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 photosyn-I 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%...

Fish from new reservoirs often have elevated concentrations of methylmercury (MeHg) that they primarily accumulate from food such as zooplankton. The objectives of this research were (i) to determine the effect of reservoir creation on total mercury (THg) and MeHg in zooplankton and (ii) to examine how variations in community structure and water chemistry affect MeHg bioaccumulation by zooplankton. Beginning in June 1992, we measured concentrations of THg and MeHg in zooplankton from an experimental reservoir (L979) and an unmanipulated reference pond (L632). After flooding of L979 in June 1993, mean concentrations of MeHg in zooplankton increased from 32 to >300 ng g-1 dw and THg increased from 87 to >500 ng g-1 dw. Annual fluxes of MeHg through the zooplankton community increased 10−100× after impoundment. MeHg concentra tions in zooplankton, seston, and water were strongly correlated with each other (r > 0.92). Bioaccumulation factors relating MeHg in zooplankton to MeHg in water or seston did not change after impoundment, despite large changes in water chemistry and zooplankton community structure. Concentrations of Hg in zooplankton from Lake 632 did not change dramatically over the 4 years of study.

The Flin Flon, Manitoba copper smelter was Canada's largest point source of mercury emissions until its closure in 2010 after ~80 years of operation. The objective of this study was to understand the variables controlling the local ground-level air mercury concentrations before and after this major point source reduction. Total gaseous mercury (TGM) in air, mercury in precipitation, and other ancillary meteorological and air quality parameters were measured pre- and postsmelter closure, and mercury speciation measurements in air were collected postclosure. The results showed that TGM was significantly elevated during the time period when the smelter operated (4.1 ± 3.7 ng m(-3)), decreased only 20% during the year following its closure, and remained ~2-fold above background levels. Similar trends were observed for mercury concentrations in precipitation. Several lines of evidence indicated that while smelter stack emissions would occasionally mix down to the surface resulting in large spikes in TGM concentrations (up to 61 ng m(-3)), the largest contributor to elevated TGM concentrations before and after smelter closure was from surface-air fluxes from mercury-enriched soils and/or tailings. These findings highlight the ability of legacy mercury, deposited to local landscapes over decades from industrial activities, to significantly affect local air concentrations via emissions/re-emissions.