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

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.