We studied the effect of increasing hydrogen ion (H+) concentration on the uptake of mercury (Hg(II)) by an aquatic bacterium. Even small changes in pH (7.3-6.3) resulted in large increases in Hg(II) uptake, in defined media. The increased rate of bioaccumulation was directly proportional to the concentration of H+ and could not be explained by assuming that the source of Hg to the bacteria was diffusion of neutrally charged species such as HgCl2. Thus, pH appeared to affect a facilitated mechanism by which Hg(II) is taken up by the cells. Lowering the pH of Hg solutions mixed together with natural dissolved organic carbon, or with whole lake water, also increased bacterial uptake of Hg(II). These findings have several potential implications for mercury cycling, including effects on elemental mercury production, mercury sedimentation, and microbial methylation of Hg(II), and could be part of the explanation for the observed positive correlation between lake acidity and methyl mercury levels in fish.

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%...

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From 1987 to 1992, Cd was added during the ice-free season to Experimental Lakes Area (ELA) Lake 382 at loadings of 1.7-4.2 mg⋅m -2 ⋅year -1 . Cd was lost rapidly from the water column to sediments. Sediment cores (1987)(1988)(1989)(1990)(1991)(1992)(1993) document increasing (from <1 to >5 mg⋅kg dry mass -1 ) sediment Cd concentration over time, sediment storage accounting for 90-95% of added Cd. Sedimentation of Cd occurred in both depositional and erosional sediment zones. Sandy sediments at shallow sites, despite having low sediment Cd concentrations, were important sinks for Cd because of their great mass per unit area. Progressive transport of Cd into the mixed sediment layer (8-10 cm thick, mixing in a decade or more) at deep sites was another important sink. Selective extractions suggest weak surface adsorption of Cd to Mn and Fe oxyhydroxides in periphyton and sedimenting particles, with high K d . Bottom sediments have lower K d , although Cd tends to be associated with more strongly bound organic or sulfide fractions. This may indicate loss of easily mobilized Cd from bottom sediment during diagenesis. Because Cd is readily released from fresh sediment, we hypothesize that recycling of sediment Cd to the water column will slow the recovery of Lake 382 to premanipulation conditions. Résumé : De 1987 à 1992, on a ajouté du Cd à l'eau du lac 382 de la Région des lacs expérimentaux, pendant la saison d'eaux libres, à raison de 1,7 à 4,2 mg⋅m -2 ⋅an -1 . Le Cd passait rapidement de l'eau aux sédiments. Des carottes (1987-1993) permettent de suivre la hausse de concentration (de <1 à >5 mg⋅kg -1 de masse sèche) du Cd dans les sédiments avec le temps; l'accumulation du Cd dans les sédiments correspond à 90-95% du Cd ajouté dans l'eau. Sa sédimentation a été observée tant dans les zones de dépôt que dans les zones d'érosion. Malgré une faible concentration du Cd dans les sédiments, les sédiments sablonneux des stations situées en eaux peu profondes sont des puits importants de Cd, du fait de leur grande masse par unité de surface. Aux stations situées en eaux profondes, le transport progressif du Cd jusque dans la couche de mélange sédimentaire (8-10 cm d'épais, le mélange se faisant en une décennie ou plus) est également un puits important de Cd. Les essais d'extraction sélective indiquent une faible adsorption du Cd sur les oxyhydroxydes de Fe et de Mn fixés au périphyton et aux particules qui sédimentent, et un K d élevé. Le K d des sédiments profonds est inférieur même si le Cd tend à être associé aux fractions organiques ou aux sulfures fortement liés. Cela peut indiquer une perte par les sédiments du Cd facilement mobilisé au cours de la diagenèse. Puisque le Cd est facilement libéré par les sédiments meubles, nous formulons l'hypothèse à l'effet que la recirculation du Cd sédimenté et repris dans l'eau ralentira le retour du lac 382 aux conditions précédant les traitements.[Traduit par la Rédaction]