Microbial Communities on Leaf Material Protected from Macroinvertebrate Grazing in Acidic and Circumneutral Streams

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Abstract
To determine how low pH and high aluminum concentrations directly affect microbial activity on decomposing leaves in streams, we examined microbial communities on leaf surfaces using in situ flow-through chambers which exclude macroinvertebrates. Bacterial production (thymidine incorporation) on leaf material was significantly higher at pH 6.4 than at 4.9 after 4 wk of incubation, and this difference persisted for the last 5 wk of the experiment. Microbial respiration was greater at the high pH site after 6 wk, and there were no significant differences in ATP standing stock. Concentrations of oxalate-extractable aluminum on the leaf material increased significantly within the first 4 wk of incubation and stabilized at significantly greater levels at low pH (4.66 mg∙g−1) than at high pH (1.70 mg∙g−1). Transferring leaf material from the high-pH to the low-pH stream had negative effects after only 1 wk of incubation. However, transferring from low pH to high pH did not result in significant positive effects until 4 wk after the transfers. The reduced microbial production and respiration in the low-pH stream imply that decomposition rates may be slower in acidified systems due, at least in part, to direct effects of acidification on microbial activity.