Longevity and efficacy of lanthanum-based P remediation under changing dissolved oxygen availability in a small eutrophic lake.

We set out to study the seasonal variations in porewater phosphorus and lanthanum concentrations in the dated sediment cores from a small eutrophic lake that has been treated with Phoslock, a lanthanum-modified bentonite (LMB) amendment. Three sites were sampled when the hypolimnion was either oxygenated or anoxic: (i) the lake's deepest point, (ii) a littoral site receiving inflows from the catchment, and (iii) a littoral site influenced by nearby septic tanks. Phosphate (PO43--P), lanthanum (La), iron (Fe), dissolved organic carbon (DOC) and sulfate (SO42-) were measured in porewater samples. An inverse diagenetic model was used to quantify fluxes of dissolved elements across the sediment-water interface as well as the net rate of their reactions along the porewater concentration gradients. Results show that porewater P and Fe underwent strong seasonal dynamics, while La did not. P fluxes, 20-fold higher at the deepest site than elsewhere in the basin, were influenced by anoxic conditions in the hypolimnion during summer and winter, suggesting that P mobility remained sensitive to redox fluctuations despite the addition of La. At the deepest site, fluxes of P across the sediment-water interface increased from 1 to 9 × 10-9 µmol cm-2 s-1 between spring and summer, while the rate of P production to the porewater also increased a hundredfold. These increases were concurrent with Fe mobilization. Finally, sediment dating shows that the fraction of P sequestered by La is buried under freshly deposited sediment at a rate of 2-3 mm per year. These results indicate that external P fluxes and erosion control remain crucial to maintain the longevity of the LMB treatment.

Experimental testing of two urban stressors on freshwater biofilms.

Aquatic ecosystems and their communities are exposed to numerous stressors of various natures (chemical and physical), whose impacts are often poorly documented. In urban areas, the use of biocides such as dodecyldimethylbenzylammonium chloride (DDBAC) and their subsequent release in wastewater result in their transfer to urban aquatic ecosystems. DDBAC is known to be toxic to most aquatic organisms. Artificial light at night (ALAN) is another stressor that is increasing globally, especially in urban areas. ALAN may have a negative impact on photosynthetic cycles of periphytic biofilms, which in turn may result in changes in their metabolic functioning. Moreover, studies suggest that exposure to artificial light could increase the biocidal effect of DDBAC on biofilms. The present study investigates the individual and combined effects of DDBAC and/or ALAN on the functioning and structure of photosynthetic biofilms. We exposed biofilms in artificial channels to a nominal concentration of 30 mg.L-1 of DDBAC and/or ALAN for 10 days. ALAN modified DDBAC exposure, decreasing concentrations in the water but not accumulation in biofilms. DDBAC had negative impacts on biofilm functioning and structure. Photosynthetic activity was inhibited by > 90% after 2 days of exposure, compared to the controls, and did not recover over the duration of the experiment. Biofilm composition was also impacted, with a marked decrease in green algae and the disappearance of microfauna under DDBAC exposure. The integrity of algal cells was compromised where DDBAC exposure altered the chloroplasts and chlorophyll content. Impacts on autotrophs were also observed through a shift in lipid profiles, in particular a strong decrease in glycolipid content was noted. We found no significant interactive effect of ALAN and DDBAC on the studied endpoints.

Enhanced Bioaccumulation and Transfer of Monomethylmercury through Periphytic Biofilms in Benthic Food Webs of a River Affected by Run-of-River Dams.

Run-of-river (ROR) power plants impound limited terrestrial areas compared to traditional hydropower plants with large reservoirs and are assumed to have reduced impacts on mercury cycling. We conducted a study on periphyton and benthic communities from different habitats of the St. Maurice River (Québec, Canada) affected by two ROR power plants and their effect on the bioaccumulation and biomagnification of monomethylmercury (MMHg). Proportion of total mercury as MMHg reached maximum values about 2.9 times higher in flooded sites compared to unflooded sites. Impoundment by ROR would therefore provide favorable environments for the growth of periphyton, which can produce and accumulate MMHg. Periphyton MMHg concentrations significantly explained concentrations in some benthic macroinvertebrates, reflecting a local transfer. Through the analysis of δ13C and δ15N signatures, we found that flooding, creating scattered lenthic habitats, led to modifications in trophic structures by the introduction of new organic matter sources. The computed trophic magnification slopes did not show significant differences in the transfer efficiency of MMHg between sectors, while intercepts of flooded sectors were higher. Increases in MMHg concentrations in flooded areas are likely due to the impoundment, combined with watershed disturbances, and the creation of small habitats favorable to periphyton should be included in future predictive models.

Food web differences between two neighboring tropical high mountain lakes and the influence of introducing a new top predator.

High mountain lakes (HMLs) are considered unique and comparable ecosystems for monitoring global climate change. The food web structure can indicate the response of these ecosystems to ecological threats, such as fish introduction, by analyzing the trophic dynamics. Nonetheless, the food webs of tropical HMLs are less well-studied than temperate HMLs. The present study assessed the food webs of two neighboring (600 m apart) tropical HMLs, El Sol and La Luna, inside the crater of the Nevado de Toluca volcano, Mexico. It used stable isotopes (δ13C and δ15N) and Bayesian mixing models with different trophic discrimination factors and priors to assess the impacts of introduced rainbow trout, persisting only in the larger lake, El Sol. The food web in Lake El Sol was more complex than in Lake La Luna, mainly due to its larger size, extensive vegetated littoral zone, and being fueled by autochthonous primary production. In contrast, the smaller and fishless Lake La Luna has a reduced and bare littoral zone that harbored a simple food web substantially sustained by allochthonous carbon inputs. The persistence of introduced rainbow trout in Lake El Sol but not in Lake La Luna accentuated the differences between the lakes. The models suggested that rainbow trout fed on key consumers of littoral macroinvertebrates (70-80%) and pelagic zooplankton (20-30%), increasing the linkage between sub-networks. In both tropical HMLs, the species richness and herbivorous fraction were elevated compared with temperate HMLs, while the linkage density and omnivorous fraction were lower. Basal nodes dominated these tropical HMLs, and the vegetated littoral zone of Lake El Sol had more intermediate (omnivore) nodes. Our results showed the convenience of food web analysis to compare the effects of introduced fish in originally fishless lakes in different latitudes.

How do metals interact with periphytic biofilms?

Extracellular matrix of periphyton has complex structural and chemical composition regulating metal transfer within biofilms with consequences for metal transfer to aquatic food webs. We investigated which metal species were retained in the loosely (LB) and the tightly bound (TB) fractions of the periphyton matrix from three pristine lakes at different growth stages. We measured the fluorescent dissolved organic matter (FDOM) composition with parallel factor analysis (PARAFAC) and the co-occurrence of essential and non-essential metals with FDOM in the two matrix fractions. The LB and TB fractions of periphyton had distinct fluorescence composition from the water column. The PARAFAC model identified five components, including two (C2 and C4) appearing to be of periphytic origin. The humic-like C2 was almost exclusive to periphyton and the tryptophan-like C4, associated to biofilm phototrophy, represented up to 47.0 ± 7.3 % of total fluorescence in the LB fraction. Most metals had significant positive relationships with four FDOM components in the LB fraction while C2 was the only component in the TB fraction to show such significant relationships. Components in the LB fraction seemed to act as scavengers for metals, preventing them from reaching the cellular fraction, while C2 from the TB fraction was likely promoting the bioavailability of essential metals for microorganisms inside periphyton. This study highlights the contrasting roles of the extracellular matrix on metal mobility beyond the usually proposed protection mechanisms. We suggest an experimental model for the study of metal regulation processes of the periphytic extracellular polymeric substances with a focus on the components produced by microorganisms within periphyton and their distribution in the different matrix fractions.

Response of sediment phosphorus partitioning to lanthanum-modified clay amendment and porewater chemistry in a small eutrophic lake.

Sustained eutrophication of the aquatic environment by the remobilization of legacy phosphorus (P) stored in soils and sediments is a prevailing issue worldwide. Fluxes of P from the sediments to the water column, referred to as internal P loading, often delays the recovery of water quality following a reduction in external P loads. Here, we report on the vertical distribution and geochemistry of P, lanthanum (La), iron (Fe) and carbon (C) in the culturally eutrophied Lake Bromont. This lake underwent remediation treatment using La modified bentonite (LMB) commercially available as Phoslock™. We investigated the effectiveness of LMB in decreasing soluble reactive phosphorus (SRP) availability in sediments and in reducing dissolved fluxes of P across the sediment-water interface. Sediment cores were retrieved before and after LMB treatment at three sites representing bottom sediment, sediment influenced by lakeside housing and finally littoral sediment influenced by the lake inflow. Sequential extractions were used to assess changes in P speciation. Depth profiles of dissolved porewater concentrations were obtained after LMB treatment at each site. Results indicate that SRP extracted from the sediments decreased at all sites, while total extracted P (PTOT) bound to redox-sensitive metal oxides increased. 31P NMR data on P extract reveals that 20-43% of total solid-phase P is in the form of organic P (Porg) susceptible to be released via microbial degradation. Geochemical modelling of porewater data provides evidence that LaPO4(s) mineral phases, such as rhabdophane and/or monazite, are likely forming. However, results also suggest that La3+ binding by dissolved organic carbon (DOC) hinders La-phosphate precipitation. We rely on thermodynamic modelling to suggest that high Fe2+ would bind to DOC instead of La3+, therefore promoting P sequestrations by LMB under anoxic conditions.

Microbial Diversity and Mercury Methylation Activity in Periphytic Biofilms at a Run-of-River Hydroelectric Dam and Constructed Wetlands.

Periphytic biofilms have the potential to greatly influence the microbial production of the neurotoxicant monomethylmercury in freshwaters although few studies have simultaneously assessed periphyton mercury methylation and demethylation rates and the microbial communities associated with these transformations. We performed a field study on periphyton from a river affected by run-of-river power plants and artificial wetlands in a boreal landscape (Québec, Canada). In situ incubations were performed on three sites using environmental concentrations of isotopically enriched monomethylmercury (MM198Hg) and inorganic mercury (200Hg) for demethylation and methylation rate measurements. Periphytic microbial communities were investigated through 16S rRNA gene analyses and metagenomic screenings for the hgcA gene, involved in mercury methylation. Positive mercury methylation rates ([5.9 ± 3.4] × 10-3 day-1) were observed only in the wetlands, and demethylation rates averaged 1.78 ± 0.21 day-1 for the three studied sites. The 16S rRNA gene analyses revealed Proteobacteria as the most abundant phylum across all sites (36.3% ± 1.4%), from which families associated with mercury methylation were mostly found in the wetland site. Metagenome screening for HgcA identified 24 different hgcA sequences in the constructed wetland site only, associated with 8 known families, where the iron-reducing Geobacteraceae were the most abundant. This work brings new information on mercury methylation in periphyton from habitats of impacted rivers, associating it mostly with putative iron-reducing bacteria.IMPORTANCE Monomethylmercury (MMHg) is a biomagnifiable neurotoxin of global concern with risks to human health mostly associated with fish consumption. Hydroelectric reservoirs are known to be sources of MMHg many years after their impoundment. Little is known, however, on run-of-river dams flooding smaller terrestrial areas, although their numbers are expected to increase considerably worldwide in decades to come. Production of MMHg is associated mostly with anaerobic processes, but Hg methylation has been shown to occur in periphytic biofilms located in oxic zones of the water column. Therefore, in this study, we investigated in situ production of MMHg by periphytic communities in habitats impacted by the construction of a run-of-river dam by combining transformation rate measurements with genomic approaches targeting hgcAB genes, responsible for mercury methylation. These results provide extended knowledge on mercury methylators in river ecosystems impacted by run-of-river dams in temperate habitats.

Understanding Food Web Mercury Accumulation Through Trophic Transfer and Carbon Processing along a River Affected by Recent Run-of-river Dams.

Unlike large dams which favor methylation of Hg in flooded soils over long periods, run-of-river dams are designed to flood a limited area of soils and are therefore not expected to significantly affect mercury (Hg) cycling or carbon processing. We studied the Hg and carbon cycles within food webs from several sectors along the Saint-Maurice River, Quebec, Canada, that differ in how they are influenced by two run-of-river dams and other watershed disturbances. We observed peak Hg concentrations in fish five-year postimpoundment, but these levels were reduced three years after this peak. Methylmercury concentrations in low trophic level fish and invertebrates were related to their carbon source (δ13C) rather than their trophic positions (δ15N). Biomagnification, measured by trophic magnification slopes, was driven mainly by methylmercury concentrations in low-trophic level organisms and environmental factors related to organic matter degradation and Hg-methylation. River sectors, δ13C and δ15N, predicted up to 80% of the variability in food web methylmercury concentrations. The installation of run-of-river dams and the related pondages, in association with other watershed disturbances, altered carbon processing, promoted Hg-methylation and its accumulation at the base of the food web, and led to a temporary increase in Hg levels in fish.

Mercury and selenium distribution in key tissues and early life stages of Yellow Perch (Perca flavescens).

Whereas early life stages are usually considered as particularly sensitive to both organic and inorganic contaminants, field studies assessing contaminant bioaccumulation in these stages are scarce. Selenium (Se) is thought to counteract Hg toxic effects when it is found at Se:Hg molar ratios above 1. However, the variation of this ratio in key fish tissues of different early life stages is mostly unknown. The present study therefore aimed to assess Hg and Se content in gravid female tissues (gonads, muscle, liver, gut, and brain) and different life stages (egg masses, newly hatched larvae (NHL), larvae and juvenile) of Yellow Perch (YP) in a large fluvial lake (Lake Saint-Pierre, Québec, Canada). Se:Hg molar ratios were measured for each compartment in order to fill associated knowledge gaps. Total Hg (THg) and methylmercury (MeHg) concentration varied between tissue according to the following trend: Muscle > Liver > Gut > Brain > Gonads. During YP early life stages, MeHg values increased according to an ontogenetic pattern (mg/kg dw) (mean ±â€¯SEM): Egg masses (0.01 ±â€¯0.002) < NHL (0.015 ±â€¯0.001) < Larvae (0.14 ±â€¯0.01) < Juveniles (0.18 ±â€¯0.01). Se concentrations in different YP tissues showed the following trend (mg/kg dw) (mean ±â€¯SEM): Gut (3.6 ±â€¯0.1) > Liver (2.5 ±â€¯0.1) > Gonads (1.92 ±â€¯0.06) > Brain (1.26 ±â€¯0.03) > Muscle (1.23 ±â€¯0.06). In YP early life stages, Se concentrations were highest in NHL (3.0 ±â€¯0.2), and then decreased as follows: Egg masses (2.8 ±â€¯0.1) > Larvae (1.37 ±â€¯0.04) > Juveniles (0.93 ±â€¯0.05). Se:Hg molar ratios varied considerably and were systematically above 1. This is the first study to simultaneously report Hg and Se bioaccumulation through fish life cycle.

The fish or the egg: Maternal transfer and subcellular partitioning of mercury and selenium in Yellow Perch (Perca flavescens).

Mercury (Hg) is a trace element of particular concern since it is ubiquitous in the environment and because its methylated form (MeHg) readily bioaccumulates and biomagnifies in food webs. This latter process leads to elevated Hg concentrations in fish and may thus induce toxicity. Maternal transfer of bioaccumulated contaminants to offspring is a suggested mechanism of impaired reproductive success in fish. The purpose of this study was to assess the toxicity potential of Hg during maternal transfer in Yellow Perch from Lake Saint-Pierre (Quebec, Canada) using a subcellular partitioning approach. We also evaluated potential protective effects of selenium, as this element has been shown to alleviate Hg toxicity through sequestration. A customized subcellular partitioning protocol was used to separate liver and gonad of Yellow Perch into various subcellular fractions. Results show that, in the liver, MeHg was primarily (51%) associated to the subcellular fraction containing cytosolic enzymes. Furthermore, 23% and 15% of MeHg was found in hepatic and gonadal mitochondria, respectively, suggesting that Yellow Perch is not effectively detoxifying this metal. There was also a strong relationship (R2 = 0.73) between MeHg bioaccumulation in the liver and MeHg concentrations in gonadal mitochondria, which corroborates the potential risk linked to MeHg maternal transfer. On the other hand, we also found that selenium might have a protective effect on Hg toxicity at a subcellular level. In fact, Se:Hg molar ratios in subcellular fractions were systematically above 1 in all tissues and fractions examined, which corresponds to the suggested protective threshold. This study provides the first assessment of subcellular Se:Hg molar ratios in fish. Since early developmental stages in aquatic biota are particularly sensitive to Hg, this study represents a step forward in understanding the likelihood for toxic effects in wild fish through maternal transfer.

Relationship between Extracellular Low-Molecular-Weight Thiols and Mercury Species in Natural Lake Periphytic Biofilms.

The uptake of mercury by microorganisms is a key step in the production of methylmercury, a biomagnifiable toxin. Mercury complexation by low-molecular-weight (LMW) thiols can affect its bioavailability and thus the production of methylmercury. Freshwater biofilms were sampled in the summer using artificial Teflon substrates submerged for over a year to allow natural community colonization in the littoral zone of a Boreal Shield lake. Inside biofilms, concentrations of different extracellular thiol species (thioglycolic acid, l-cysteine-l-glycine, cysteine, and glutathione) were up to 3 orders of magnitude greater than in the surrounding water column, potentially more readily controlling mercury speciation than in the water column. All biofilm thiols except thioglycolic acid were highly correlated to chlorophyll a, likely indicating an algal origin. Extracellular total mercury represented 3 ± 1% of all biofilm mercury and was preferentially found in the capsular fraction. Levels of LMW thiols of presumed algal origins were highly correlated with total mercury in the mobile colloidal fraction of biofilms. We propose that periphytic phototrophic microorganisms such as algae likely affect the bioavailability of mercury through the exudation of LMW thiols, and thus they may play a key role in the production of methylmercury in biofilms.

Mercury methylation and demethylation by periphyton biofilms and their host in a fluvial wetland of the St. Lawrence River (QC, Canada).

Wetlands in large rivers are important sites of production of the neurotoxin methylmercury (MeHg), and the periphyton growing on wetland macrophytes are increasingly recognized as key players in this production and transfer in food webs. Information is lacking about mercury methylation (Km) and demethylation (Kd) rates in periphytic biofilms from the Northern Hemisphere, as well as about the drivers of net MeHg production, hampering ecosystem modeling of Hg cycling. Mercury methylation and demethylation rates were measured in periphytic biofilms growing on submerged plants in a shallow fluvial lake located in a temperate cold region (St. Lawrence River, Quebec, Canada). Incubations were performed in situ within macrophyte beds using low-level spikes of (199)HgO and Me(200)Hg stable isotopes as tracers. A direct relationship was observed between Km (0.002 to 0.137 d(-1)) and [MeHg] in periphyton. A similar relationship was found between Kd (0.096 to 0.334 d(-1)) and [inorganic Hg]. Periphyton of Lake St. Pierre reached high levels of net MeHg production that were two orders of magnitude higher than those found in local sediment. This production varied through the plant growing season and was mainly driven by environmental variables such as depth of growth, available light, dissolved oxygen, temperature, plant community structure, and productivity of the habitat.

Spatio-temporal variations in biomass and mercury concentrations of epiphytic biofilms and their host in a large river wetland (Lake St. Pierre, Qc, Canada).

Within wetlands, epiphytes and macrophytes play an important role in storage and transfer of metals, through the food web. However, there is a lack of information about spatial and temporal changes in their metal levels, including those of mercury (Hg), a key priority contaminant of aquatic systems. We assessed total mercury (THg) and methylmercury (MeHg) concentrations of epiphyte/macrophyte complexes in Lake St. Pierre, a large fluvial lake of the St. Lawrence River (Québec, Canada). THg and MeHg concentrations were ten fold higher in epiphytes than in macrophytes. THg concentrations in epiphytes linearly decreased as a function of the autotrophic index, suggesting a role of algae in epiphyte Hg accumulation, and % of MeHg in epiphytes reached values as high as 74%. Spatio-temporal variability in THg and MeHg concentrations in epiphytes and macrophytes were influenced by water temperature, available light, host species, water level, dissolved organic carbon and dissolved oxygen.

Assessing factors underlying variation of CO2 emissions in boreal lakes vs. reservoirs.

Reservoirs and lakes were compared to test the hypothesis that they are similar with respect to factors driving the variation in CO(2) emissions to the atmosphere. Understanding this variation is necessary for the assessment of the contribution of these freshwater ecosystems to the global carbon cycle. This study, in contrast to previous ones, included analyses of the relationships between CO(2) emissions and microbial communities. Pooled data (lakes and reservoirs) showed that variations in CO(2) emissions were strongly related to variations in temperature, dissolved organic matter (DOM) quality, and bacterial production (BP). Results also showed that lakes were characterized by higher water temperature, lower DOM quality, larger size of Daphnia, and enriched δ(13) C zooplankton compared to reservoirs. Moreover, interactions within plankton communities and relationships between CO(2) emissions and zooplankton δ(13) C signatures differed in lakes vs. reservoirs, indicating among-system type differences in food web structure and carbon cycling. As a result of these ecosystem-type characteristics, CO(2) emission variation was mainly explained by temperature and BP in lakes, and by DOM quality and the ratio of phytoplankton biomass to microheterotroph biomass in reservoirs. These results showed that differences in temperature and DOM quality between lakes and reservoirs translate into differences in microbial interactions and ultimately in the importance of factors driving CO(2) emissions to the atmosphere. They indicated that considering microbial communities and environmental variables such as temperature and DOM quality can help improve our understanding of the variation in CO(2) emissions from freshwater ecosystems.

Methanogens: principal methylators of mercury in lake periphyton.

Mercury methylation and demethylation rates were measured in periphyton biofilms growing on submerged plants from a shallow fluvial lake located along the St. Lawrence River (Quebec, Canada). Incubations were performed in situ within macrophytes beds using low-level spikes of (199)HgO and Me(200)Hg stable isotopes as tracers. To determine which microbial guilds are playing a role in these processes, methylation/demethylation experiments were performed in the absence and presence of different metabolic inhibitors: chloramphenicol (general bacteriostatic inhibitor), molybdate (sodium molybdate, a sulfate reduction inhibitor), BESA (2-bromoethane sulfonic acid, a methanogenesis inhibitor), and DCMU (3-(3,4-dichlorophenyl)-1,1 dimethyl urea, a photosynthesis inhibitor). Active microbes of the periphytic consortium were also characterized using 16S rRNA gene sequencing. Methylation rates in the absence of inhibitors varied from 0.0015 to 0.0180 d(-1) while demethylation rates ranged from 0.083 to 0.217 d(-1), which corresponds to a net methylmercury balance of -0.51 to 1.28 ng gDW periphyton(-1) d(-1). Methylation rates were significantly decreased by half by DCMU and chloramphenicol, totally inhibited by BESA, and were highly stimulated by molybdate. This suggests that methanogens rather than sulfate reducing bacteria were likely the primary methylators in the periphyton of a temperate fluvial lake, a conclusion supported by the detection of 16S rRNA gene sequences that were closely related to those of methanogens. This first clear demonstration of methanogens' role in mercury methylation in environmental periphyton samples expands the known diversity of microbial guilds that contribute to the formation of the neurotoxic substance methylmercury.

Mercury methylation in the epilithon of boreal shield aquatic ecosystems.

Methylation rates by periphyton growing on the rocky shore of a remote boreal shield lake were measured over diurnal cycles at temperatures representative of summer and fall conditions. The measurements were carried out in vitro with natural communities grown on artificial Teflon substrates submerged along the lake's shore for 1-2 years. At temperatures above 20 degrees C, epilithon Hg methylation rates were fast and reached a steady state within 12 h upon exposure to 2 ng L(-1) of inorganic mercury. A variety of inhibitors were used to identify which microorganisms in the epilithic biofilm are responsible for the methylation. The addition of molybdate, which is believed to suppress the activity of sulfate-reducing bacteria, decreased methylmercury production rates by 60% in both light and dark experiments. The prokaryote inhibitor chloramphenicol reduced the methylation rate by 40% only during dark periods whereas an algal inhibitor (DCMU), which suppresses photosynthesis, decreased the methylation rate by 60% during light periods. Results of this study reveal that epilithon communities may be a significant source of MeHg to higher aquatic organisms in lakes and that the integrity of the epilithic biofilm is important for its ability to methylate Hg.

Total mercury and methylmercury accumulation in periphyton of Boreal Shield lakes: influence of watershed physiographic characteristics.

Little is known about Hg accumulation in littoral communities, especially in periphyton biofilm of unperturbed lakes. The objectives of this study were to investigate and establish relationships between total mercury (THg) and methylmercury (MeHg) concentrations in periphyton communities of Boreal lakes and watershed physiographic and lake morphometric characteristics. This study was carried out on 23 Boreal Canadian Shield lakes located between 47-50 degrees N and 73-77 degrees W. Periphyton was sampled on rocks, the dominant periphyton substrate in the littoral zone of these lakes. Periphyton algal biomass (Chla) ranged from 12 to 164 mg m(-2) whereas THg concentrations varied from 42 to 271 ng g(-1) DW and MeHg levels varied from 3 to 55 ng Hg g(-1) DW. Periphyton biomass was positively correlated to latitude, watershed wetland area and negatively correlated to watershed slope and depth of the lake. THg concentrations in periphyton were negatively correlated to watershed wetland area whereas MeHg concentrations were negatively correlated to latitude and positively correlated to watershed slope, dissolved sulfate concentration and the presence of beavers in the lake. This study confirms that periphyton can accumulate large amounts of Hg and the accumulation is strongly influenced by watershed characteristics and periphyton biomass.

[Potential activity of methane production in soil, peat, and lacustrine sediments and in the Robert Bourassa hydro-electric reservoir in northern Canada]. / Activité potentielle de méthanogenèse dans les sols, tourbières, sédiments lacustres et du réservoir hydroélectrique Robert-Bourassa dans le moyen Nord-Canadien.

Flooding of land associated with the creation of reservoirs may increase, at least in the short term, methane flux to the atmosphere. To evaluate the potential contribution of such land use on methane production, field samples were studied in vitro for the potential activity of methanogenic bacteria in unflooded or flooded boreal forest soils, together with lacustrine sediments. From this comparative study, periodically flooded or flooded peats contribute more to methane production than do unflooded peats, soils, and natural lake sediment. The intensity and temporal changes in the activity of methanogenic archaea in the different systems depended on a combination of environmental factors, such as the amount and quality of organic carbon, the water level, and the concentration of oxidizing ions (SO42-, Fe3+).