Mercury concentrations in fish of the Negro River-Basin,
Amazon, Brazil
Antonio C. Barbosa, IBAMA (Brazilian Environmental Agency), Brasília,
Brazil.
Jurandir de Souza, Instituto
de Química, Universidade de Brasília, Brasília, Brazil.
Íris Ferrari, Faculdade de Ciências
da Saúde, Universidade de Brasília, Brasília, Brazil.
José G. Dórea, Faculdade de
Ciências da Saúde, Universidade de Brasília, Brasília, Brazil;
dorea@rudah.com.br
Fátima Barretto, Instituto de
Química, Universidade de Brasília, Brasília, Brazil.
ABSTRACT:
The population living along the riverbank of the Amazon Basin depends heavily
on fish for its nutritional support, and daily per capita consumption has been estimated as 200g. Fish is an important concentrator of methyl
mercury (Me-Hg). Therefore the MeHg contamination depends not only on the
amount of fish consumed but also on the chosen species, and its trophic level,
as well as, mercury content of the environment. We sampled 6 specific locations
on two rivers, Negro River and Branco River, and determined total mercury on
fish representative of the food chain: herbivorous, omnivorous, detritivorous,
and piscivorous. Mercury concentration on samples of fish muscle was determined
by the cold vapor atomic absorption spectrometry (AAS-CV). Mercury
concentration varied according to fish trophic level, and season. The highest mean concentration was found in
the piscivorous species (688.9 ng/g) followed by omnivorous (190,3 ng/g),
detritivorous (136.04 ng/g), and herbivorous (70.39 ng/g). Piranhas (Serrasalmus spp) and Tucunarés (Cichla spp) were the highest
concentrators of Hg with respectively 562.8 ng/g and 690.75 ng/g. The results
of mercury concentrations in species of the Negro River, with no history of
gold mining activity, is similar to other Amazon rivers with intensive gold
mining activities.
INTRODUCTION
Mercury is
widespread in the environment and
constitutes an important contaminant that causes neuropathies. It occurs naturally in three oxidation
states Hg0, Hg+1, and Hg+2. A series of complex chemical transformations
allows Hg to cycle in the environment.
Methylation is the most important step for Hg entrance in the food chain
with aquatic organisms as the most important bioconcentrators. The riverine
inhabitants of the Amazon Basin depend heavily on fish to meet their
nutritional needs and are naturally at risk of heavy mercury body loads. Per capita fish intake has been estimated as
200 g/day (Barbosa et al, 1995). Fish eating populations are exposed to
methylmercury contamination with fetus and infants at greater risk of
neurotoxic effects. Mercury concentration in fish depends on feeding habits and
age/size of fish. As a result
contamination of fish eating populations will depend not only on the quantity
of fish consumed but also on the species of choice. Bioaccumulation of
methylmercury in fish depends on the organism trophic level. We sampled 6
specific locations on two rivers, one with clear waters (Branco River) and the
other with peculiarly dark waters that gives its name (Negro River), and
determined total mercury on fish species commonly consumed by river-bank
populations. These fish were representative of the food chain: herbivorous,
omnivorous, detritivorous, and piscivorous.
METHODS
The Negro
River-Basin have distinctive physico-chemical characteristics as well as gold
mining activities among its tributaries (Figure 1). We selected sampling sites
that included waters with such differences. The fishes were caught by a local
professional fisherman, and were weighed,
had the length measured, and approximately 10 g of muscle samples were
immediately taken, and frozen to be transported and analyzed in the laboratory
at the University of Brasilia. Fish samples were digested with concentrated HNO3
and H2SO4. For 1g of sample, 5 mL of each one of the
acids is added and the pre-digestion is accomplished during one hour at room
temperature. The solution is then submitted to reflux with constant temperature
(90oC) for 3 hours. After reflux, the sample is cooled until the
temperature reaches 250C (room temperature) and a 6% KMnO4
solution is added until the appearance of rose-coloured solution. After the
fading of the solution with solid hydroxylamin, the solution is transferred to
a volumetric flask. The digest is then transferred into a 50.0 mL volumetric
flask and completed to volume with tri-distilled and deionized water. All
measurements were accomplished by cold vapor atomic absorption spectroscopy
(AAS-CV), by using a mercury monitor of LDC analytical, model 1255.
RESULTS
AND DISCUSSION:
In this riverbank population a survey of the sub
sample of adult population informed that fish is eaten at least once a day
(7.1%) but most of them (78.6 %) consumed it at least twice a day.
Table 1. Mercury concentrations (µg/g of wet weight)
in fish species of the Negro River according to trophic level.
------------------------------------------------------------------------------------------------------
Piscivorous
Omnivorous Detritivorous Herbivorous
------------------------------------------------------------------------------------------------------
N 289 80 57 44
Mean 688.10 190.3 136.04 70.39
Range 14.98-5,437.36 10.12-849.49 21.10-778.36 62.39-384.61
-----------------------------------------------------------------------------------------------------------------------------
Riverbank inhabitants of the Amazon are heavily dependent on fish for their daily nutritional sustenance and therefore are exposed to mercury contamination. Most of the mercury found in fishes is in the methylmercury form. The fish samples of Negro River-Basin were classified by trofic level and they showed that the concentration of Hg is larger in the piscivorous species. The mercury concentration in fishes of Negro River is comparable to the one of the fishes collected in another areas of the Amazon Basin.
The fishes collected in black water, Negro River, have
averages of total Hg larger than those collected in clear waters, Branco River,
suggesting the influence of the physical-chemical parameters of the waters in
the accumulation of mercury by fish species.
The results showed that for two piscivorous species, Tucunarés
and Piranhas, the average of total mercury changed with regime of rains.
Piranhas (Serrasalmus spp) presented
the following average concentrations, 780.8, 478.4 and 759.9 ng/g for the
months of February of 98, July of 98 and January of 99 respectively, while
Tucunarés (Cichla spp) in the same
periods showed the following average concentrations 1,036.4, 469.8 and 545.5
ng/g.
Although the studied area in the Negro River does not show record of gold mining activity, contrary to the sampling sites of the Branco River, the results obtained showed that mercury contamination cannot be attributed to antropogenic activities. Recent studies indicates that the Negro River Basin baths 11% of the Amazones, and that 99,74% of its mercury would be of natural origin (Fadini and Jardim, 2000).
Individuals
at risk of mercury intoxication, pregnant and lactating women, should avoid
consuming piscivorous species of fish.
REFERENCES
Barbosa AC, Boischio AA, East GA, Ferrari I, Gonçalves
A, Silva PRM, Cruz TME.
(1995) Mercury contamination in
the Brazilian Amazon. Environmental
and
occupational aspects. Water Air Soil
Poll. 80:109-121.
Fadini PS, Jardim WF. Is the Negro River Basin
(Amazon) impacted by naturally occurring
mercury?
Sci Total Environ 2000 (in press).
Figure 1. Map of the Amazon Basin with sampling sites.
