Progress Report for Research Project
A substantial part of the diet of many communities along the St. Lawrence River is composed of fish. This
is particularly true for the community of Lake St. Pierre, a large fluvial lake below Montreal. We selected
this location as our study site to determine the extent to which these people are exposed to mercury in their
diet. This lake is surrounded by an extensive wetland area. Since Hg cycling in river wetlands is a key gap
of knowledge in our understanding of fish contamination and exposure to human population, we proposed
an integrated study of biogeochemical transformations and fluxes within the Bay St. François wetland that
drains into Lake St. Pierre. We also monitored other sites in Lake St. Pierre, to extend the results of the
detailed studies to the whole lake. To link fish contamination to human exposure and health effects, we
also began a participative research initiative with the community to exchange knowledge about fishing
spots and feeding habits. This information will be used to integrate the biogeochemical modeling with
environmental modeling. Our general approach is therefore to conduct innovative research projects on
critical gaps in biogeochemical cycling and to link our findings to community concerns and needs. So far,
main achievements are discussed below.
We have shown that a major Hg input to the wetland is through the atmosphere by wet deposition (dew
and rain), and dry deposition of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and
total particulate mercury (TPM) in this system. Rain deposition explained more than 41% of the mercury
concentration at the BSF’s outlet. We are now able to report the first synchronized, automated, high-time
resolution, fluxes of mercury species over wetlands. Although RGM should be rapidly deposited, as
predicted by its solubility, no significant RGM flux was observed at night. Time series of canopy wetness
and RGM fluxes suggested that RGM fluxes are not driven by RGM solubility since canopy wetness
values were zero during larger RGM deposition periods observed during daytime. It is suggested that
vegetation directly takes up RGM during daytime through stomata. It is suggested a bio-transfer
(conversion) from a short transported mercury species (Hg (II)) to a long transport mercury species (Hg°)
occurs within vegetation canopy. This should imply a light-driven oxidation / reduction process which
transfers two electrons within plant foliar. Some evidence showed that photochemical reactive air masses
can lead to RGM production, RGM being then rapidly removed by dry deposition to the surface.
It should be recalled that methyl mercury (MeHg) is by far the most toxic form of Hg and the only form of
mercury that is biomagnified in food webs to levels of a million times higher in the top predator fish that
are used for food. The production of MeHg in sediments has long been recognized but details of the
process and the interaction of sediment chemistry with microbiology is still poorly known. We conducted
geochemical (pore water) and microbiological/biochemical (microbial community structure by analysis of
fatty acid methyl ester, polar lipid fatty acid, methyltransferase assay, and by DNA/RNA/PCR)
measurements in sediments cores collected near the site where atmospheric fluxes were determined.
Also, to study the distribution pattern of sulfides across the sediment-water interface, we have improved a
recently developed DGT-CID (diffusive gradients in thin films – computer imaging densitometry)
technique which can profile the two-dimensional distribution of sulfide at a spatial resolution at
submillimeters. Using this technique we have observed unprecedented heterogeneity in sulfide distribution
across the sediment-water interface. A similar degree of heterogeneity is expected in mercury speciation
and in the distribution of methylmercury. In addition to sulfide, new techniques are also being developed
to identify and quantify biogenic thiols (e.g., cysteine and phytochelatins) across the sediment-water
interface, which are expected to play a significant role in determining the speciation and bioavailability of
mercury. The objectives were to: i) to obtain information on the implication of Hg-sulfur species in the
mechanism of Hg methylation; ii) to obtain information on the transport of HgT and Me-Hg across the
sediment-water interface; iii) to determine the reproducibility of Me-Hg profiles; iv) to determine the
seasonal effects (growth and decay cycle) on the profiles and transport of Hg and Me-Hg. Samples are
currently being processed. Up to now, the main conclusions are that 1) we obtain reproducible pore water
Hg and Me-Hg profiles with peepers; 2) MeHg is produced and accumulates in the sediment pore water,
and it diffuses slowly to the overlying water; 3) dissolved HgS may not be involved in Hg methylation as
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Progress Report for Research Project
currently thought; 4) there is a need to reassess the stability constant of HgS to accurately model Hg
cycling in ecosystems.
Levels of methyl mercury are very low in the St Lawrence River water and it is difficult to determine why
the fish have the level of mercury that they have. We are currently determining the amount of methyl
mercury bound to dissolved organic carbon in water such as that found in Bay St. François with that in the
main channel of the St. Lawrence River and values compared with other sites in Ontario and Quebec. We
predict that the dissolved organic carbon in water samples which have been exposed to solar radiation do
not bind methyl mercury as readily as water samples fresh from wetlands. In other words, methyl mercury
is more bioavailable when not bound to dissolved organic carbon. This will aid in developing predictions
for food web biomagnification models. Currently, models indicate that algae bioconcentrate methyl
mercury by about a million times more than that in the water sample, but our measurements indicate that
most of it is indeed bound to dissolved organic carbon. This information is also important in predicting the
rate of photodegradation of methyl mercury.
A major feature of wetlands is the presence of macrophytes that may: 1) methylate Hg; 2) create
favourable conditions for bacterial Hg methylation or provide a substrate for methylating epiphytes; 3)
reduce or promote Hg(II) reduction to volatile Hg(0) by bacteria or epiphytes, or by releasing reducing
compounds; 4) affect the regional Hg budget at senescence, through the transfer of tissue-bound Hg. The
objectives were to: 1) determine depth profiles of dissolved gaseous mercury (DGM, mainly Hg(0)) in
different macrophyte beds and relate them to other variables; 2) measure DGM production rates in and
out of macrophyte beds and under different types of canopy; 3) establish the tissue distribution of Hg in
different species of macrophytes; 4) establish the importance of epiphytes on main submerged and
emergent macrophyte species; 5) determine [Hg] and [MeHg] in epiphytes, and the associated
methylation/demethylation rates. Although many of the samples remain to be processed, we can draw the
following preliminary conclusions: 1) highest DGM levels were usually found at the sediment/water
interface, suggesting that bacterial reduction may be important; 2) subsurface DGM levels measured in
waters heavily covered by macrophytes (e.g. Ceratophyllum beds) and receiving little light were similar to
those from uncovered areas, suggesting that reduction of Hg(II) induced by macrophytes is of similar
importance to photoreduction in this system; 3) clear patterns in [Hg] could be found when examining
tissue distribution every 2 cm along the roots, stems and leaves; 4) the highest peaks in DGM production
were observed, immediately after sunrise, not around noon, suggesting a fast depletion of substrate for Hg
photoreduction; 5) MeHg concentration in epiphytes varied with macrophytes species and, possibly light;
Highest MeHg levels were found on epiphytes growing near the surface on floating macrophytes; 6)
Epiphytes growing on submerged macrophytes methylate mercury.
Once Hg has been deposited to wetlands, transformed in the sediments, the water column and the plants, it
may reach the food web and be accumulated by fish. The main objectives of our fish studies were: 1) to
obtain data on mercury levels in fish that people consume; 2) to evaluate fishing pressure by commercial
and sport fishermen; 3) to assess, from these combined data, community exposure to Hg by fish
consumption, in collaboration with the Environmental Education and Health research teams (who will try
to establish diet patterns in the population, including store bought fish). The main conclusions from our
work up to now are: 1) Fishing pressure is intense: about 700 t/yr just for the commercial part and we
estimate that the biomass captured by sport fishermen might be between 300 and 700 t/yr; 2) Commercial
fishermen are mainly fishing sturgeon, yellow perch and catfish, whereas sport fishermen prefer walleye,
northern pike and yellow perch; 3) the bay where the biogeochemical studies are conducted is not
intensively fished; although logistically convenient, this could hamper our efforts to model the whole
ecosystem and the addition of new sites could be appropriate; 4) walleye and northern pike (piscivorous
fish at the top of the food web) had low Hg levels compared to the ones from boreal forest lakes or from
Lac Saint-Jean. These values are below the 0.5 ppm actual Canadian threshold for commercialization.
These low Hg concentrations are similar to previous studies from Ministère de l’environnement du
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Progress Report for Research Project
Québec; they are significant in reassuring the community. Also, even though levels are low, a portion of
the population is eating large quantities of fish and may still be exposed to a high cumulative dose. They
might also exposed to other contaminants, through fish consumption (Ex: other heavy metals, DDT,
PCB...), which can act in synergy with Hg. In the study carried out on Hg exposure of pregnant women
and their new-borns in the Upper St. Lawrence River, strong relations were found between the frequency
of fish consumption during pregnancy and mother's blood organic Hg levels. At birth, the organic Hg in
cord blood was higher than in maternal blood. In Lac Saint-Pierre, we established contacts with sports
fisher's associations and with the public Health Centers and hospitals, in order to investigate in 2003 the
relation between diet and Hg exposure of anglers and pregnant women.
In 2003, we will study the reasons leading to these low Hg levels in fish from Lac St-Pierre. Two
hypotheses will be retained: 1) over fishing may induce biodilution of Hg in fish that people consume; 2)
this environment may not be prone to high methylation rates.
New research avenue
The role of reduced sulfur species in mercury methylation: A comparative in situ study
Feiyue Wang (University of Manitoba) and Marc Amyot (University of Montreal)
Methylmercury (MeHg) is the most common contaminant of fish in Canada. Different from most
contaminants, the majority of the MeHg in aquatic systems is not introduced from external sources, but
produced in situ. It is this in situ methylation process that determines the amount of MeHg reaching higher
trophic levels in a food web.
In situ Hg methylation is carried out predominantly by sulfate-reducing bacteria (SRB) and modulated by
sulfide. Laboratory studies suggest that sulfide can both promote and prohibit Hg methylation. In
particular, the neutral HgS0 complex has been proposed to be the most bioavailable Hg species for
methylation. However, so far no model has been successful in predicting in situ MeHg production in
natural waters, mainly because (i) the speciation of Hg in such sulfidic waters is poorly understood; and
(ii) most of the studies were conducted in the laboratory and whether the results can be extrapolated to in
situ conditions remains unknown.
We hypothesize that MeHg in an aquatic system is produced primarily within a thin layer (“mercury
methylation layer” or MML) across or near the sediment-water interface. The delicate chemical (e.g.,
sulfide, organic matter, pH, Eh) and microbial (e.g., SRB) conditions in the MML lead to an optimal
transformation of certain Hg species to MeHg, which can then diffuse to the overlying water, be
demethylated, or taken up by biota. We further hypothesize that the concentration and distribution of
reduced sulfur species (RSS, including sulfide, polysulfides, and thiols) play a central role in Hg
methylation in the MML. Several reasons lead us to this hypothesis: (i) sulfide is produced from sulfate
reduction by SRB and is thus present wherever SRB are present; (ii) the speciation of Hg in the sulfidic
MML is dominated by Hg-RSS complexes, due to the extremely high binding affinities between Hg and
RSS; (iii) the neutral Hg-RSS species such as HgS0 and Hg-cysteine have been suggested to be highly
bioavailable; and (iv) the concentration of sulfide also reflects the nature and concentration of organic
matter, as the microbial reduction of sulfate is dependent on the nature and influx of organic matter; it is
also a sensitive indicator of pH and Eh.
The objective of this project is to uncover the central role of RSS in Hg methylation process in the MML.
Instead of focusing on laboratory culturing studies which are lacking of chemical gradients, we propose to
probe the role of RSS by conducting comparative in situ studies in two drastically different wetlands:
Delta Marsh on the south shore of Lake Manitoba (Manitoba) and Baie St. Francois of Lac St. Pierre
(Quebec). Wetlands are chosen because they are areas of rapid MeHg production. The two wetlands to be
studied are representative of Prairie and Shield wetlands, respectively. Most notably, the sediment
interstitial waters in the two wetlands differ by three orders of magnitude in sulfide concentration (~800
µM at Delta Marsh vs. ~1 µM at Baie St. Francois) and will provide a unique natural contrast for studying
the role of RSS in Hg methylation.
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At each wetland, we propose to use an in situ micro profiling technique recently improved by us to profile
the two-dimensional distribution of sulfide across the MML at a resolution of sub-millimeters. Similar
profiling techniques will be developed for Hg, MeHg, pH, thiols, polysulfides, and organic matter. At
Delta Marsh, enclosures will also be set up in an isolated pond (Crescent Pond) where different amounts
of Hg will be added. The high-resolution temporal and spatial datasets will then be used to model Hg
speciation and to relate it to the production of MeHg. The results will not only reveal unambiguously the
role of RSS in in situ Hg methylation, but also allow for the prediction of the MeHg production in a wide
range of water bodies. In Years 4 and 5, further studies will be conducted to study the inter- and extracellular processes of Hg-RSS uptake by SRB and epiphytes in the two drastically different wetlands. This
project will be incorporated into the St. Lawrence River case study (project 324).
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