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The Machado River is the main tributary of the Madeira River in the state of Rondônia, a region that has been impacted by deforestation, livestock, urban development, alluvial gold mining and urban and industrial effluents that likely contribute to the introduction of mercury (Hg) in the aquatic system. We aimed to determine the concentrations of total Hg (THg) in muscle and liver of Plagioscion squamosissimus, a main fishing resource in the region. Fish were sampled for two years at five sites, along a 90-km stretch of the middle Machado River. THg concentrations were analyzed in 64 muscle and 54 liver samples by cold vapor atomic absorption spectrophotometry (FIMS-400). THg levels were related with fish size and sex and with periods of the hydrological cycle. THg concentrations in both organs were positively and significantly related with fish body size. There was no significant variation in THg among periods of the hydrological cycle. Mean THg concentrations in muscle (1.09 ± 0.72 mg kg-1) and liver (1.28 ± 1.23 mg kg-1) were higher than the limit established by the WHO. Our results suggest that residual mercury from former alluvial gold extraction and lixiviation of mercury naturally occurring in the soil due to large-scale deforestaion is accumulating in the aquatic food chain in the Machado River. We conclude that it is necessary to monitor the levels of mercury in commercially important fish species in the region.(AU)

O Rio Machado é o principal afluente do Rio Madeira no estado de Rondônia, uma região impactada por desmatamento, pecuária, desenvolvimento urbano, mineração de ouro e efluentes urbanos e industriais, que provavelmente contribuem para a introdução de mercúrio (Hg) no sistema aquático. Nosso objetivo foi determinar as concentrações de Hg total (THg) em músculo e fígado de Plagioscion squamosissimus, um importante recurso pesqueiro na região. Os peixes foram amostrados durante dois anos em cinco locais ao longo de 90 km no médio Rio Machado. As concentrações de THg foram analisadas em 64 amostras de músculos e 54 de fígado por espectrofotometria de absorção atômica com vapor frio (FIMS-400). As concentrações de mercúrio foram relacionadas ao tamanho e sexo dos peixes, e com períodos do ciclo hidrológico. As concentrações de THg em ambos os órgãos foram relacionadas positiva e significativamente com o tamanho corporal dos peixes. Não houve variação significativa do THg entre os períodos do ciclo hidrológico. As concentrações médias de THg no músculo (1,09 ± 0,72 mg kg-1) e no fígado (1,28 ± 1,23 mg kg-1) foram superiores ao limite estabelecido pela OMS. Os resultados sugerem que o mercúrio residual da extração de ouro e lixiviação de mercúrio que ocorre naturalmente no solo desmatado está se acumulando na cadeia alimentar aquática do Rio Machado. Concluímos que é necessário monitorar os níveis de mercúrio em espécies de peixes comerciais na região.(AU)

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In the Amazon, the leaching from soil left unprotected by deforestation increases the entry of iron, among other elements, in aquatic ecosystems, which can cause cyanobacterial blooms. This study aimed to investigate the physiological response of a strain of Microcystis panniformis to iron variation. The strain was isolated from a reservoir located in the Western Amazon and produces microcystin-LR. After a period of iron deprivation, the cultures were submitted to three conditions: control (223 µgFe.L-1), treatment with 23 µgFe.L-1, and absence of iron. At regular intervals for eight days, the cell density, levels of chlorophyll a and microcystins were determined. On the second and fourth day, transcription of genes responsive to iron limitation was quantified. Starting on the fourth day of the experiment, the different iron concentrations affected growth, and on the eighth day in the iron-free condition cell density was 90% lower than in control. Chlorophyll cell quota in 23 µgFe.L-1 and control presented similar values, while without iron the cells became chlorotic as of the fourth day Toxin concentration in cells grow in 0 µgFe.L-1 in relation to the control. Higher transcription levels of the feo and fut genes were observed in the 0 µgFe.L-1 and 23 µgFe.L-1 treatments, indicating that the cells were activating high-affinity capture systems to reestablish an adequate concentration of intracellular iron. The increasing deforestation in the Jamari River Basin (Amazon region), can contribute to the occurrence of toxic cyanobacterial blooms due to the greater entrance of iron in water bodies.

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The energy transfer in the aquatic food chain is an important way for mercury (Hg) to enter other trophic levels. The objective of this work was to evaluate the Hg concentrations in plankton upstream and downstream of the Samuel Hydroelectric Reservoir, Rondônia, Brazil. Phytoplankton and zooplankton samples were collected with 20-µm and 68-µm nylon nets. An aliquot was removed for taxonomic analysis and another for total mercury determination, performed by cold vapor atomic absorption spectroscopy. Water physical-chemical parameters were also measured. The Hg concentrations in total plankton (phytoplankton and zooplankton samples) obtained at the three sampling upstream stations showed the same behavior, with the highest values registered in June 2005 (232 µg kg-1, 118 µg kg-1, 128 µg kg-1). The lowest values at stations J1 and M1 were recorded in November 2005 (4 µg kg-1 and 22 µg kg-1, respectively), while the lowest values at stations M4 and M8 were recorded in October 2005 (22 µg kg-1 and 5 µg kg-1, respectively). The Hg results found in the plankton in this study corroborate the results of other recent studies in the same region. The statistical analyses revealed that Hg concentrations in plankton do not explain the distribution of these organisms at the four sampling stations of Samuel Reservoir. Graphical Abstract.

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This work presents quality control results on the mercury concentrations in different environmental (river sediments, forest soils, river suspended matter and fish) and human samples from the lower Madeira River, Amazon sampled between 2001 and 2003, about 15-20 years after the nearly cessation of gold mining activities in the region, which reached its peak in the late 1980s. The study aimed to compare mercury concentrations in these environmental samples with those reported by other authors during the gold rush of the Madeira River Basin. Today, in the Madeira River the releases of mercury register a sudden reduction due the gold price fall in the international trade. However, about 100 t of Hg were released to the atmosphere and to aquatic systems in the region during the gold rush. The present survey shows that notwithstanding the reduction of Hg emissions to the Madeira River Basin from gold mining proper, concentrations in fish and humans are similar to those measured during the gold rush. Reduction in Hg concentrations is restricted to areas close to old point sources and only for abiotic compartments (air and sediments). Remobilization of Hg from bottom sediments plus re-emission from soils due to land use changes are probably responsible for keeping high Hg concentrations in biological samples.

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