There is a need to assess the role of selenium in mercury accumulation in tropical ecosystems. Without further studies of the speciation of selenium in livers of fishes from this region, the precise role of this element, if any, cannot be verified in positively affecting mercury accumulation. Further studies of this element in the study of marine species should include liver samples containing relatively high concentrations of mercury. A basin-wide survey of selenium in fishes is also recommended.

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Recebido em 4/2/10; aceito em 7/10/10; publicado na web em 26/1/11 BIOCONCENTRATION AND BIOMAGNIFICATION OF METHYLMERCURY IN GUANABARA BAY, RIO DE JANEIRO.Methylmercury was determined in water and aquatic biota from Guanabara Bay. Trophic transfer of methylmercury was observed between trophic levels from prey (microplankton, mesoplankton and fish with different feeding habits) to top predator (pelagicdemersal fish). Top predator fish presented the highest methylmercury concentrations (320.3 ± 150.7 µg kg -1 dry wt.), whereas microplankton presented the lowest (8.9 ± 3.3 µg kg -1 dry wt.). The successive amplification of methylmercury concentrations and its bioconcentration factor with increasing trophic levels from base to top indicate that biomagnification may be occurring along the food web. Results suggest the importance of feeding habits and trophic level in the bioaccumulation of methylmercury by aquatic biota.Keywords: biotransference of methylmercury; water and organisms; trophic levels. INTRODUÇÃOExiste uma grande preocupação com relação ao aumento da poluição ambiental resultante do incremento do emprego intensivo e extensivo de produtos químicos, da liberação de substâncias potencialmente tóxicas, dos lançamentos de efluentes industriais e urbanos, bem como de seus efeitos para os ecossistemas e, consequentemente, para o homem. 1 Com o aumento das atividades industriais, agrícolas e mineradoras, bem como o crescimento populacional observados no último século, uma série de contaminantes (matéria orgânica, poluentes orgânicos e um grande número de compostos metálicos) vem sendo liberada para o meio ambiente, principalmente para a atmosfera e os ecossistemas aquáticos. 2 Os contaminantes de compostos metálicos presentes nos efluentes das atividades antrópicas apresentam pouca solubilidade na água resultando, então, em baixas concentrações, mesmo em locais próximos às suas emissões pontuais, via efluente. Entretanto, em ambientes estuarinos, os contaminantes metálicos estão preferencialmente associados ao material particulado em suspensão, que lhes serve de veículo para áreas mais distantes das suas fontes. 3 Dentre os diferentes metais que despertam interesse ambiental, presentes nos efluentes industriais e agrícolas e que, também, em diversas regiões apresentam concentrações naturais relativamente elevadas, encontra-se o mercúrio (Hg). A exposição ambiental ao Hg via cadeia alimentar, principalmente para consumidores de nível trófico superior, incluindo-se o homem, é significativamente maior, uma vez que este metal apresenta alta toxicidade e capacidade de sofrer biomagnificação ao longo das cadeias tróficas. 4 Este metal na sua forma orgânica mais tóxica, metilmercúrio (MeHg), é bioacumulado em até um milhão de vezes ao longo da cadeia trófica aquática, desde a sua base (microorganismos e plâncton) até os organismos de topo (peixes predadores e mamíferos) através de adsorção na superfície corporal e, principalmente, pela ingestão de alimento (peixes, crustáceos e cefalópodes). 5 O metilmercúrio é um age...

Mercury content, as methylmercury, in aquatic biota varies greatly among species from the same location. Many parameters may affect its accumulation and concentration in fish tissues. The study assessed total mercury (T-Hg) and methylmercury (MeHg) in the muscle, liver and gonad of Micropogonias furnieri -carnivorous fish, Bagre spp. -omnivorous fish and Mugil lizailiophagous fish from a polluted eutrophic estuary in the Brazilian Southeast coast, Guanabara Bay. Fish were collected during the years 1990, 1999 and 2003. T-Hg was determined by CV-AAS with sodium borohydride as a reducing agent. MeHg was identified and quantified in the toluene layer by GC-ECD. In all cases, the liver appears to be the preferential organ for mercury accumulation. T-Hg in muscle was higher and more variable in carnivorous than in omnivorous and iliophagous fishes. Carnivorous and omnivorous fishes presented a similar percentage of MeHg (99% and 97%) in the muscle. Iliophagous fish, which is at the lower level of this food chain, presented the lowest % MeHg in the muscle and liver, 52% and 9% respectively. However, the percentage of MeHg to T-Hg was around 25% in the liver of carnivorous and omnivorous fishes. In all cases, the gonad presented the lowest T-Hg, and the ratio of MeHg to T-Hg was around 1. In the year 1999, the samples of carnivorous and iliophagous fishes presented the highest T-Hg in the muscle. In this year, the fish specimens of both species showed sexual maturity and the highest total length. The sex of the specimens did not show any influence in the accumulation of mercury by the fish. The total length of the fish specimens presented a significant relationship with T-Hg in the muscle. The different feeding habits of the studied species are important for the accumulation of mercury and methylmercury by the organisms. Mercury, as methylmercury, is biomagnified through this food chain.

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