Microbial consumption of nitric and sulfuric acids in acidified north temperate lakes’

Rates of sulfate reduction and denitrification were measured in the sediments of unacidified, experimentally acidified, and atmospherically acidified lakes in North America and Norway. These data, plus profiles of porewater and sediment chemistry, demonstrated that in all of the lakes Hb was being actively consumed by both sulfate reducers and denitrifiers. Both of these microbial activities were assayed in sediments overlaid by oxygenated water, demonstrating that anoxic hypolimnia are not required for in situ alkalinity production. Neither short term experimental acidification nor long term atmospheric acidification had detectably inhibited the activity of these two types of bacteria. Both processes were active at pH 4.5. In lakes that were receiving significant quantities of both nitric and sulfuric acids, short term H b consumption from denitrification was 1.5-2 times faster than H-’ consumption by sulfate reduction. However on an annual basis, because of loss of reduced sulfur during fall and winter, long term H+ consumption by denitrification was estimated to be 4-5 times as large as H+ consumption by sulfate reduction. Atmospheric deposition of nitric and sulfuric acids has increased during the last several decades in various regions of the world, endangering a wide variety of aquatic organisms (Natl. Res. Count. Can. 198 1). However, not all of the acid entering the watershed of a lake causes an increase in acidity. H+ ions are consumed by chemical weathering processes both in the terrestrial catchment area and in the lake itself. In acidsensitive regions of the world, these chemical buffering processes are of limited capacity because of the slow rate of chemical weathering. In addition to chemical buffering, there are also biological mechanisms that provide buffering as long as the biological processes remain active and substrates are available. In the case of nitric acid, nitrate can be reduced to nitrogen gas by denitrifying bacteria or to organic material by photosynI Funded by NSERC grant A2671 and by the Department of Fisheries and Oceans, Canada. 2 Present address: Royal Ontario Museum, Univ. Toronto. thetic processes; both processes result in the consumption of H+ (alkalinity production) in an amount proportional to the consumption of N03(Kelly et al. 1982). Similarly, sulfate can be reduced by bacteria and stored either as iron sulfides (e.g. Berner 1984; Rudd et al. 1986) or as organic sulfides (Landers et al. 1983; Nriagu and Soon 1985; Rudd et al. 1986). As long as these sulfurcontaining compounds remain reduced, there is a net consumption of H+ that is related to the net loss of SOd2-. In the terrestrial catchment area, nitrate is normally retained much more efficiently than is sulfate (Likens et al. 1977; Jeffries et al. 1984) and most of the terrestrial biological alkalinity production involves nitrate reactions. However, eventually the terrestrial ecosystem appears to become saturated with nitrate, at which time nitrate retention in the watershed decreases from nearly 100% to about 13% (Likens et al. 1977). At that point, both nitrate and sulfate enter the lake in large amounts. Within lakes, nitrate and sulfate are reduced by algae or bacteria, providing additional alkalinity (Hongve 1978). These