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.     

The application ofin situ dialysis samplers for close interval investigations of total dissolved mercury in interstitial waters

The suitability of the dialysis technique for the close interval sampling of total dissolved mercury (Hg) in interstitial waters was evaluated through field tests conducted in flooded soils of two hydroelectric reservoirs, as well as in lake sediments and peat bogs. In these shallow water and weakly contaminated environments, we have demonstrated that peepers offer simplicity and yield more consistent results when compared with the squeezed sediment technique; that the presence of O₂ in the initial filling solution does not significantly influence total dissolved Hg concentrations, [Hg_tot]D, in samples collected from anoxic environments; and that an equilibration period of 8 to 9 days yields slightly less variable results than one of 6.5 days. Using the dialysis technique, we were able to generate close interval vertical profiles of [Hgtot]D, in interstitial waters. A comparison of preliminary results indicates that [Hgtot]D in porewaters collected in both artificial and natural systems are similar and independent of the sampling environment, the depth of the overlying water column, and the history of reservoir impoundment.     

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.     

The partitioning of mercury in the solid components of dry and flooded forest soils and sediments from a hydroelectric reservoir,Quebec (Canada)

Upon inundation, the soils in a hydroelectric reservoir are subjected to several years of physical, biological, and chemical changes as the transition from a terrestrial to an aquatic ecosystem is achieved. It is suspected that changes in soil Eh and pH alter the metal binding capacity of organic matter, reactive iron (Fe) oxides/oxyhydroxides, and clay minerals, and may cause the mercury associated with these phases to be remobilized. Four cores were collected along a transect from an unflooded forest soil to a pre-impoundment lake bottom sediment. They were subjected to a customized sequential extraction procedure to determine the distribution of Hg between three operationally-defined solid compartments: organic carbon, reactive Fe oxides/hydroxides, and the solid (clay and sulfide) residue. Results indicate that up to 80% of the Hg in the O-horizon of forest soils and flooded soils and up to 85% of the Hg in lake sediments is bound to the NaOH-extractable organic carbon fraction. Furthermore, it was observed that the highest Hg concentrations are associated with degraded organic matter. In the B-horizon of a podzol, 40–60% of the total Hg was found associated with reactive Fe minerals. In contrast, the flooded podzol contains almost no reactive Fe at any depth and associated Hg concentrations are low. We propose that upon inundation, Fe oxides are reduced and Hg released to the pore waters where it is rapidly bound to other available substrates. Analyses of the extractions residues suggest that there is an enrichment of Hg in this fraction immediately above the B-horizon in a flooded soil.