The potential importance of bacterial processes in regulating rate of lake acidification1,2

Rates of microbial reduction of O 2 , Fe 3+ , Mn 4+ , NO 3 − , and SO 4 2− , and total generation of CO 2 and CH 4 were measured in the hypolimnia of three Canadian Shield lakes. Methanogenesis accounted for 72–80% of anoxic carbon generation, while sulfate reduction contributed 16–20%. The remainder of anoxic carbon generation (2–8%) originated from all of the other processes combined (nitrate, iron, and manganese reduction). In lakes affected by acid deposition, inputs of sulfate and nitrate will increase, and it is expected that reducing power normally going to methane production will be diverted to nitrate and sulfate reduction. The last two reduction reactions can result in alkalinity production, whereas methane production does not. A model was developed to predict the significance of hypolimnetic alkalinity production which could result from these reactions in lakes with known hypolimnetic reducing power (methane production). The model showed that the hypolimnia of two ELA lakes which have been made eutrophic artificially could potentially produce enough persistent alkalinity to neutralize “typical” acid deposition, while a lake that was not eutrophic could not. Besides trophic state, other factors important in determining a lake’s capability for hypolimnetic alkalinity production were watershed area: surface area ratio, the watershed retentions of H + , SO 4 2− , NO 3 − , and NH 4 + , and the degree of precipitation of FeS in the sediment.