Epilithic Microbial Populations and Leaf Decomposition in Acid-Stressed Streams

Palumbo, Anthony; Mulholland, Patrick; Elwood, Jerry

ABSTRACT: The number of bacteria on decomposing leaf material (direct counts of acridine orange-stained bacteria) and bacterial production on leaf material and on rock surfaces (incorporation of tritiated thymidine into dioxyribonucleic acid, DNA) were measured as part of a study of the effects of acidification on microbial communities in streams. The number of bacteria on leaf material ranged from 3.3 x 10" 1 to 12.4 x 1010 cells per gram dry weight, it was not significantly different among streams of different pH and aluminum concentration, it increased over the 1st 4 weeks of incubation, and it was relatively constant therafter at all sites. There was no significant dilution of added thymidine by unlabeled internal or external pools of thymidine in measurements of bacterial production on leaf material. Baterial production opn both leaf and rock surfaces was significantly greater in streams with higher pH (>5.5) than at matched sites in nearby streams with lower pH (<5.0). However, differences in productivity among sites were not evident until after 2 weeks of incubation of leaf material in the streams, and significant differences could not be generated in short-term (<48-h) transplants of rock material from high to low pH sites. Therefore, the effects of acidification on bacteria do not appear to be due to acute toxicity. Because Al concentration and pH covary in these streams (high Al concentrations at low pH), the observed depression of bacterial production at lower pH sites may be due to chronic effects of high concentrations of both H+ and Al3+ or may be indirectly due to the effects of acidification on the invertebrate grazing community in the streams.