin ation of antibiotic resistances were used in microcosms as tracers of bacterial exchanges. The role of macroinvertebrates in movement of bac teria from leaves to water was investigated by including either mayfly , stonefly, or dragonfly nymphs or glass shrimp in microcosms. The pre sence of macroinvertebrates did not alter bacterial exchange between h abitats. There were considerable variation among replicates for some m acroinvertebrate treatments, indicating invertebrates may cause large, catastrophic releases of bacteria from biofilms on leaves or fecal pe llets. Alternatively, invertebrates may not be directly involved in th ese releases if pulses of bacteria result from disintegration of fecal pellets. The effect of invertebrate density on exchange was measured by varying mayfly nymph density. No significant differences among dens ities were detected. The ability of macroinvertebrates to serve as vec tors for transfer of bacteria between leaf packs was also investigated . Although transfer by invertebrates was detected in some experiments, transfer was not consistently observed.