Mercury bioaccumulation in fishes of a paddy field in Southern of Brazil

: Aim : The aim of present study was to evaluate the Hg concentration in two species of fish ( Astyanax sp and Corydoras paleatus ) and its potential use as a biomonitor, in order to know if the use of pesticides and fertilizers in paddy can enhance the Hg contamination to adjacent aquatic environment. Methods: Soil, suspended particulate matter and fish samples were sampled in a paddy field in South Brazil . A cold vapor system, coupled with a GBC 932 atomic absorption spectrophotometer was used for total Hg determinations in samples. Results: The paddy soil shows Hg concentration 2-fold higher (mean 31 ng g -1 ) in comparison to background areas (not cultivated). Suspended particle matter Hg concentration in paddy channels (mean 232.5 ± 44.2 ng g –1 ) are 1.5 times higher than the regional background. The analyzed fish specimens Astyanax sp in paddy showed Hg concentration 4-fold higher and significant different to background area. The mean Hg concentration in fish was: 51.7 ± 19.5 ng g –1 in Astyanax sp and 156.8 ± 44.0 ng g –1 in Corydoras paleatus . Conclusions: Considering the linear regression and Man whitney test hypothesis to Hg concentration in fish tissue from paddy suggests that Astyanax sp . can be a good biomonitor of Hg contamination, whereas Corydoras paleatus is a potential biomonitor. However, more studies with Corydoras are necessary in order to aggregate consistency to this hypothesis.

The Pampa has a great biodiversity including endemic species, some of them endangered or at risk of extinction (Marques et al., 2002).Notwithstanding the considerable number of endangered species, this region has received little attention in terms of environmental research (Bencke, 2010).
The paddy has a considerable impact on aquatic animals, since many species that inhabit nearest areas, swamps and lagoons, move to them after the flood.Furthermore, this environment has great ecological significance because it represents an important feeding, resting and breeding area for birds, including migratory species that comes from North and South America's (Dias & Burger, 2005).
Studies with fish demonstrated that this animal group is a good indicator of environmental health (Van der Oost et al, 2003;Raimundo et al., 2011).In this group, the main Hg incorporation in tissue occurs through feeding (Kidd et al., 1995;Snodgrass et al., 2000).The MeHg is absorbed by the gut and accumulated in tissues, where it can be biomagnified because of the long half-life of this compound (Lacerda & Malm, 2008).Therefore, as predator fish are in the top of the food web, they have the highest concentrations of this metal in relation to species which are at the bottom.Mirlean et al. (2005), who investigated Hg levels in lakes near the paddy region in the Southern Brazil, found that the main source of Hg in this area is the atmospheric deposition.In this same study, the authors have proposed the use of Astyanax sp as a bioindicator of Hg contamination in studied area, once these species have shown a good correlation with Hg levels in the environment.
The genus Astyanax has been appointed by several authors as an excellent biomonitor of aquatic ecosystems, responses of countless biomarkers, such as histopathological assays (Prado et al., 2011;

Introduction
Mercury (Hg) is considered a highly toxic metal and has been used in the composition of pesticides utilized in rice fields (Smart & Hill, 1968).In Brazil, the use of pesticides containing Hg was banned in the 70's.Although the input of anthropogenic Hg had been stopped at these sites after the banishment, its effects persist in the environment and biota.Studies on paddies in Japan have reported an insignificant reduction of Hg soil contamination over eight years of study (Nakagawa & Yumita, 1998).Furthermore, more recent studies in paddies reported high concentration of methylmercury (MeHg) in rice grains (Zhang et al., 2010a,b;Zhao et al., 2010;Zhu et al., 2011;Peng et al., 2012;Rothenberg et al., 2012;Li et al., 2013).
In Brazil, the study developed by Silva et al. (2010) found Hg concentrations ranging from 2.2 to 4.4 ng g -1 in rice grains.Actually the large use of fertilizers in the rice production can intensify the methylation process in paddy fields due to bacteria growth stimulation.
Amongst the three Hg forms, the organic form MeHg is the most toxic to humans even in low concentrations (Zahir et al., 2005).Although Hg presents low concentration in water, it can be potentially biomagnified, reaching very higher concentrations in predatory fish, which can result in an increased risk of developing health problems when consumed by humans (NRC, 2000).
According to the report of the Committee on the Toxicological Effects of Methylmercury in the United States, the major source of human exposure to MeHg is the consumption of contaminated fish (NRC, 2000).Innumerous studies have been demonstrated that more than 80% of the total Hg present in fish tissue is in the form of MeHg (Silva et al., 2011;Kannan et al., 1998).
The characteristics of paddy are favorable to the process of Hg conversion into MeHg by anaerobic microorganisms (Rothenberg & Feng, 2012).Moreover, some studies have demonstrated that this process can also occur in the intestine of some fish species when they are fed with inorganic mercury (Rudd et al., 1980;Leaner & Mason, 2002).
The Astyanax sp and Corydoras paleatus are widely found in paddies in Southern Brazil, and are a food source to a large number of birds and mammals that use the paddy as a feeding area.These fish species are in the base of the aquatic food web.Therefore, they occupy a key position in the Hg bioaccumulation and transfer in the food web of this environment.
The aim of present study was to evaluate the Hg concentration in two species of fish (Astyanax sp and Corydoras paleatus) and its potential use as a biomonitor, in order to know if the use of pesticides and fertilizers in paddy can enhance the Hg contamination to adjacent aquatic environment.
We hypothesize the use of pesticides and fertilizers in rice paddy fields are a potential Hg source to adjacent aquatic environment.

Study sites and species
The studied site is located at the municipality of Rio Grande-RS, in the region of the swamppampas, Coastal Southern Brazil (Figure 1).This region is characterized by a coastal plain where major soil types are Planosol, Gleysol, Podzol and quartz sands, comprised of medium/fine sand and clay (Tagliani, 1997).Around the paddy fields, there are artificial channels used for drainage and irrigation, connecting this environment to natural lakes and wetlands with great biodiversity (fish, crustaceans, amphibians, aquatic birds, etc.) and allows the migration of the aquatic fauna between these sites.
During the growth period, the rice farms remain flooded for 80-100 days and many wild aquatic organisms inhabit this environment.Also in this period fertilizers, pesticides and herbicides are applied into the crops.In the harvest period the crops are drained and the animals are found in the channel, lakes and wetlands.
The sampling campaign was carried out in the summer of 2005, in a farm located at the boarder of the highway RS-473 in the period immediately after the rice harvest.The water used for rice irrigation comes by diversion from São Gonçalo River.One 40 cm core of paddy soil was collected.In the paddy drainage channel, four samples of suspended particulate matter (SPM) and the two wild fish species (Astyanax sp and Corydoras paleatus) were collected.
Regarding the biology of the fish species, the Astyanax genus feeding in the water column, consuming mainly zooplankton, fish eggs, and insects, whereas the Corydoras paleatus are bottom feeders, eating insects, small invertebrates and organic detritus (Moresco & Bemvenuti 2005).In the swamp-pampas of Southern Brazil the most abundant species are: Astyanax jacuhiensis, Astyanax eigenmanniorum and Astyanax fasciatus (Moresco & Bemvenuti 2005).
The paddy soil core was obtained employing acrylic tube sampler.In the laboratory, the soil core was fractionated in: 0-15 cm, 15-30cm and 30-40cm.Each part of the soil core was homogenized and an aliquot of this homogenates were utilized in the total and <63µm fractions for mercury determination.
The paddy drainage water was sampled in 1.5 L plastic bottles previous cleaned with HCl 10% (v/v).The water was maintained at temperature of 4 °C in cooler boxes until arrival to laboratory.At the laboratory, the water was filtered in acetate cellulose membranes (0.45µm) in a vacuum system for suspended particulate matter (SPM) quantification.
The fish species were collecting by netting.A total of 25 specimens of Astyanax sp and 10 of Corydora paleatus were obtained.In the laboratory, the fork lengths were determined, and then muscle tissue samples were removed for Hg analysis.
The results obtained in the present study were compared to literature data from background area (Federally-protected Taim Ecological Station).This area is located 80 Km away from farming investigated in the present work.

Sample digestion and total mercury analysis
Digestion methods for soil, SPM and fish tissues were the same proposed by Zhou & Wong (2000).Briefly, aliquots (0.5 g) of the soil and SPM samples were digested in 15 ml of concentrated H 2 SO 4 and HNO 3 (2:1 v/v) in a 60 ºC water bath.Digestion was proceeded until the solution was clear, then the digestion flasks were transferred to an ice bath, and 6% KMnO 4 solution was added slowly, until the digest turned purple.Five milliliters of 5% K 2 S 2 O 8 was then added to each flask to ensure complete oxidation of organic mercury compounds.
Fish tissue samples were pre-digested in 8 ml of concentrated HNO 3 and H 2 SO 4 (2:1 v/v) at 25 °C for 3h, then at 60 °C for 5 h.Five milliliters of 30% H 2 O 2 were added to the samples in 0.5 ml increments, with time enough for decreasing of foaming between additions.The temperature was then raised to 65 °C, and digestion proceeded until the samples turned colorless or light yellow.A cold vapor system, coupled with a GBC 932 atomic absorption spectrophotometer, was used for total Hg determinations in digested soil, SPM and fish samples.
During the analysis, the Hg liberated by the reduction was purged with argon, and collected on a gold wool trap connected to a GBC HG3000 hydride generation system.An additional gold trap in the gas line stripped mercury from the carrier gas.Concentrations were then determined with the GBC atomic absorption spectrophotometer.The spectrophotometer has a detection limit of 0.4 ng L -1 .
All samples were analyzed in triplicate.Coefficients of variation for all triplicates were <6%.The accuracy and precision of Hg analysis were verified by sequential digestion and analysis of certified reference materials (PACS-2 for sediment, and IAEA 350 tuna homogenates for fish muscle).Mean Hg recovery was within the 97% confidence interval for both materials, indicating high accuracy, and the coefficient of variation for six analysis was <5%, indicating high precision.
Mercury concentrations in fish are expected to increase with age due to cumulative exposure; however, to determine fish ages is not often possible.Fish lengths and weights are typically used as surrogates for age, and positive relationships between Hg concentrations and both size and age have been reported for lake fishes (Grieb et al., 1990).A linear regression was computed to assess the relationship between tissue Hg and fork length.Furthermore, a Man Whitney test was employed in Statistica 8.0 to test the differences of Hg concentration in fish from different areas are significant.

Mercury in the Soil and Suspended particulate matter
The mercury concentration in the soil (fraction <63 µm) was around 2 times higher than found in the total fraction (Table 1).
This result agrees with that found by Conceição (2005) for the regional background.It was not found any considerable variability in the Hg concentration in the different soil depths evaluated.This is probably because the soil is plowed every year for rice planting, leading to a homogenization among the soil layers.
According to Mirlean & Oliveira (2006), the unpolluted soils (fraction <63 µm) in this region presents average Hg concentration of 27 ng g -1 (ranging from 10 to 50 ng g -1 ).The results found in the paddy soil indicate that it contains a small enrichment in Hg in comparison to the regional background.The same result is found in the total soil fraction, since Conceição (2005) found a mean Hg concentration of 15 ng g -1 in this fraction.
The probable source of mercury enrichment in the paddy soil is the use of fertilizers and/or pesticides.Some studies with fertilizers described mercury concentrations of 147,000.0±38,235,000.0±98and 196,000.0±65ng g -1 in superphosphate, triple superphosphate, and NPK fertilizer, respectively (Mirlean et al., 2008).The low Hg enrichment of the soil could be related to the leaching process.
After the soil fertilization and rice planting, the area is flooded and due to the predominance of fine grained soil, the SPM in water column allows the Hg transport to adjacent aquatic environments.These processes were found by Aomine et al. (1967), which showed the mercury sprayed onto crops would be partly retained by the soil in paddy conditions for a considerable period whereas another fraction would be removed with percolation and run-off water and also be absorbed by plant roots.Moreover, Aomine et al. (1967) showed that largest part of Hg applied in the paddy is transported by and deposited in the rivers bottom sediment from the adjacent watershed.
The mean Hg concentration in suspended particulate matter in the paddy was 232.5±44.2ng g -1 .This value is about 5 times higher than that found in the fine soil fraction (<63 µm) and lower than that found in SPM at hydric compartments contaminated by gold mining and urban effluent (Table 2).However, it is similar to that found by Kütter (2006) from an industrial area in the same region (Table 2).The Hg concentrations in SPM paddy was about 1.5 times higher than the regional background and up to 4 times higher compared to remote areas in Amazon (Brabo et al., 2003).Silva et al. (2009) demonstrated that watershed land use influences Hg levels in fishes of Amazon biome.According to this work, watershed with highest aquatic vegetation density and lowest forest cover showed ichthyofauna with highest Hg concentration.Conversely, the watershed with the highest forest cover and low aquatic vegetation density showed the lowest mercury concentration in fish community.Lacerda et al. (2012), demonstrated that the change of land use in Amazon region is the main cause of Hg increase in top food web predatory fish.Furthermore, the SPM represents 90% of the total Hg present in the water river.According to Lacerda et al. (2012) the SPM has a key role in the process of fish Hg increase.
In Amazon biome, soils under any type of local cultivation (fallow, pasture, orchards, banana plantations) were characterized by cation enrichment associated with slash-and-burn activities, leading to loss of Hg, compared to levels measured in soils under forest cover (Farella et al., 2006(Farella et al., , 2007)).Pampa biome soils under rice cultivation are characterized by adduction of fertilizers and calcareous (pH soil corrector) by the periodic flooding, leading to Hg availability for biota incorporation.

Mercury in fishes
The linear regression of Hg concentrations in the fish tissue versus fish size show an increasing trend to bioaccumulation in both species investigated (Figure 2).
In Astyanax sp the Hg average concentration was 51.7 ± 19.5 ng g -1 (26.1 to 51.7 ng g -1 ), whereas Corydoras paleatus has averaged of 156.8 ± 44.0 ng g -1 (ranging from 85.4 to 222.3 ng g -1 ).This difference in Hg concentration between Astyanax sp and Corydoras paleatus may be related to feed habits as well as to distinct physiological characteristics of the two species.The Corydoras paleatus is detritivorous feeding organic matter and invertebrates from bottom, on the other hand, the Astyanax sp is omnivorous feeding plankton and invertebrates from water (Moresco & Bemvenuti, 2005).
The maximum size attained by Astyanax sp as an adult is greater than the Corydoras paleatus.The highest concentration of Hg found in Corydoras paleatus may be related to the fact that this fish species grows less in comparison to Astyanax sp.
The Hg concentration in Astyanax sp is approximately 4-fold higher in the paddy area when compared to individuals of the same size of the background area (Mirlean et al., 2005) (Figure 3).Moreover, these differences between the paddy field and background area are significantly different p< 0.05 (Man-Whitney test).However, the Hg concentration in Astyanax sp from paddy is 2.6 and 1.3 times lower than observed in individuals from industrial and suburban areas, respectively (Table 3).The highest Hg concentrations in Astyanax sp are reported in hydroelectric reservoir from Amazon region (Durrieu et al., 2005).These values are related to methylation process that is increased in those environments (Hylander et al., 2006;Montgomery et al., 2000).Even in the background areas from the Amazon region the Hg values in Astyanax sp are at least 5 times higher than that found in paddy fields in Southern Brazil (Kwon et al., 2012;Fujimura et al., 2012) (Table 3).
In another study developed in a reservoir in Southeastern Brazil, Kasper et al. (2009) found Hg concentration 1.8 to 3.0 times higher in Astyanax sp in comparison to paddy field from Pampa (Table 3).These reservoirs receive water from a polluted river, Paraíba do Sul, that cross the biggest Brazilian industrial park.
Due to the fact that the present work is the first study to show data on Hg accumulation in Corydoras paleatus, we opted to compare the data from paddy field with others detritivores species.The Corydoras paleatus from paddy showed Hg concentration 3 times higher than that found in Rineloricaria cadeae in an unpolluted area from Pampa biome (Rodrigues & Formoso, 2006).On the other hand, tissue Hg concentrations in Rineloricaria cadeae from tanneries impacted area are 1.4 times higher than Corydoras paleatus (Table 3) in our study.Other detritivorous fish, Hypostomus sp showed tissue Hg concentration 4 times lower in unpolluted area than Corydoras paleatus from paddy (Table 3).

Conclusion
The results demonstrated that the levels of Hg in fish found in the paddy are higher than in near sites without human activities ( control area,Taim reserve).This result, suggest that the use of pesticides and fertilizers in paddy fields can enhance the mercury contamination to adjacent aquatic ecosystems.Furthermore, considering the linear regression and Man whitney test hypothesis to Hg concentration in fish tissue from paddy suggests that Astyanax sp. can be a good biomonitor of Hg contamination, whereas Corydoras paleatus is a potential biomonitor.However, more studies with Corydoras are necessary in order to aggregate consistency to this hypothesis.

Figure 1 .
Figure 1.Location of the sampling area.

Figure 3 .*
Figure 3. Mercury concentration in Astyanax sp from paddy in comparison to data from Mirlean et al. (2005) nature reserve.

Table 1 .
Mercury concentration (ng g -1 ) in soils from the study area.

Table 2 .
Comparison among different sources of Hg in SPM for different sites in Brazil.