Dynamics of (total and methyl) mercury in sediment, fish, and crocodiles in an Amazonian Lake and risk assessment of fish consumption to the local population.

Increasing concerns have been raised about the toxicity of mercury (Hg) to humans, especially for those that consume a great amount of fish. High Hg concentrations have previously been measured in Amazonian waterbodies, both resulting from natural and anthropogenic sources. However, few studies have been conducted so far in Amazonian lakes that are fished by local populations. In addition, few of those studies included methylmercury (MeHg), the most toxic and bioaccumulative Hg form, and evaluated the influence of physico-chemical conditions and season on Hg dynamics. In the present study, total Hg (THg) and MeHg concentrations were measured in bottom sediment as well as in two fish and two crocodile species of the Amazonian Cuniã Lake. Bottom sediment MeHg concentrations were higher in the dry season than in the wet season, which is related to differences in physico-chemical (pH and electrical conductivity) conditions. Diet appeared to be related with animal tissue MeHg concentrations, with the herbivorous fish having lower MeHg levels than the predatory fish and crocodiles. Based on the measured tissue concentrations and published data on local person weight and fish consumption, MeHg risk to Cuniã Lake populations was estimated. Although the MeHg fish tissue concentrations did not exceed national and international standards, a significant risk to the local population is anticipated due to their high fish consumption rates. Graphical abstract.

Aquatic oligochaeta (Annelida: Clitellata) in extractive reserve Lake Cuniã, Western Brazilian Amazon / Oligoquetos aquáticos (Annelida: Clitellata) na reserva extrativista Lago do Cuniã, Amazônia Ocidental Brasileira

Abstract Oligochaeta is one of the most common and abundant taxon in continental aquatic fauna. However, knowledge of their distribution in Brazilian ecosystems is still incomplete and fragmented. Thus, the aim of this study was to develop an inventory of aquatic oligochaetes in the Extractive Reserve of Lake Cuniã in the State of Rondônia, Brazil. Collections of data were performed during the dry (August 2015) and rainy seasons (February 2016). The sediment samples were collected near the lakeside region using the kick sampling method and a kick-net sampler (mesh size of 0.25mm). This paper provides a catalog with 12 taxon from a total of 383 specimens, distributed into two families: Naididae (95.08%) and Opistocystidae (4.92%). The Pristininae subfamily was the most significant (85.68%), followed by subfamilies: Naidinae (8.36%), Tubificinae (0.52%) and Rhyacondrilinae (0.52%). In addition, some species such as Allonais inaequalis, Aulophorus furcatus, Dero nivea, Pristina synclites, Pristina menoni and Opistocysta serrata, were recorded for the first time in the Brazilian Amazon region. Therefore, the results of this study contribute to increase knowledge on the distribution of the Oligochaeta class in Brazil, particularly in the North of the country, which is so extensive and rich in water resources, but not extensively studied.

Resumo Oligochaeta é um dos táxons mais comuns e abundantes na fauna aquática continental. No entanto, o conhecimento da sua distribuição nos ecossistemas brasileiros ainda é incompleto e fragmentado. Assim, o objetivo deste estudo foi desenvolver um inventário de oligoquetos aquáticos na Reserva Extrativista do Lago Cuniã no estado de Rondônia, Brasil. Foram realizadas duas coletas, cobrindo o período seco (agosto 2015) e chuvoso (fevereiro 2016). As amostras de sedimento foram coletadas perto da região marginal do lago pelo método de amostragem Kick sampling usando um amostrador Kick-net (malha de 0,25 mm). Nós catalogamos 12 táxons do total de 383 espécimes, distribuídos em duas famílias: Naididae (95,08%) e Opistocystidae (4,92%). A subfamília Pristininae foi a mais significativa (85,68%), seguida das subfamílias: Naidinae (8,36%), Tubificinae (0,52%) e Rhyacondrilinae (0,52%). Além disso, algumas espécies, como Allonais inaequalis, Aulophorus furcatus, Dero nivea, Pristina synclites, Pristina menoni e Opistocysta serrata, tiveram o primeiro registro na Amazônia brasileira. Assim, os resultados deste estudo contribuem para aumentar o conhecimento da distribuição da classe Oligochaeta no Brasil, particularmente no Norte do país, que é tão extenso e rico em recursos hídricos, porém pouco estudado.