Mercury methylation by sulphate-reducing bacteria from sediments of an acid stressed lake

The role of freshwater sulphate-reducing bacteria in McHg production was examined by adding specific microbial inhibitors to anoxic lake sediments spiked with ²⁰³HgCl₂ and measuring net methylation. The effect of increased sulphate (such as would arise from acid deposition in the area) on the activity of sulphate-reducing bacteria both in terms of sulphate reduction rate and methylation of Hg was examined by adding sulphate to ²⁰³HgCl₂ spiked sediments. Sodium molybdate (10 mM), a specific inhibitor for sulphate-reducing bacteria, reduced the amount of MeHg produced from anoxic lake sediments by 75% compared with controls over a 7 d period. In contrast, 2-bromoethane sulfonic acid (15 mM), a specific inhibitor for methanogenic bacteria, did not alter the amount of McHg produced. Additions of sulphate that were “realistic” in terms of the normal range of the area (5 to 30 mg.L^?), increased the sulphate reduction rate of sediment slurries. However, in the experimental system, these additions did not stimulate McHg production. In our study, methylation of Hg in sediments was primarily due to the activity of sulphate-reducing bacteria. However, the methylation rate does not appear to be sensitive to the concentration of sulphate over the range typical of softwater, Precambrian Shield lakes. This could be due to reduced availability of Hg due to the formation of insoluble HgS or to the fact that the overall activity of sulphate reducers not being stimulated even if sulphate reduction rate is, or both.     

Natural Mercury Levels in Geological Enriched and Geological Active Areas: Case Study of Katun River and Lake Teletskoye,Altai (Siberia)

Natural geological Hg deposits control the Hg levels inthe upper Katun river. Very high levels of total Hg areobserved in the watercolumn (up to 20 ng L^-1) and thesediments (up to 244 μg g^-1) close to the depositarea, but almost normal levels (1.8 ng L^-1 in the watercolumn and 0.14 μg g^-1 in the sediments) are reached60 km downstream of that zone. In general, low dissolvedmethylmercury (MMHg) concentrations were found (0.04–0.05 ngL^-1) due to unfavourable methylation conditions. The MMHgconcentrations in the sediments vary from 23.3 ng g^-1, inthe vicinity of the geological Hg deposits, to 0.17 ng g^-1 60 km downstream.Total Hg levels in Lake Teletskoye (a geological activearea) are slightly increased (1.1–1.8 ng L^-1) compared toLake Baikal and fairly constant alover the Lake, suggestingmultiple sources. High mercury concentrations in springs andsoils coincide with high radon concentrations in the samecompartments as well as high soil exhalation fluxes. Theseresults in combination with the fact that Lake Teletskoye islocated in an active fault zone suggest that the Rn and Hgsources may be fault aligned spring waters and deep seatedgases escaping through open cracks. Methylmercuryconcentrations in the Lake (0.03–0.1 ng L^-1) werecomparable to the concentrations found in Katun river butrelative to the total Hg burden this means a higher percentage.     

Involvement of bacteria in methylmercury formation in anaerobic lake waters

The potential net MeHg formation in water bodies and surface sediments was investigated. The radiochemical method was applied to the methylation assays performedin situ. The role of sulfate reducing bacteria and overall bacteria activity in the methylation process was examined by molybdate and formaldehyde addition, respectively. The impact of light conditions was tested by light and dark incubations. Low but detectable methylation was found in the oxic epilimnion of all lakes. The rates were affected neither by molybdate nor by formaldehyde. The highest rates were observed in the hypolimnion within the sulfide maxima. Near the sediment surface the rates again declined. Usually, the methylation rates in sediments were of the same level as the rates the anoxic water. Formaldehyde prevented methylation in the anoxic water and sediment. Molybdate inhibited methylation in sediments but in the sulfidic waters the inhibition was only partial or the methylation process was stimulated by molybdate. No significant differences in methylation activity were found between the natural light and the dark bottle incubations. The highest methylation rates were not always concomitant with the highest MeHg concentrations, the highest concentrations more likely to be found in the particle rich water layers. The results show that MeHg in sulfidic waters was formed biotically by sulfate reducing bacteria which were competing with methanogens for acetate and hydrogen under sulfate limiting conditions.     

Sulfur cycling in a dutch moorland pool under elevated atmospheric S-deposition

Atmospheric S deposition, sulfate reduction and the accumulation of reduced S components were investigated in sediments of a Dutch moorland pool. Laboratory ³⁵SO42? tracer experiments indicated that the variation in reduction rate with depth in the sediment could be described by an exponential fit, suggesting first order kinetics. Sulfate reduction rates calculated from a kinetic transport model were highest in June and July (>0.6 mmol m^?2 day^?1), and decreased towards the end of the summer (<0.2 mmol m^?2 day^?1). Winter reduction rates were low, but increased towards the beginning of the spring. Concentrations of total, organic and inorganic S decreased with depth, consistent with an increasing depositional flux of S over the last century. Increased atmospheric sulfate deposition rates and consumption in the upper sedimentary layers caused a continuous increase in subsurface S concentrations. Present day S-immobilization rates were estimated between 25 to 30 mmol S in^?2 yr^?1, in the order of the annual input of S from wet atmospheric deposition. Sulfate metabolism in anaerobic moorland pool sediments may be the most important mechanism of long-term S-accumulation.     

Controls on microbial mercury transformations in contaminated sediments downstream of the Idrija mercury mine (West Slovenia) to the Gulf of Trieste (northern Adriatic)

Purpose

Concentrations and transformations of mercury were measured in river, estuarine, and marine sediments to determine factors affecting the fate of mercury entering the northern Adriatic Sea.

Materials and methods

Radiotracer methodology was used to compare rates of mercury methylation (²⁰³Hg), MeHg demethylation (¹⁴C), and sulfate reduction (³⁵S) in sediment depth profiles to concentrations of total and dissolved mercury species in the lower freshwater region of the Isonzo River, the coastal lagoons, and in the Gulf of Trieste, northern Adriatic Sea.

Results and discussion

Mercury was readily methylated and demethylated in all sediments, but the relative activity of these processes varied greatly with location. Methylation activity increased greatly from freshwater to the marine regions; however, demethylation was extremely high in the estuarine and lagoon sites. Ratios of methylation to demethylation were low in these coastal sites but increased further offshore in the gulf, which agreed with increased ratios of MeHg to total Hg (%MeHg) in gulf sediments. Comparisons of microbial activities indicated that sulfate reduction strongly controlled both methylation and demethylation. However, Hg methylation in coastal lagoon sediments was controlled by rapid demethylation and the bioavailability of Hg that was affected by Hg adsorption and precipitation. Methylation in offshore marine sites correlated with sulfate reduction but not the partitioning of Hg between pore water and solid phases. The decrease in sulfide production offshore exacerbated Hg methylation.

Conclusions

The freshwater to marine gradient in the Idrija/So?a/Isonzo/Adriatic region is dynamic, exhibiting horizontally variable rates of microbial activities and Hg transformations that create “hot spots” of MeHg accumulation that are controlled differently in each region.

     

Seasonal variability in the Mercury speciation of Onondaga Lake (New York)

Dissolved and particulate Hg speciation was determined on four occasions in the Spring to Fall interval of 1989, at three depths of the water column of Onondaga Lake, New York; an urban system in which the sediments and fish flesh are contaminated with H_g. Species determined included total Hg (Hg_t), reactive (‘ionic’) Hg (Hg_i), monomethylmercury (CH₃HgX), elemental Hg (Hg°) and dimethylmercury (CH₃)₂Hg). Onondaga Lake was found to contain very high levels of Hg_t (2 to 25 ng L^?1 Hg), Hg_j (0.5 to 10 ng L^?1 Hg), and CH₃HgX (0.3 to 7 ng L^?1 Hg), which generally increased with depth in the lake. These concentrations represent a significant level of contamination, based upon comparisons with other polluted and pristine sites. Elemental Hg levels were typically about 0.05 ng L^?1 and (CH₃)₂Hg was near the limits of detection (?0.001 ng) L^?1 in most samples. The greatest CH₃HgX concentrations in the hypolimnion, as well as the largest gradients of both CH₃HgX and (Hg_t), were observed upon the first onset of stratification, in early summer. These concentrations did not become more pronounced, however, as stratification and H₂S levels in the hypolimnion increased throughout the summer. The very low concentrations of (CH₃)₂Hg in these MeHg and sulfide-rich waters calls into question the belief that CH₃HgX and H₂S will react to yield volatile dimethyl-mercury, which can then escape to the atmosphere by diffusion. Mercury speciation was highly dynamic, indicating active cycling within the lake, and an apparent sensitivity to changes in attendant Iimnological conditions that track the stratification cycle.     

Mercury Cycling in Lake Gordon and Lake Pedder,Tasmania (Australia). I: In-lake Processes

The processes affecting the concentrations of total mercury (total Hg) and methylmercury (MeHg) in a freshwater system comprising two connected reservoirs in southwest Tasmania were investigated. Surface concentrations of total mercury (total Hg)were temporally and spatially uniform in both Lake Gordon (2.3±0.4 ng L^-1, n = 27) and Lake Pedder (2.3±0.3 ng L^-1, n = 11). The surface concentrations of MeHg in Lake Gordon (0.35±0.39 ng L^-1, n = 25) were more variable than total Hg and MeHg typically comprised 10–20% of total Hg. The relatively high amount of total mercury present as MeHg in Lake Gordon was attributed to the high proportion of wetlandsin the upper catchment (50% of total area) and in-lake contributions (ca. 40% of total MeHg). Despite the close proximity of the two lakes, MeHg concentrations in Lake Pedder were consistently lower than in Lake Gordon. This phenomenon canbe explained in part by the greater contribution of direct rainfall to Lake Pedder leading to the dilution of MeHg. Water column MeHg concentrations were higher in warmer months in bothlakes, reflecting increased net methylation of inorganic mercury.Unlike previous studies of seasonally anoxic lakes, depth profiles of total mercury and MeHg in Lake Gordon were uniform and were not affected by water column stratification occurringin the summer months, and oxygen depletion with depth. This suggests that redox cycling and accumulation of MeHg in the hypolimnion following seasonally-induced anoxia is not a significant part of the mercury cycle in Lake Gordon. The primary location of MeHg production within the lake's water column is not conspicuous. Mercury speciation measurements made above and below the lake system over a period of 19 months indicates that after 20 yr of impoundment, the reservoirs are not significantly affecting MeHg concentrations in the downstreamriverine environment.     

Microbial methylation of mercury in upper Wisconsin river sediments

The microbial methylation of Hg was studied in water and sediments from the Upper Wisconsin River by quantifying the methylation of radioactive Hg(II) (²⁰³Hg[NO₃]₂). Methylation activity was near detection limits in the water, highest in surface sediments (0 to 4 cm), and decreased with increasing sediment depth. Methylation had a temperature optimum of 35 °C. Organically enriched sediments exhibited higher methylation activity than less eutrophic sediments. Methylation activity in sediments was stimulated by the addition of peptone but not by glucose or starch. Oxygenation of sediments inhibited methylation activity. A summertime peak in methylation activity, observed in water, floc, and sediments, was related to factors other than temperature. More than 98% of the added ²⁰³Hg(II) was bound to sediments within 4 hr of inoculation, while more than 3% was methylated during a 10-day incubation. As much as 7% of the added ²⁰³Hg(II) was methylated in other experiments, suggesting that bound Hg is available for methylation. These data suggest that organic-rich surficial sediments in the Upper Wisconsin River have the potential to produce significant amounts of toxic methylmercury during late summer months.     

Mercury cycling in a northern wisconsin seepage lake: The role of particulate matter in vertical transport

During summer stratification, total mercury (Hg_τ) reached maximum concentrations in the O₂ :depleted, hypolimnion of Little Rock Lake, Wl. Initially, the hypolimnetic increase was attributed solely to redox-controlled release of Hg from bottom sediments. However, subsequent depth profiles of Hg indicated that hypolimnetic Hg enrichment could also result from the downward transport and recycling of particulate Hg prior to incorporation in the sediments. Contrasts between Fe and Hg cycles in this lake reinforce this notion. Increases in hypolimnetic Fe were observed during both summer and winter O₂ decreases. In contrast, hypolimnetic Hg concentrations declined during winter. In the ice-free season, the distribution of particulate mercury (Hg_p) correlated with the distribution of chlorophyllous particulates in this lake, re-emphasizing the importance of biotic processes in controlling Hg cycling in the hypolimnion.     

Mercury Cycling in Lake Gordon and Lake Pedder,Tasmania (Australia). II: Catchment Processes

The sources and concentrations of total mercury (total Hg) and methylmercury (MeHg) in the upper catchment of the Lake Gordon/Lake Pedder system in Tasmania, Australia were investigated. The catchment area, which contains over 50% wetlands, is located in a temperate region with no obvious point sources of mercury. Surface waters in the region had concentrations of total Hg ranging from 1.2 to 14.4 ng L^-1 and MeHg from < 0.04 to 1.4 ng L^-1. MeHg concentrations were seasonally dependent, with the highest concentrations occurring in summer. Sediments/soils in the catchment had concentrations of total Hg ranging from 4.0 to 194 ng g^-1 and MeHg from <0.02 to 20.1 ng g^-1. The low concentrations of total Hg confirmed that this region is pristine as regards mercury and has no geological enrichment of total Hg. The highest total Hg and MeHg concentrations in both sediment/soils and waters were found in bogs whereas the lowest concentrations typically occurred on the wetlandplains. MeHg concentrations, in bog and swamp sediments were correlatedwith the organic matter content (r = 0.942, P < 0.001). Acid volatile sulfide (AVS) measurements indicate that in most sediments AVS was greater than total Hg. Given the high reactivity of inorganic mercury and sulfide, this suggests that most of the particulate mercury in sediments is present as mercuric sulfide. The yield of MeHg from the catchment was estimated to be 3.2 mg ha^-1 yr^-1 and is higher than published rates measured in non-contaminated temperate catchments in the northern hemisphere. The higher yield was attributed to the generally warmer climatic conditions that favour net methylation and the relatively high rainfall (2–3 m yr^-1) of the region, which supplies reactive inorganic mercury to the active zones ofmercury methylation and also flushes MeHg from the catchment.     

Factors controlling the removal of sulfate and acidity from the waters of an acidified lake

Although Lake Anna, an impoundment in Central Virginia, receives acid mine drainage (AMD) from Contrary Creek, the effects of the AMD pollution on the lake are less severe than expected. Previous work at Lake Anna has shown that bacterial sulfate reduction in the lake sediments plays an important role in the recovery of the lake from the AMD inputs. Sulfate removal rates were measured in sediment microcosms under a variety of experimental conditions to determine the factors controlling the rate of sulfate and acidity removal from the lake water. Sulfate removal rates were not significantly different over the short term (3 weeks) in summer sediment microcosms incubated at 6 vs 26 °C. Winter sediment microcosms showed no significant sulfate removal during the 18 day experiment when incubated at either 6 or 28 °C. Thus there is a strong seasonal temperature effect in Lake Anna sediments but no significant short term effect. Simulated AMD, with and without Fe, was added to sediment microcosms collected from an unpolluted part of the lake. The microcosms with Fe had significantly higher rates of sulfate removal indicating that Fe plays an important role in transporting sulfate to the sediment and/or in preventing oxidation of the reduced sulfide. After 27 days, from 54 to 96% of the added sulfate in the simulated AMD was recovered as FeS or S⁰ in the top 4 cm of sediment. In a separate experiment, ³⁵S-SO _inf4 ^sup2? was found to attach to precipitating Fe oxyhydroxides (1.5 to 4.7 mol SO _inf4 ^sup2? mol^?1 Fe precipitate) upon mixing Contrary Creek (AMD) and Lake Anna waters. Results of this study suggest that sulfate removal may be more rapid in metal rich AMD systems thans in metal poor systems characteristic of those which receive acidic deposition.     

Total cadmium concentrations in the water and littoral sediments of Central Ontario Lakes

Lakes within 20 km of Sudbury, Ontario, have significantly higher Cd concentrations in surface waters (geometric mean 122 ng L^?1; n = 7) than lakes elsewhere in central Ontario (10.8 ng L^?1; n = 57). Cadmium concentrations in water from lakes beyond the Sudbury halo were negatively correlated (r = 0.797; p < 0.001) with pH. A weak correlation between fluoride and Cd concentrations leads to speculation that some Cd may be mobilized from watersheds with Al. Cadmium concentrations in littoral sediments are not elevated near Sudbury. The geometric mean Cd concentration of littoral sediments in central Ontario lakes is 0.08 mg Cd kg^?1 dry mass (n = 75). Cadmium concentrations in littoral sediments are strongly correlated with sediment loss on ignition (r = 0.860; p < 0.001). After correction for differences in organic content, littoral sediments are less enriched with Cd than profundal sediments, as reported in the literature. The difference between littoral and profundal sediments, and the sensitivity of Cd concentrations in water to pH, may be due to the importance of Cd binding by Fe/Mn hydrous oxides in the profundal zone, while organic matter binds Cd in the littoral zone. The lack of sensitivity of Cd concentrations in littoral sediments to acidification may be due to the incorporation of much of the Cd in those sediments into organic particulates.     

Measurement of Hg methylation in sediments using high specific-activity203Hg and ambient incubation

Two methods were developed for estimating the rate ofin situ methylmercury (MeHg) formation in sediments. One method is based on incubation of intact sediment cores without added Hg over a period of days. The second method uses²⁰³HgCl₂ with a specific activity high enough to be used as a tracer (relative to bulk Hg). Use of high-specific activity²⁰³HgCl₂ allowed measurement of methylation rate in hours at ambient total Hg concentrations.²⁰³HgCl₂ was pre-equilibrated with pore water before injection into intact cores, to allow complexation with dissolved ligands. Methylation rates were measured with²⁰³HgCl₂ additions as low as 0.02 μCi and 1.2 ng Hg per g wet weight sediment. These methods were tested in epilimnetic and littoral sediments of two pristine seepage lakes in Northern Wisconsin, and found to compare well.In situ methylation rates in Pallette and Little Rock Lake sediments ranged from 0.1 to 0.4 ug/m² d. Use of²⁰³Hg gave lower errors with shorter incubation times than the ambient incubation method. A method for extraction of Me²⁰³Hg from bulk sediments is given.     

Methyl-mercury affects microbial activity and biomass,bacterial community structure but rarely the fungal community structure

Monomethyl-mercury is one of the most toxic compounds. Methylation of Hg usually appears under anoxic conditions. In Swiss forest soils, methyl-Hg concentrations of up to 3 μg kg⁻¹ soil dw have been observed, but the impact of methyl-Hg on soil microorganisms have rarely been examined so far. In this study, we investigated the effect of increasing concentrations of methyl-Hg (0, 5, 20, 90 μg kg⁻¹ soil dw) on the microbial communities in various forest soils differing in their physico-chemical properties. Experiments were conducted in microcosms under controlled conditions and the basal respiration (BR), the microbial biomass carbon (MBC) and the bacterial and fungal community structures using T-RFLP-profiling were investigated. BR was significantly affected by methyl-Hg. In general, the BR increased with increasing methyl-Hg concentrations, whereas the MBC was significantly reduced. Bacterial communities were more sensitive to methyl-Hg than fungal communities. In five out of seven soils, the bacterial community structures differed significantly between the treatments whereas the fungal communities did not. The impact of methyl-Hg on the soil bacterial communities was site specific. In one soil, a methyl-Hg concentration of already 5 μg kg⁻¹ soil dw significantly affected the relative abundance of 13% bacterial operational taxonomic units (OTU), whereas in other soils concentrations of even 90 μg kg⁻¹ soil dw rarely affected the abundance of OTUs. In this study, for the first time, the impact of methyl-Hg on soil bacterial and fungal communities in forest soils was assessed. We showed that its impact strongly depends on the physico-chemical conditions of the soil and that bacterial communities were more sensitive to methyl-Hg than fungi.     

Mercury uptake patterns of biota in a seasonally anoxic northern California Reservoir

Biotic uptake of mercury (Hg) in Davis Creek Reservoir, California increased dramatically in conjunction with the entrainment of anoxic hypolimnetic water into the mixed layer. This indicated a seasonal pulse increase of bioavailable Hg associated with thermal destratification. The effect was more pronounced in juvenile bass (70–200% seasonal increases in muscle Hg concentration), as compared to adults (15–25% increases), and was most distinct in zooplankton, which spiked to concentrations of 3–6 mg/kg, dry weight, immediately following fall destratification (130–270% seasonal increases over pre-fall levels). In addition to the general buildup of methyl Hg in the hypolimnion under anoxic conditions, a dense layer of photosynthetic anaerobic bacteria just beneath the thermocline is implicated as a potentially important seasonal source of methyl Hg to reservoir fish. Hg increases in juvenile and adult fish correspond to late summer and fall entrainment of upper hypolimnetic water, while zooplankton spike increases may be partially related to ingestion or adsorption of Hg-scavenging manganese oxides, which precipitate following full turnover. A simple and effective, syringe-based cold vapor atomic absorption method for total Hg is also described.     

Physicochemical Behavior of Tetracycline and 17α-Ethinylestradiol with Wastewater Sludge-Derived Humic Substances

Concentration of methylmercury (MeHg) in different habitats and associated food chains may vary because of habitat characteristics that determine methylation and MeHg transfer. We examined MeHg levels in primary consumers from littoral, pelagial and profundal habitats of a boreal humic lake, and measured total mercury (TotHg) and MeHg in surface sediments at increasing depths. MeHg concentrations in primary consumers increased from profundal to littoral, a pattern which was mirrored by the surface sediment concentrations. Methylation potential (expressed as the ratio of MeHg to TotHg) was lower in profundal than in littoral sediments, suggesting that littoral sediments have higher net methylation rates. No specific MeHg-enriched entrance point in the littoral food chain was identified, however. High MeHg concentrations in littoral primary consumers and sediments suggest that shallow lake sediments are important for MeHg transfer to the aquatic food web in boreal humic lakes. Lake morphometry, most specifically the fraction of littoral, is hence likely to add to differences in MeHg bioaccumulation rates in lake food webs.     

Specific rates of net methylmercury production in lake sediments

Specific rates of Hg (²⁰³HgCl₂) methylation and McHg (¹⁴CH₃HgI) demethylation in aerobic and anaerobic conditions were determined in samples of surface sediments (0 to 2 cm) taken from five small headwater lakes in Southern Finland. The highest rates of methylation were measured in anaerobic conditions. However, the importance of aerobic methylation increased with increasing Fe and Mn content in sediment. There was little difference between aerobic and anaerobic demethylation. The results demonstrate that the net McHg production in lake sediments depends on the individual characteristics of the lake, particularly pH and and sediment properties. These characteristics seem to affect demethylation in anaerobic conditions and methylation in aerobic conditions.     

Evaluating the mercury distribution and release risks in sediments using high-resolution technology in Nansi Lake,China

Purpose

Mercury (Hg) and methylmercury (MeHg) are easily released from sediments to overlying water and cause secondary contamination. In general, Hg concentrations are low in natural aquatic environments, but Hg toxicity is high. Therefore, it is important to assess the mobility and release risks of Hg and MeHg from surface sediment using in situ high-resolution sampling techniques.

Methods

The profile distribution of Hg and MeHg was obtained for samples from Weishan sub-lake (WL) and Dushan sub-lake (DL) of Nansi Lake, China, by high-resolution dialysis (HR-Peeper probes) and the diffusive gradients in thin films (DGT) technique at mm-resolution. Furthermore, Hg mobility and release risks in sediments were evaluated by combining BCR (European Community Reference Bureau) extraction and DGT-measured data.

Results

The soluble concentrations of Hg in surface sediments in WL and DL were 21.70 and 19.38 ng L^?1 and the DGT-labile concentration of Hg were 8.21 and 10.30 ng L^?1, respectively. The soluble and labile Hg and MeHg concentrations were higher in the surface sediments (from??40 to 0 mm) than in deep sediments. The distribution of the labile-Hg was controlled by the ferrimanganic (hydr)oxide and total nitrogen rather than organic carbon content. The non-residual components accounted for a greater proportion of the interface, which further confirmed Hg was more active on the surface layer of the sediment. The resupply ability indicated that the release of Hg from sediment was insufficient to maintain the initial concentration in the porewater before consumption. The MeHg fluxes in WL (6.18 ng m^?2 day^?1) were twice those in DL (2.89 ng m^?2 day^?1), and the risk assessment code revealed a higher risk in the surface layer (25.21–61.88%) than in the deep layer (0–27.75%).

Conclusions

Dissolved Hg and MeHg accumulated on the surface of the sediments and were more active than in the deeper sediments. The DGT-labile state can be used for a better understanding of the bioavailability and mobility of Hg. The diffusion direction of Hg and MeHg was from sediment to the overlying water. The release risks of Hg and MeHg from surface sediments (especially in WL) were found to be worthy of concern.

     

Depletion of epilimnetic oxygen and accumulation of hydrogen sulfide in the hypolimnion of Onondaga Lake,NY, U.S.A.

The depletion of epilimnetic DO during fall, and the accumulation of H₂S in the hypolimnion during summer, are documented for polluted hypereutrophic Onondaga Lake, NY, U.S.A. The fall epilimnetic DO depletion is so severe that in many, if not all years, the New York State standard for minimum DO of 4 mg- L^?1 is violated for periods as long as 1 mo. The depletion is caused by the transport of O₂ demanding species, particularly H₂S, from the hypolimnion into the epilimnion. Hydrogen sulfide accumulates progressively through the summer, mostly via SO _inf4 ^sup2? reduction, to unusually high concentrations; a maximum of 1.65 mM has been observed. These conditions are apparently manifestations of hypereutrophy, and thus may be subject to remediation through P management efforts.