Mina Nasr

Mercury in freshwater ecosystems of the Canadian Arctic: recent advances on its cycling and fate.

The Canadian Arctic has vast freshwater resources, and fish are important in the diet of many Northerners. Mercury is a contaminant of concern because of its potential toxicity and elevated bioaccumulation in some fish populations. Over the last decade, significant advances have been made in characterizing the cycling and fate of mercury in these freshwater environments. Large amounts of new data on concentrations, speciation and fluxes of Hg are provided and summarized for water and sediment, which were virtually absent for the Canadian Arctic a decade ago. The biogeochemical processes that control the speciation of mercury remain poorly resolved, including the sites and controls of methylmercury production. Food web studies have examined the roles of Hg uptake, trophic transfer, and diet for Hg bioaccumulation in fish, and, in particular, advances have been made in identifying determinants of mercury levels in lake-dwelling and sea-run forms of Arctic char. In a comparison of common freshwater fish species that were sampled across the Canadian Arctic between 2002 and 2009, no geographic patterns or regional hotspots were evident. Over the last two to four decades, Hg concentrations have increased in some monitored populations of fish in the Mackenzie River Basin while other populations from the Yukon and Nunavut showed no change or a slight decline. The different Hg trends indicate that the drivers of temporal change may be regional or habitat-specific. The Canadian Arctic is undergoing profound environmental change, and preliminary evidence suggests that it may be impacting the cycling and bioaccumulation of mercury. Further research is needed to investigate climate change impacts on the Hg cycle as well as biogeochemical controls of methylmercury production and the processes leading to increasing Hg levels in some fish populations in the Canadian Arctic.

Quantifying Hg within ectomycorrhizal fruiting bodies, from emergence to senescence.

Ectomycorrhizal fruiting bodies (basidiomata) collected from forested areas in southwestern New Brunswick were analyzed for total mercury, sulphur, nitrogen, and carbon concentrations (THg, TS, TN, and TC, respectively). This analysis was done for caps and stalks and by development stage (emergent, mature, senescent) across 27 species associated with five classes, eight families, and 13 genera. Across the species, THg correlated positively with TN and TS, thereby implying N as well as S mitigated transfer of Hg from the mycelia into the basidiomata, with THg ranging from 3 to 10 457 ppb. TS, TN, and TC varied from 0.07 to 1, 1 to 11, and 43 to 53 %, respectively. Cap and stalk THg, TS, TN, and TC were also correlated to one another, with mean stalk/cap ratios of 0.59, 0.76, 0.71, and 0.98, respectively. Soil availability indexed by THg, TS, TN, and TC within the forest floor contributed to basidiomatal THg as well. THg, THg/TS, and THg/N varied strongly by species. These variations involved: (i) no growth dilution and no volatilization (Group I), (ii) growth dilution only (Group II), (iii) growth dilution followed by loss during senescence (Group III), and (iv) growth dilution combined with loss from emergence onward (Group IV). Depending on species, TN and TS remained the same or declined from 100 % at emergence to about 80 and 70 % at senescence. Lack of THg decline for the Group I species would be due to HgS encapsulation. Reanalyzing the freeze-dried samples revealed that THg continued to drop during the first year of air-dry storage for the Group II, II, and IV species, but TS, TN, and TC remained stable. The results were quantified by way of best-fitted regression models.

Modeling hydrothermal regimes and potential impacts of climate change on permafrost within the South Mackenzie Plain, Northwest Territories, Canada

Abstract: Hydrothermal processes are key components of permafrost dynamics and hydrological and carbon cycles in northern forest ecosystems. A forest hydrology model, ForHyM, was used to evaluate these processes by determining how the depth and duration of frost penetration into the soil would vary daily over the course of several decades. This was done for chosen upland/wetland conditions within the Mackenzie Plain south of Fort Simpson, where permafrost is currently sporadic to discontinuous. The model calculations were done using daily weather records from November 1963 to 2010, starting with a hypothetical no-frost condition within the soil and subsoil. Model performance was evaluated by comparing modeled and measured temperatures at different soil depths (upland and peat plateau modelling, R2 = 0.95 and 0.94, respectively). It was found that well-drained upland forests within the general area would experience deep and complete freeze—thaw cycles each year. In contrast, poorly drained wetlands would develop gradually deepening permafrost that would at first stabilize in depth over the course of 10 to 20 y, with thaw depth limited to <1 m each year. However, recent increases in recorded air temperature (more so in winter than in summer) would destabilize the permafrost layer, and this would especially occur in areas with insufficient surface insulation by local peat, moss, forest litter, and snow accumulations. These estimates are consistent with (i) reported thawing depths and (ii) the widening encroachment of collapse scars towards the poorly drained portions of the South Mackenzie Plain.

Mercury and Organic Matter Concentrations in Lake and Stream Sediments in relation to One Another and to Atmospheric Mercury Deposition and Climate Variations across Canada

This article focuses on analyzing the Geological Survey of Canada (GSC) data for total mercury concentrations (THg) in lake and stream sediments. The objective was to quantify how sediment THg varies by (i) sediment organic matter, determined by loss on ignition (LOI) at 500∘C, (ii) atmospheric Hg deposition (atm.) as derived from the Global/Regional Atmospheric Heavy Metals Model GRAHM2005, and (iii) mean annual precipitation and mean monthly July and January temperatures (, ). Through regression analyses and averaging by National Topographic System tiles (NTS, 1:250,000 scale), it was found that 40, 70, and 80% of the sediment THg, LOI, and atm. variations were, respectively, related to precipitation, , and . In detail, lake sediment THg was related to atm. and precipitation, while stream sediment THg was related to sediment LOI and . Plotting sediment THg versus sediment LOI revealed a curvilinear pattern, with highest Hg concentrations at intermediate LOI values. Analysing the resulting 10th and 90th log10THg percentiles within each 10% LOI class from 0 to 100% revealed that (i) atm. contributed to the organic component of sediment THg and (ii) this was more pronounced for lakes than for streams.

Biomonitoring and assessing total mercury concentrations and pools in forested areas

Abstract: This article focusses on the bio-monitoring of total Hg (THg), sulfur (TS) and carbon (TC) concentrations and pool sizes in forest vegetation and soil layers within the context of a maritime-to-inland transect study in southwestern New Brunswick. This transect stretches from the Grand Manan Island in the Bay of Fundy to the mainland coast (Little Lepreau to New River Beach) and 100 km northward to Fredericton. Along the Bay, frequent summer fogs are thought to have led to increased THg concentrations in forest vegetation and soils such that island THg > coast THg > inland THg concentrations. Transect sampling was done in two phases: (i) a general vegetation and soil survey, and (ii) focusing on specific soil layers (forest floor, top portion of the mineral soils), and select moss and mushrooms species. By way of multiple regression, it was found that soil, moss and mushroom THg and TS were strongly related to one another, with THg decreasing from the island to the inland locations. The accumulated Hg pool within the mineral soil, however, far exceeded (i) the estimated THg pools of the forest biomass (trees, moss and mushrooms) and the forest floor, and (ii) the literature-reported and case-study inferred net input/output rates for annual atmospheric Hg deposition and sequestration, Hg volatilization, and Hg leaching. Partitioning the total soil Hg pool into geogenically and atmospherically derived portions suggested that mineral soils in temperate to boreal forest regions have accumulated and retained atmospherically derived Hg over thousand years and more. These results are summarized in terms of further guiding forest THg monitoring and modelling efforts in terms of specific vegetation and soil sampling targets.

Total Mercury Concentrations in Lake and Streams Sediments Related to Wet-Area Coverage and Geogenic Sources within Upslope Basins

ABSTRACT Total mercury concentrations (THg) in lake and stream sediments generally decrease with wet-area coverage (AW) per upslope basin area (AB). This was determined by delineating the wet-area component of 12,653 basins above as many sediment-sampling locations of the Geological Survey of Canada. These locations represent four climate regions (maritime, boreal, arctic, alpine) comprising six stream and six lake study areas. The dependence of sediment THg on AW/AB was examined by dividing the 0 < AW/AB < 1 range into 40 equal segments, and obtaining the mean sediment THg value for each segment. The results were evaluated by way of regression analysis using the following equation: mean sediment THg = a (1 − AW/AB)b + c AW/AB, with a, b and c as area-specific coefficients. The “a” and “c” coefficients could – in part – be inferred from bedrock type, annual atmospheric Hg deposition, and mean monthly air temperatures, and mean annual precipitation. Both “a” and “c” increased with increasing atmospheric Hg deposition for lake sediments. For stream sediments, only “a” did so. The geogenic influence on the THg variations per study area was addressed through multiple regression analyses, using sediment concentrations of other heavy elements and organic matter as independent variables.