Microbial and chemical responses to pulsed and gradual acid additions in dark- and clear-lake enclosures

Abstract

Responses of lake bacteria, protozoans, and chlorophyll a, to two applications of sulfuric acid were observed in lake enclosure experiments. Acid was applied all at once (pulsed addition) or gradually over a 2 week period (gradual addition) to decrease pH to 3.5. These experiments were carried out simultaneously in a dark-dystrophic lake and a clear-oligotrophic lake on the Katherine Ordway Preserve, near Gainesville, Fla. Pulsed acid addition in the clear lake caused bacterial densities and biomass to increase, and bacterial growth to decrease. This indicated decreased grazing of bacteria by protozoans and/or zooplankton. Gradual addition caused some positive effects on bacterial biomass and metabolic processes indicating changes to more acid tolerant bacteria. Bacteria did not have time to adapt in the pulsed acid addition, while they did in the gradual acid addition. Acid additions had no significant effects on protozoan densities or chlorophyll a. Acid additions did not significantly affect bacterial densities or biomass in the dark lake. Glucose assimilation and glucose mineralization were significantly higher on day 6, while only glucose assimilation was higher on day 12 in the pulsed addition. Pulsed acid addition caused a decline in protozoans and chlorophyll a. Gradual acid addition caused increased protozoan densities and decreased chlorophyll a. Either the bacteria were better adapted to acid conditions or the dark lake chemistry buffered them against the effects of acidification.