Methylmercury and total mercury in estuarine organisms from Rio de Janeiro, Brazil

Guanabara Bay (GB), located in the Rio de Janeiro State, is still a product ive estuary on the south-eastern Brazilian coast. It is an ecosystem heavil y impacted by organic matter, oil and a number of other toxic compounds, in cluding Hg. The present study aimed to comparatively evaluate the aquatic t otal mercury (THg) and MeHg contamination, and the ratios of MeHg to THg (% MeHg), in 3 species of marine organisms, Micropogonias furnieri-carnivorou s fish (N=81), Mugil spp. - detritivorous fish (N=20) and Perna perna- filt er-feeding bivalves (N=190), which are widely consumed by the population. A total of 291 specimens were collected at the bay in different periods betw een 1988 and 1998. THg concentrations were determined by cold vapour AAS wi th stannous chloride as a reducing agent. MeHg was extracted by dithizone-b enzene and measured by GC-ECD. Analytical quality was checked through certi fied standards. All organisms presented both low THg and MeHg concentration s and they were below the maximum limit of 1,000 mug Hg(.)kg(-1) wet wt. as established for human intake of predatory fish by the new Brazilian legisl ation. Carnivorous fish showed higher THg and MeHg concentrations, and also % MeHg in muscle tissues, than organisms with other feeding habits and low er trophic levels. The average of THg concentrations in carnivorous fish wa s 108.9 +/- 58.6 mug(.)kg(-1) wet wt. (N=61) in 1990 and 199.5 +/- 116.2 mu g(.)kg(-1) wet wt. (N=20) in 1998, but they presented different total lengt h and body weights. The average THg content in detritivorous fish was 15.4 +/-5.8 mug(.)kg(-1) wet wt., whereas THg concentrations ranged from 4.1 to 53.5 mug(.)kg(-1) wet wt. for the molluscs. The THg and MeHg contents of mu ssel varied according to the sampling point and water quality. MeHg concent ration in detritivorous fish was similar to MeHg concentration in molluscs, but there was a significant difference in the MeHg/THg ratio: the carnivor ous fish presented higher MeHg percentages (98 %) than the detritivorous fi sh (54%) and the molluscs (33%). Weight-normalised average concentration of THg in carnivorous fish collected in 1990 (0.18 0.08 mug(.)g(-1)/0.7 kg we t wt.) and in 1998 (0.16 +/-0.09 mug(.)g(-1)/0.7 kg wet wt.) presented no s ignificant difference (t=1.34; P <0.5). In conclusion, the low THg and MeHg concentrations in the organisms from the GB ecosystem, are related to its eutrophic conditions and elevated amounts of suspended matter. In this situ ation, Hg could be strongly complexed or adsorbed by the particulate, which would dilute the Hg inputs and reduce its residence time in the water colu mn, with a consequent decrease in its availability to organisms.