Mercury (Hg) levels are alarmingly high in fish from lakes across Fennoscandia and northern North America. The few published studies on the ways in which silviculture practices influence this problem indicate that forest operations increase Hg in downstream aquatic ecosystems. From these studies, we estimate that between one-tenth and one-quarter of the Hg in the fish of high-latitude, managed forest landscapes can be attributed to harvesting. Forestry, however, did not create the elevated Hg levels in the soils, and waterborne Hg/MeHg concentrations downstream from harvested areas are similar to those from wetlands. Given the current understanding of the way in which silviculture impacts Hg cycling, most of the recommendations for good forest practice in Sweden appear to be appropriate for high-latitude regions, e.g., leaving riparian buffer zones, as well as reducing disturbance at stream crossings and in moist areas. The recommendation to restore wetlands and reduce drainage, however, will likely increase Hg/MeHg loadings to aquatic ecosystems.

We compared diurnal variations in dissolved gaseous mercury (DGM) concentration and in losses of dissolved organic carbon fluorescence (DOCF) in four boreal Canadian Shield lakes, in one beaver pond, and in filtered and unfiltered water from a wetland in Lake St. Pierre, a fluvial lake of the St. Lawrence River. These systems were chosen to represent a spectrum of DOC. We also determined the contribution of UVB, UVA and visible light on DGM photo-induced production in the four lakes. Our results showed a strong relationship between DGM concentrations and light intensity and between DGM production and losses in DOCF, in all study sites. We also observed higher rates of DGM formation and of DOCF bleaching in the presence of UV radiation. Under UVB light, production of DGM was higher in clear lakes than in the humic ones. Inversely, in the UVA range, DGM production tended to be higher in humic lakes. We suggest that DOCF bleaching can be used as a proxy for the rate of formation of reactive species that may alter the redox state of mercury in surface waters. We also have indications that DGM production is more important in clear than in humic waters.

The contamination of polar regions due to the global distribution of anthropogenic pollutants is of great concern because it leads to the bioaccumulation of toxic substances, methylmercury among them, in Arctic food chains. Here we present the first evidence that microbes in the high Arctic possess and express diverse merA genes, which specify the reduction of ionic mercury [Hg(II)] to the volatile elemental form [Hg(0)]. The sampled microbial biomass, collected from microbial mats in a coastal lagoon and from the surface of marine macroalgae, was comprised of bacteria that were most closely related to psychrophiles that had previously been described in polar environments. We used a kinetic redox model, taking into consideration photoredox reactions as well as mer-mediated reduction, to assess if the potential for Hg(II) reduction by Arctic microbes can affect the toxicity and environmental mobility of mercury in the high Arctic. Results suggested that mer-mediated Hg(II) reduction could account for most of the Hg(0) that is produced in high Arctic waters. At the surface, with only 5% metabolically active cells, up to 68% of the mercury pool was resolved by the model as biogenic Hg(0). At a greater depth, because of incident light attenuation, the significance of photoredox transformations declined and merA-mediated activity could account for up to 90% of Hg(0) production. These findings highlight the importance of microbial redox transformations in the biogeochemical cycling, and thus the toxicity and mobility, of mercury in polar regions.

We measured total Hg and stable isotopes (δ13C and δ15N) in northern pike (Esox lucius) from 19 Boreal Shield lakes with undisturbed, logged, or burned watersheds. Average Hg level in standard 560-mm northern pike, on a dry weight basis, was significantly higher in logged lakes (3.4 µg·g-1) than in reference lakes (1.9 µg·g-1). Average Hg concentrations in burned lakes (3.0 µg·g -1) did not differ significantly from those in logged and references lakes. Concentrations of Hg normalized to trophic position determined from isotopic composition yielded similar results. Mercury levels were above the WHO safe consumption limit in all logged lakes. Mercury in northern pike was correlated with methyl mercury in zooplankton (+), total N (+), pH (-), alkalinity (-), sulfate (+), dissolved organic C loading (+), and light attenuation in lake water (+). Stepwise multiple regressions explained 79% of the variability in Hg in fish and included methyl mercury in zooplankton, pH, and sulfate as independent variables. Explained variability increased to 92% when a second-order lake with an exceptionally large drainage area was excluded. Our results suggest that extensive logging activities may disrupt the natural cycling of Hg in watersheds and increase Hg levels in the aquatic biota.

Atmospheric mercury deposition on snow at springtime has been reported in polar regions, potentially posing a threat to coastal and inland ecosystems receiving meltwaters. However, the post-depositional fate of Hg in snow is not well known, and no data are available on Hg partitioning in polar snow. During snowmelt, we conducted a survey of Hg concentrations, partitioning and speciation in surface snow and at depth, over sea ice and over land along a 100 km transect across Cornwallis Island, NU, Canada. Total Hg concentrations [THg] in surface snow were low (less than 20 pmol L À1 ) and were significantly higher in marine vs. inland environments. Particulate Hg in surface snow represented up to 90% of total Hg over sea ice and up to 59% over land. At depth, [THg] at the snow/sea ice interface (up to 300 pmol L À1 ) were two orders of magnitude higher than at the snow/lake ice interface (ca. 2.5 pmol L À1 ). Integrated snow columns, sampled over sea-ice and over land, showed that particulate Hg was mostly bound to particles ranging from 0.45 to 2.7 lm. Moreover, melting snowpacks over sea ice and over lake ice contribute to increase [THg] at the water/ice interfaces. This study indicates that, at the onset of snowmelt, most of the Hg in snow is in particulate form, particularly over sea ice. Low Hg levels in surface snow suggest that Hg deposited through early spring deposition events is partly lost to the atmosphere from the snowpack before snowmelt. The sea ice/snow interface may constitute a site for Hg accumulation, however. Further understanding of the cycling of mercury at the sea ice/ snow and sea ice/seawater interfaces is thus warranted to fully understand how mercury enters the arctic food webs.

Agriculture in Brazil is booming. Brazil has the world's second largest cattle herd and is the second largest producer of soybeans, with the production of beef, soybeans, and bioethanol forecast to increase further. Questions remain, however, about how Brazil can reconcile increases in agricultural production with protection of its remaining natural vegetation. While high hopes have been placed on the potential for intensification of low-productivity cattle ranching to spare land for other agricultural uses, cattle productivity in the Amazon biome (29% of the Brazilian cattle herd) remains stubbornly low, and it is not clear how to realize theoretical productivity gains in practice. We provide results from six initiatives in the Brazilian Amazon, which are successfully improving cattle productivity in beef and dairy production on more than 500,000 hectares of pastureland, while supporting compliance with the Brazilian Forest Code. Spread across diverse geographies, and using a wide range of technologies, participating farms have improved productivity by 30-490%. High-productivity cattle ranching requires some initial investment

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