Sediments, located near Cornwall have been historically contaminated with Hg and other metals by local industries. Cornwall was designated an area of concern by the International Joint Commission (IJC) in 1985. The concentrations of mercury in these sediments exceeded the sediment quality guideline (SQG), of 170 ng g-1 set by Environment Canada for the protection of aquatic biota. To identify the role of these contaminated sediments on mercury dynamics in the river, I measured concentrations of total mercury (THg) and methyl mercury (MeHg) in both the porewater and solid phase of sediment cores and in the overlying water to determine whether sediments are a net source or sink for Hg. A comparison of porewater profiles in June, July and August of 2007 revealed little seasonal variation in MeHg concentrations. I also compared THg and MeHg vertical profiles in sediments with complimentary redox-dependent variables, including sulfate, sulfide, and Fe2+ distributions which showed that zones of active sulfate reduction and Fe reduction have little effect on the distribution of dissolved MeHg in the sediments. THg in sediment cores was related to the sediment accumulation rates by 210Pb radiochronology which showed the history of industrial Hg emissions to the river. MeHg contributed 4% to 100% of the THg in the porewater samples, whereas in the solid phase it contributed less than 1% of the THg. There was little to no diffusion of THg and MeHg from sediments to the overlying water. I have concluded that sediments are a major sink for THg and MeHg to the St. Lawrence River near Cornwall.

Photodegradation rates of methylmercury (MeHg) were measured in water samples from several lakes in the Lake Berthelot region in Quebec in order to investigate the impact of drainage basin logging and associated changes in dissolved organic carbon (DOC) concentrations on MeHg levels in lake water. The lakes were selected on the basis of the amount of logging that had previously taken place in the drainage basin, and on DOC levels which were correspondingly higher in logged lakes due to increased runoff. Lakes DA9 and DF9 had DOC concentrations of 12.3 and 20.9 ppm, respectively and were designated as high DOC for the purposes of this study. Lakes N55 and N70 had DOC concentrations of 6.9 and 6.8 ppm, respectively and were designated as low DOC lakes. Experiments were conducted with both ambient and 2 ppt spiked MeHg levels in the high and low DOC lakes to determine whether the spiked levels reflect photodegradation patterns occurring in nature. The effect of different sized DOC fractions on photodegradation rates were also examined. Upon spiking, the binding of added MeHg reached equilibrium virtually immediately (within minutes) for all size fractions. It was hypothesized that less photodegradation of MeHg would be found in high DOC lakes than in low DOC lakes. In contrast to expected results, no photodegradation was observed in the two lower DOC lakes, but there was photodegradation in the two higher DOC lakes. Hypothesizing that larger size fractions of DOC would result in lower methylmercury (MeHg) photodegradation rates, water samples were fractionated into three DOC size fractions (300 kDa, 30 kDa, and 5 kDa). In the two high DOC lakes in which photodegradation was detected, DOC size fractions had an effect on photodegradation rates, though this effect varied. Average k values (hr-1) for DA9 were -1.25 x 10-1 for the 5 kDa fraction, -1.16 x 10 -1 for the 30 kDa fraction, and -9.21 x 10-2 for the 300 kDa fraction, indicating a decrease in photodegradation rates with larger fractions. For DF9, average k values were -1.69 x 10-1 for the 5 kDa fraction, -6.38 x 10-2 for the 30 kDa fraction, and -1.34 x 10-1 for the 300 kDa fraction, which does not indicate a clear trend in photodegradation rates with respect to DOC size fraction. A comparison of samples from a wetland outflow using ambient MeHg levels demonstrated that 300 kDa DOC fractions showed no photodegradation while 5 kDa DOC showed photodegradation with a k value of -9.12 x 10-2 (hr-1). Overall, watershed logging status was found not to affect MeHg photodegradation rates in the lakes studied. A possible contributor to the variation in photodegradation rates between lakes was postulated to be pH, with H+ ions displacing MeHg from binding sites on DOC and rendering it available for photodegradation. To study the relationship between rates of photodegradation and pH, water samples were adjusted to pH values of approximately 4.5, 6.5, and 7.5 and incubated in sunlight. Rates of photodegradation increased with decreasing pH conditions in all lakes, except for one lake (N55) which also had no detectable photodegradation during the initial field trials. In the remaining three lakes studied, however, lower pH values (4.1-4.6) resulted in greater photodegradation than higher pH values that resulted in little or no photodegradation. It appears that this effect only occurs in lakes with a potential for photodegradation. As results suggested that the size of DOC controls rates of photodegradation, samples were analyzed for the size of DOC and corresponding MeHg content using tangential flow ultrafiltration (TFUF). The efficacy of the TFUF system as a means of both removing microbes as well as fractionating DOC into differing size fractions was evaluated. Results indicated that this is an effective field-portable method. Mass balances for MeHg recovery after filtering large volumes of water were found to be 96% for the 5 kDa filter, 81% for the 30 kDa filter, and 109% for the 300 kDa filter, while flow rates decreased by 26%, 17%, and 50% respectively. Potential artifacts of DOC fractionation using the TFUF were investigated. The amount of DOC passing through the filter was found to decrease with increasing volume of filtrate that passed through the membrane.

Methane (CH_4) and nitrous oxide (N_2O) are the second- and third-most important long-lived greenhouse gas species after carbon dioxide (CO_2) in terms of radiative forcing. This thesis describes the magnitude, spatial distribution, and seasonality of N_2O and CH_4 sources over North America using atmospheric data. We also investigate the environmental drivers and/or anthropogenic source sectors that can explain these emissions patterns. Overall, this thesis provides information on the magnitude, distribution, and likely drivers of greenhouse gas emissions to aid existing or future climate change mitigation policies in the US and Canada. We estimate anthropogenic N_2O and CH_4 emissions that greatly exceed most existing inventory estimates. Our US budgets for N_2O and CH_4 are approximately 2.8 and 1.5 times higher, respectively, than inventory estimates from the US EPA. Much of the discrepancy in methane appears to stem from oil and natural gas industry and agricultural emissions. In contrast, we estimate natural CH_4 sources that are smaller than most existing process-based biogeochemical models. These estimated fluxes have a spatial distribution centered around the Hudson Bay Lowlands. Most existing models estimate fluxes that are far more spatially distributed across the Canadian shield. These estimates provide negative information on the spatial distribution of fluxes relative to a spatially-constant model. We find that a simple model using only three environmental variables can describe flux patterns (as seen by the atmospheric observations) as well as any process-based estimate. / Earth and Planetary Sciences

Numerous applications of heavy metal have caused to their wide contamination in the environmental system and raised serious concerns over potential harmful effects on public health and the environment. Water, sediment, and dietary food are the main exposure media of heavy metal pollution and key determinants of adverse human and environmental health effects. Heavy metal(s) toxicity and speciation involve various mechanistic features with specific media and some of them are not clearly investigated. In particular, biological effects such as toxicity are not related to the total concentration of heavy metals in media, and many laboratory and field studies have supported this supposition. Organisms respond to the bioaccessible and bioavailable fraction of metals only, not the total concentration. The bioaccessibility and bioavailability of toxicants are dependent on chemical properties of the contaminant, the many exposure pathways, and temporal variability of these variables with respect to uptake by the target organism. Usually, bioavailable fractions are estimated using chemical or biological approaches. For this study, biological approaches were performed to better ascertain the toxic effects of heavy metals on organisms. A better understanding of bioaccessibility and bioavailability can be a useful tool in exposure and risk assessment. Therefore, this study presents experimental designs focusing on assessing of the bioaccessibility and bioavailability of metals in aquatic, benthic organisms and dietary food. This study also examines the role of metal mixtures on the adverse effects of metals.

Methylmercury (MeHg) is a neurotoxin found in fish and shellfish, that poses risks to human and ecological health. Exposure to MeHg adversely affects neurodevelopment of children and cardiovascular health in adults. Seafood consumption is the primary exposure route to MeHg in North America. An understanding of the link between environmental MeHg sources and human exposures is needed to determine the impacts of ongoing environmental change. However, few data exist for relating environmental exposures to human health outcomes. Imprecision in dietary recall data on fish consumption and variability in MeHg concentrations within and across seafood species consumed have made it challenging to accurately identify sources of human MeHg exposure. In addition, the diverse environmental sources of MeHg production in ecosystems make it more difficult to quantitatively attribute human exposures to specific environments where methylation is taking place. My doctoral dissertation uses naturally occurring mercury (Hg) stable isotopes to characterize sources of MeHg exposure in aquatic biota and human populations. The objectives of my work are to (1) explore the utility of Hg stable isotopes in human hair as a novel method for tracing sources of MeHg exposure to humans; (2) examine drivers of the internal body burden of MeHg in frequent seafood consumers; (3) refine understanding of environmental MeHg sources for estuarine fish. My first dissertation chapter characterizes the magnitude of mass-dependent fractionation between seafood and consumers and shows Hg stable isotopes in human hair is a promising tool for estimating different Hg exposure sources (e.g., coastal vs. oceanic fish). My second chapter uses dietary survey data and Hg isotopes in hair from high-frequency seafood consumers to show that differences in in vivo demethylation do not explain variability in biomarker concentrations. I infer that absorption efficiencies for MeHg in seafood are very low for some high-frequency fish consumers and hypothesize that this is caused by interactions with co-ingested foods. The last chapter investigates diverse Hg stable isotope signatures in benthic, riverine and pelagic estuarine fish and uses these signatures to better characterize the relative importance of different environmental MeHg sources.

<p> Fecal stanol analysis is an emergent geoarchaeological method that provides a proxy of human population change within a watershed. We provide an overview of the method, summarizing previous research and biochemical, geological, and archaeological considerations necessary for the method&rsquo;s success. We apply the method on cores from lake sediments near Cahokia, Illinois, the largest and most well-studied prehistoric mound center in North America. We find fecal stanol data closely track independently established population reconstructions from multiple sources, confirming the utility of the method and demonstrating its viability in warm climates. We compare the Horseshoe Lake fecal stanol record with paleoenvironmental data from this study and others to evaluate the role of flooding, droughts, and environmental degradation in Cahokia&rsquo;s decline. We find Mississippi River flooding and warm season droughts occurred shortly after Cahokia&rsquo;s population maximum, but we find no conclusive evidence of prehistoric environmental degradation in the watershed.</p><p>

Photodegradation rates of methylmercury (MeHg) were measured in water samples from several lakes in the Lake Berthelot region in Quebec in order to investigate the impact of drainage basin logging and associated changes in dissolved organic carbon (DOC) concentrations on MeHg levels in lake water. The lakes were selected on the basis of the amount of logging that had previously taken place in the drainage basin, and on DOC levels which were correspondingly higher in logged lakes due to increased runoff. Lakes DA9 and DF9 had DOC concentrations of 12.3 and 20.9 ppm, respectively and were designated as high DOC for the purposes of this study. Lakes N55 and N70 had DOC concentrations of 6.9 and 6.8 ppm, respectively and were designated as low DOC lakes. Experiments were conducted with both ambient and 2 ppt spiked MeHg levels in the high and low DOC lakes to determine whether the spiked levels reflect photodegradation patterns occurring in nature. The effect of different sized DOC fractions on photodegradation rates were also examined. Upon spiking, the binding of added MeHg reached equilibrium virtually immediately (within minutes) for all size fractions. It was hypothesized that less photodegradation of MeHg would be found in high DOC lakes than in low DOC lakes. In contrast to expected results, no photodegradation was observed in the two lower DOC lakes, but there was photodegradation in the two higher DOC lakes. Hypothesizing that larger size fractions of DOC would result in lower methylmercury (MeHg) photodegradation rates, water samples were fractionated into three DOC size fractions (300 kDa, 30 kDa, and 5 kDa). In the two high DOC lakes in which photodegradation was detected, DOC size fractions had an effect on photodegradation rates, though this effect varied. Average k values (hr-1) for DA9 were -1.25 x 10-1 for the 5 kDa fraction, -1.16 x 10 -1 for the 30 kDa fraction, and -9.21 x 10-2 for the 300 kDa fraction, indicating a decrease in photodegradation rates with larger fractions. For DF9, average k values were -1.69 x 10-1 for the 5 kDa fraction, -6.38 x 10-2 for the 30 kDa fraction, and -1.34 x 10-1 for the 300 kDa fraction, which does not indicate a clear trend in photodegradation rates with respect to DOC size fraction. A comparison of samples from a wetland outflow using ambient MeHg levels demonstrated that 300 kDa DOC fractions showed no photodegradation while 5 kDa DOC showed photodegradation with a k value of -9.12 x 10-2 (hr-1). Overall, watershed logging status was found not to affect MeHg photodegradation rates in the lakes studied. A possible contributor to the variation in photodegradation rates between lakes was postulated to be pH, with H+ ions displacing MeHg from binding sites on DOC and rendering it available for photodegradation. To study the relationship between rates of photodegradation and pH, water samples were adjusted to pH values of approximately 4.5, 6.5, and 7.5 and incubated in sunlight. Rates of photodegradation increased with decreasing pH conditions in all lakes, except for one lake (N55) which also had no detectable photodegradation during the initial field trials. In the remaining three lakes studied, however, lower pH values (4.1-4.6) resulted in greater photodegradation than higher pH values that resulted in little or no photodegradation. It appears that this effect only occurs in lakes with a potential for photodegradation. As results suggested that the size of DOC controls rates of photodegradation, samples were analyzed for the size of DOC and corresponding MeHg content using tangential flow ultrafiltration (TFUF). The efficacy of the TFUF system as a means of both removing microbes as well as fractionating DOC into differing size fractions was evaluated. Results indicated that this is an effective field-portable method. Mass balances for MeHg recovery after filtering large volumes of water were found to be 96% for the 5 kDa filter, 81% for the 30 kDa filter, and 109% for the 300 kDa filter, while flow rates decreased by 26%, 17%, and 50% respectively. Potential artifacts of DOC fractionation using the TFUF were investigated. The amount of DOC passing through the filter was found to decrease with increasing volume of filtrate that passed through the membrane.

Physical and chemical variables were measured in riffle zones of 20 Fraser River tributaries, British Columbia, to examine which factors explain variation in algal biomass and taxonomic composition. July epilithic periphyton chlorophyll a was weakly correlated with TN, while October periphyton was significantly related to DP and conductivity. The phytoplankton biomass in July was best predicted by TP (r2 = 0.70, p &le; 0.001, n = 19), while October phytoplankton was best predicted by a multiple regression with conductivity, current velocity and TN. In terms of taxonomic composition in October, Bacillariophyta was most abundant, followed by Chlorophyta, Cyanobacteria and Phaeophyta. DP and current velocity explained 25% of the taxonomic variation among the rivers in a CCA. While changes in the relative abundance of specific diatom taxa were observed, shifts in division dominance were not. River algal biomass could be predicted from environmental variables, however the empirical models differed between July and October.

Decommissioning hydro-dams increases sediment exposure to air, altering biogeochemical cycling of metals. Hg and MeHg mobility was studied in reservoir sediments from two Eastern Ontario lakes (Stump (SL) and Black Donald (BDL)) submitted to wet/dry cycles with artificial rainwater (pH&sim;4.5) every two weeks. Leachate pH, sulphate, sulphide and MeHg, and sulphate-reducing bacteria (SRB) populations were monitored over 6 months. The sediments did buffer the rainwater at the start of the experiment, but leachate pH decreased over time for both lakes. MeHg release occurred during the first draining event (2-4 ppt), and decreased thereafter, with no relationship with pH, sulphide and sulphate. SRB populations remained constant over time. Over 70% of Hg was in the non-labile refractory organic and residual phases, where Hg moved to semi-mobile phases in SL, but shifted to more immobile phases in BDL over time. Decommissioning hydro-dam reservoirs may increase acid production and alter Hg partitioning in sediments, while MeHg release will be low.

The main objective of this study was to examine the contribution of arbuscular mycorrhizal (AM) colonization on nickel (Ni) uptake and Ni tolerance in sunflower (Helianthus annuus L.) at a vegetative or reproductive stage of development. The combined effect of AM colonization, Ni input, and nitrogen (N) fertilization on N-assimilation in sunflower plants was also investigated. Furthermore, concerns over the transfer of heavy metals (HMs) to higher trophic levels led us to investigate whether the AM colonization and accumulation of Ni within plant tissues would induce synthesis of secondary defense compounds. It was hypothesized that AM colonization increases Ni content and plant Ni tolerance, the activities of N-assimilating enzymes (nitrate reductase, NR; glutamine synthetase, GS; and glutamine dehydrogenase, GDH), and induces the accumulation of sesquiterpene lactones (STLs), in sunflower grown under soil Ni conditions. It was also hypothesized that N-type fertilization affects ammonium assimilation as the activities of GS and GDH would be enhanced in plants supplied with an NH+4 as compared to a NO-3 fertilizer. To verify these hypotheses, three greenhouse experiments were performed with sunflower cv. "Lemon Queen", with or without the AM fungus, Glomus intraradices Schenck & Smith, and treated with (1) 0 or 100 mg Ni kg-1 dry soil (DS) at the reproductive stage, and supplied with NO-3 or NH+4 fertilizer; (2) 0, 100, 200 or 400 mg Ni kg-1, at the reproductive stage and supplied with a complete NH4NO 3 fertilization; and (3) 0, 200 or 400 mg Ni kg-1, at the vegetative stage and supplied with a complete NH4NO 3 fertilizer. The overall results indicated that AM colonization significantly enhances Ni content in sunflower plants, exposed to a moderate soil Ni level of 100 mg Ni kg-1, at the reproductive stage. Furthermore, at 100 mg Ni kg-1, the AM plants had a significantly higher shoot Ni extracted %, suggesting that the AM symbiosis contributed to Ni uptake and its translocation from roots to shoots. The AM contribution to plant Ni content and Ni extracted % were significantly higher in plants supplied with NO-3 than with NH+4 . Moreover, the plant biomass and shoot height were significantly higher in plants supplied with NO-3 than with NH+4 . In late Ni exposed sunflower, the AM colonization significantly increased the Ni extracted % at 400 mg Ni kg-1, yet also resulted in a biomass reduction of 45% as compared to only 14% at 100 mg Ni kg -1. Furthermore, a soil [Ni] of 400 mg Ni kg-1 was toxic to sunflower directly seeded in Ni treated soils, as all seedlings died within four weeks after sowing. The mineral concentrations were enhanced in AM plants, especially at lower soil Ni treatments. It is therefore concluded that the AM contribution to Ni uptake was optimal at 100 mg Ni kg-1 . The AM colonization also contributed to enhance the activities of N-assimilating enzymes, especially under NH+4 fertilization. Moreover, our results showed that the effects of HM stress and N fertilization were linked, as the activities of NR, GS, and GDH were significantly enhanced in plants under NH+4 and at 100 mg Ni kg-1. These results suggest that the combined treatments of soil Ni input and NH+4 nutrition enhance N assimilation via concurrent activities of the GS/GOGAT and GDH pathways. We also observed that both soil Ni input and AM colonization lead to an accumulation of STLs in sunflower leaves. In addition, the combination of AM colonization and soil Ni input would result in a synergistic effect to maximize defense properties while minimizing energy expenditure. These findings support the hypothesis that the AM symbiosis contributes to enhanced Ni uptake and Ni plant tolerance. It is therefore concluded that sunflower, especially in association with AM fungi, shows promise as a "candidate" species in phytoremediation strategies.