Carol A. Kelly

Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

Prediction of biological acid neutralization in acid-sensitive lakes

Sulfate and nitrate removal, and the resulting sulfuric and nitric acid neutralization within acid-sensitive lakes, were predicted from a simple model requiring knowledge only of water residence time, mean depth, and average mass transfer coefficients for nitrate and sulfate removal. The model applies to lakes with oxic hypolimnia which are typical of acid-sensitive lakes. Average mass transfer coefficients for sulfate and nitrate were obtained by two independent methods which agreed well with each other. A model such as this is necessary for predicting the rates at which different lakes acidify and recover from acidification, and explains why lakes with short water residence times are especially susceptible to acidification.

Natural and man-caused factors affecting the abundance and cycling of dissolved organic substances in precambrian shield lakes

Effects of natural factors (drought and forest fire), and experimental perturbations (fertilization and acidification) on dissolved organic carbon (DOC) concentrations and ratios to other nutrients in lakes of the Experimental Lakes Area are examined using data obtained over a period of 20 years. DOC concentration, and the ratio of dissolved iron to DOC in lakes of the area were strongly correlated with the relative size of the catchment to the lake.

In situ sulphate stimulation of mercury methylation in a boreal peatland: Toward a link between acid rain and methylmercury contamination in remote environments

Recent studies have found that “pristine” peatlands have high peat and pore water methylmercury (MeHg) concentrations and that peatlands may act as large sources of MeHg to the downstream aquatic system, depending upon the degree of hydrologie connectivity and catchment physiography. Sulphate-reducing bacteria have been implicated as principal methylators of inorganic mercury in many environments with previous research focused primarily on mercury methylation in aquatic sediments. Experiments in a poor fen in the Experimental Lakes Area, northwestern Ontario, Canada, demonstrated that the in situ addition of sulphate to peat and peat pore water resulted in a significant increase in pore water MeHg concentrations. As peatlands cover a large area of the Northern Hemisphere, this finding has potentially far ranging implications for the global mercury cycle, particularly in areas impacted by anthropogenically derived sulphate where the methylmercury fraction of total mercury species may be much larger than in nonimpacted environments.

Role of the Hudson Bay lowland as a source of atmospheric methane

Based on point measurements of methane flux from wetlands in the boreal and subarctic regions, northern wetlands are a major source of atmospheric methane. However, measurements have not been carried out in large continuous peatlands such as the the Hudson Bay Lowland (HBL) (320,000 km2) and the Western Siberian lowland (540,000 km2), which together account for over 30% of-the wetlands north of 40°N. To determine the role the Hudson Bay Lowland as a source of atmospheric methane, fluxes were measured by enclosures throughout the 1990 snow-free period in all the major wetland types and also by an aircraft in July. Two detailed survey areas were investigated: one (≈900km2) was in the high subarctic region of the northern lowland and the second area (≈4,800 km2) straddled the Low Subarctic and High Boreal regions of the southern lowland. The fluxes were integrated over the study period to produce annual methane emissions for each wetland type. The fluxes were then weighted by the area of 16 different habitats for the southern area and 5 habitats for the northern area, as determined from Landsat thematic mapper to yield an annual habitat-weighted emission. On a per unit area basis, 1.31±0.11 and 2.79±0.39 g CH4 m−2 yr−1 were emitted from the southern and northern survey areas, respectively. The extrapolated enclosure estimates for a 3-week period in July were compared to within 10% of the flux derived by airborne eddy correlation measurements made during the same period. The aircraft mean flux of 10±9 mg CH4 m−2 d−1 was not statistically different from the extrapolated mean flux of 20±16 mg CH4 m−2 d−1. The annual habitat-weighted emission for the entire HBL using six wetland classes is estimated as 0.538±0.187 Tg CH4 yr−1 (range of extreme cases is 0.057 to 2.112 Tg CH4 yr−1). This value is much lower than expected, based on previous emission estimates from northern wetlands.

Flux to the atmosphere of CH4 and CO2 from wetland ponds on the Hudson Bay lowlands (HBLs)

Ponds on peatlands of the Hudson Bay lowlands (HBLs) are complex ecosystems in which the fluxes to the atmosphere of CH4 and CC2 were controlled by interacting physical and biological factors. This resulted in strong diel variations of both dissolved gas concentrations and gas fluxes to the atmosphere, necessitating frequent sampling on a 24-hour schedule to enable accurate estimates of daily fluxes. Ponds at three sites on the HBL were constant net sources of CH4 and CO2 to the atmosphere at mean rates of 110–180 mg CH4 m−2 d−1 and 3700–11,000 mg CO2 m−2 d−1. Rates peaked in August and September. For CH4 the pond fluxes were 3–30 times higher than adjacent vegetated surfaces. For CO2 the net pond fluxes were similar in magnitude to the vegetated fluxes but the direction of the flux was opposite, toward atmosphere. Even though ponds cover only 8–12% of the HBL area, they accounted for 30% of its total CH4 flux to the atmosphere. There is some circumstantial evidence that the ponds are being formed by decomposition of the underlying peat and that this decomposition is being stimulated by the activity of N2 fixing cyanobacteria that grow in mats at the peat-water interface. The fact that the gas fluxes from the ponds were so different from the surrounding vegetated surfaces means that any change in the ratio of pond to vegetated area, as may occur in response to climate change, would affect the total HBL fluxes.

Retention and release of S from A freshwater wetland

A freshwater wetland at the Experimental Lakes Area in northwestern Ontario stored most of the SO42− received annually from precipitation, runoff and experimental additions. The S budget was determined for a small fen spray irrigated with H2SO4 and HNO3. Annual S retention was greatest during the first year of experimental addition of H2SO4 (73% of input in 1983). Retention was lowest (22%) in 1984, a year of lower than average precipitation with a long hot summer. During years with hot, dry summers, SO42− was produced from the reoxidation of reduced S compounds in the peat and released to surface waters. The autumn SO42− pulse was accompanied by the release of Ca and Mg but was not accompanied by a H+ release as has been detected in eastern Ontario and southern Norway, areas which receive more acidic precipitation.

Wet deposition of methyl mercury in northwestern Ontario compared to other geographic locations

Concentrations of methyl mercury (MeHg) and total mercury (THg) in precipitation were measured at the Experimental Lakes Area (ELA), a remote field station in northwestern Ontario. We found that precipitation was a source of both MeHg and THg to boreal ecosystems, but at lower rates than in industrialized regions of North America and Scandinavia. MeHg concentrations in precipitation ranged from 0.010 to 0.179 ng L1 and were highest when events originated west of the ELA. THg concentrations in precipitation ranged from 0.95 to 9.31 ng L1 and were highest when the events came from the southeast. There was no relationship between THg and MeHg over time in precipitation. Inputs of both MeHg and THg to ecosystems were highest during summer months.

Effect of temperature on production of CH4 and CO2 from Peat in a Natural and Flooded Boreal Forest Wetland

Flooding of a small boreal forest wetland (979) in northwestern Ontario, caused the formation of peat islands, which resulted in an approximate 10 °C increase in peat temperatures at a depth of 50 cm. Peat collected from the flooded wetland and a natural unflooded wetland was incubated anaerobically at temperatures of 4 °C, 15 °C, and 20 to 25 °C. Flooding of the wetland greatly increased CH4 production rates by increasing the ratio of CH4:CO2 produced from 979 peat (40% : 60%) compared to 632 peat (20% : 80%), at both preflood and postflood temperatures, likely due to the altered hydrological and geochemical conditions within the peat mats due to flooding. CH4 and CO2 production rates approximately tripled for every 10 °C temperature increase and may have been linked to to the metabolic rate of the methanogens or the fermentors independent of the substrate quality. Methane production rates from deep peat deposits within the islands were also significant and responded well to temperature increases despite peat 14C ages of 1000 years. Due to the large quantity of carbon stored within natural wetlands, artificial reservoirs may act as a significant and long term source of CH4 to the atmosphere.

The importance of floating peat to methane fluxes from flooded peatlands

The effect of flooding on methane (CH4) fluxes was studied through the construction of an experimental reservoir in a boreal forest wetland at the Experimental Lakes Area in northwestern Ontario. Prior to flooding, the peatland surface was a small source of CH4 to the atmosphere (1.0± SD of 2.3 mg CH4 m−2 d−1). After flooding, CH4 fluxes from the submerged peat surface increased to 64±68 mg CH4 m−2 d−1 CH4 bubbles within the submerged peat caused about 1/3 of the peat to float. Fluxes from these floating peat islands were much higher (440±350 mg CH4 m−2 d−2) than from both the pre-flood (undisturbed) and the post-flood (submerged) peat surfaces.The high fluxes of CH4 from the floating peat surfaces may be explained by a number of factors known to affect the production and consumption of CH4 in peat. In floating peat, however, these factors are particularly enhanced and include decreased oxidation of CH4 due to the loss of aerobic habitat normally found above the water table of undisturbed peat and to increased peat temperatures. The extremely high fluxes associated with newly lifted peat may decrease as the islands age. However, CH4 flux rates from floating peat islands that were several years old still far exceeded those from undisturbed peat surfaces and from the water surface of a newly created reservoir.