Geochemistry of mercury in pristine and flooded ferralitic soils of a tropical rain forest in French Guiana,South America

In ferralitic soils of the dense Guianese forest, mercury (Hg) concentrations of the surface horizons varied from 122 to 318 ng/g d.w. The behaviour and accumulation of Hg is not related to the accumulation of organic matter but to the penetration of humic substances and the progressive adsorption onto iron (Fe) oxy-hydroxydes in the mineral horizons. The flooding of such soils with the creation of a small reservoir has lead to a release of 20% of the Hg initially present. This observation is related to the reduction of the Fe oxy-hydroxydes and to the migration of the organo-metallic complexes to the water column. Although physico-chemical conditions are appropriate for bacterial methylation, methyl Hg (MeHg) levels of these soils are very low. The process regulating MeHg production and its possible loss from the soil are unknown.     

Economic and conservation significance of the trophy hunting industry in sub-Saharan Africa

There is a lack of consensus among some conservation NGOs and African governments concerning the acceptability and effectiveness of trophy hunting as a conservation tool. This lack of consensus is due partly to a lack of reliable information on the economic significance and ecological impact of the industry. We provide a review of the scale of the trophy hunting industry, and assess both positive and negative issues relating to hunting and conservation in Africa. Trophy hunting occurs in 23 countries in Africa, with the largest industries occurring in southern Africa and Tanzania, where the industry is expanding. The trophy hunting industry has remained static or is shrinking in Central and West Africa. A minimum of 1,394,000 km² is used for trophy hunting in sub-Saharan Africa, which exceeds the area encompassed by national parks. Trophy hunting is thus of major importance to conservation in Africa by creating economic incentives for conservation over vast areas, including areas which may be unsuitable for alternative wildlife-based land uses such as photographic ecotourism. However, there are a number of problems associated with the industry which limit conservation benefits. Several of these problems are common to multiple countries, suggesting that if solutions were developed, conservation benefits would accrue over large areas.     

Effects of Recent Human Colonization on the Presence of Mercury in Amazonian Ecosystems

Three Hg sources were characterised and mass balance calculations were used to determine their relative contributions to the contamination of the Amazonian environment. About an order of magnitude more Hg is emitted to the atmosphere by goldmining activity than by the burning of forest biomass. However, anthropogenic atmospheric Hg cannot account for the high Hg burdens found in terrestrial ecosystems: deposition of Hg from goldmining sources is estimated to account for less than 3% of the Hg present in the surface horizons of soils. We propose that erosion of deforested soils following human colonization constitutes a major disturbance of the natural Hg cycle. Deforestation thus increases soil Hg mobilisation by runoff, which may explain the increase of Hg burdens in Amazonian aquatic ecosystems in newly colonized watersheds.     

Controls on latent heat flux and energy partitioning at a peat bog in eastern Canada

Surface and atmospheric controls on latent heat flux (Q_E) and energy partitioning were assessed during three growing seasons at the Mer Bleue peat bog. The surface consisted of a sparse canopy (maximum leaf area index 1.3) of low, mostly evergreen shrubs over moss-covered hummocks and hollows. Available energy was partitioned mostly to Q_E (Bowen ratio often less than 0.5) throughout the growing seasons over an extensive range of water table fluctuation (as much as 50 cm). Q_E was often at or below the equilibrium rate due to surface (low moss water content, strong vascular stomatal control) and/or atmospheric (low vapour pressure deficit (D_a)) factors. Turbulent energy fluxes varied with net radiation and the magnitude of the fluctuations were affected by D_a and moss water content. It is suggested that a change in source partitioning for Q_E led to a change in Q_E − D_a dynamics. Early in the growing season the moss was wet and the vascular canopy was replacing leaves, thus Q_E increased as D_a increased because moss, which reacts passively to D_a, contributed strongly to Q_E. Later in the growing season as water table declined and the evaporation load reduced moss and fibric peat water contents, moss contributed less strongly to Q_E and vascular contribution became more important. Also, stomatal control became more influential in reducing bulk surface conductance for water vapour and Q_E in response to increasing D_a.     

Trophic Structure and Bioaccumulation of Mercury in Fish of Three Natural Lakes of the Brazilian Amazon

The bioaccumulation of mercury by fish was studied in three natural lakes lining the Tapajós River, Brazilian Amazon. The Hg content variations are also reported between the rainy and the rising water seasons. Position of fish in the food chain and the source of carbon at the base of the food chain were determined using nitrogen and carbon stable isotopes ratios respectively. During the two seasons, the effect of the Hg biomagnification between the non-piscivorous and piscivorous fish of the three lakes is observed. The correlation between the mercury concentrations and the nitrogen isotopic ratios suggests that the fish mercury content variability is connected with the trophic position and size. Aside from classic observations on mercury biomagnification by certain species of fish (i.e. contents increase with size or trophic status), novel results were revealed as well, such as some species decrease of mercury contents with size of fish. Aquatic organisms in the Amazon are adapted to the complexity of trophic webs coupled with changes in environmental variables, depending on the location and/or the season, which results in a wide range of mercurial contamination among the Tapajós ichthyofauna.     

Errors in greenhouse forcing and soil carbon sequestration estimates in freshwater wetlands: a comment on Mitsch et al. (2013)

Radiative forcing feedbacks from wetlands have been an important component of past climate change and will likely be so in the future, so accurately assessing the carbon (C) and radiative balances of wetlands remains an important research priority. This commentary shows that the paper by Mitsch et al. (Landscape Ecol 28:583–597, 2013) seriously underestimated the radiative forcing effect of methane (CH₄) emissions and overestimated soil C sequestration in freshwater wetlands. The model that they used is flawed in double counting the atmospheric decay of CH₄ and incorporating a single 100 year CH₄ global warming potential. They also used a small number of sites and short-term soil dating that resulted in unrealistically high soil C sequestration rates, ignoring decay of the entire soil C pool and allochthonous inputs of C. They calculated the radiative balance instead of the radiative forcing of natural wetlands, making their calculations irrelevant to anthropogenic climate change. Irrespective of the radiative forcing of wetlands, they provide essential ecosystem services that are important to protect.     

Fish Diet and Mercury Exposure in a Riparian Amazonian Population

The incorporation of mercury into the food chain and its assimilation by humans is a universally recognized potential health hazard. Studies carried out in the Amazon Basin have shown that mercury (Hg) is present in fish and in humans, however, the relation between fish diet and human exposure has received limited attention in this region. The present study focused on a small village, Brasília Legal (3°59′00″S, 55°30′00″W), situated on the banks of the Rio Tapajós. A total of 181 fish (40 species) were captured in March, 1995 and analysed for Hg concentration. Of these, 132 fish were among species consumed by the population during the rainy season (mid-November to mid-May) and the dry season (mid-May to mid-November). Wide intra- and inter-species variations in Hg concentrations were observed. Thirty four fish (25.8% of the consumed species) had levels above 0.5 μg/g Hg fresh weight; all were among the piscivorous and omnivorous species. Hair Hg concentrations (HHg), showed that villagers with a high fish diet (n=31; median HHg=16.1 μg/g) and mixed fish diet (n=36; median HHg=14.8 μg/g) had significantly higher HHg concentrations compared to the low fish diet group (n=29; mean HHg=7.8 μg/g). Time series function of HHg measurements, made for 26 persons with over 24 cm of hair, revealed sinusoidal variations, with peaks during the rainy season and troughs during the low water period, paralleling the seasonal shift in dietary habits. Piscivorous and omnivorous fish species, with higher mercury levels, are the main component of the fish diet during the rainy season, while herbivorous fish species predominate during the dry season. Preventive actions should take into account the risk to human health, particularly for fetal and neonatal development, the importance of fish in the riparian diet, the wide intra- and inter-species variations in mercury content and seasonal fluctuations in diet.     

Methylmercury Production and Accumulation in Sediments and Soils of an Amazonian Floodplain – Effect of Seasonal Inundation

This study investigated the spatialand seasonal variations of MeHg concentrations andburdens of different sediments and soils of theTapajós river floodplain, one of the majorclear-water tributaries of the Amazon. The smallfloodplain of the Tapajós is typical of Amazonianfloodplain ecosystems. The studied lakes are borderedby inundated forest (igapó), while floatingmacrophyte mats (Paspalum sp.) develop at themargin of lakes during the flooded season. During theflood, we observed very low MeHg concentrations in theopen water lake sediments (<0.5 ng g^-1 d.w or<0.5 μg m^-2 cm^-1 d.w.) as compared to thesemi-aquatic sediments of the macrophyte zone (0.2–1.4 ng g^-1 d.w or 1–3 μg m^-2cm^-1 d.w.) and the igapó semi-terrestrial soils (0.2–3 ng g^-1 d.w or2–12 μg m^-2 cm^-1 d.w.). The litter horizon fromthe igapó soils showed the highest value of MeHg(4–8 ng g^-1 d.w.) representing 0.2–2 μg m^-2cm^-1 d.w. at the sediment/water interface during theaquatic phase. The inundation had no effect on theconcentrations and burdens of MeHg in the sediment ofthe central part of the lake. The inundation had aclear effect on the methylation of Hg at the surfaceof semi-aquatic shoreline sediments (macrophyte zone)and semi-terrestrial forest soils, where MeHgconcentrations and burdens appeared to be 3 timesgreater following inundation. In all cores, total Hgconcentrations follow those of Fe and Aloxy-hydroxides, whereas the MeHg concentrations arelinked to organic matter quality and quantity. It issuggested that organic matter and inundation controlMeHg production and accumulation at the benthicinterface. These results confirm previousobservations, in the same study area, of net²⁰³Hg methylation potentials. The fresh andlabile organic matter in the litter of the igapóforest appears as the most important factor leading tosignificant enrichment of MeHg in these particularterrestrial/aquatic sedimentary environments.