The horizontal variation in the salinity plays a fundamental role in maintaining coastal environments, especially estuaries, which are characterized by this stratification (Scully and Geyer, 2012). From monitoring carried out between 2014 and 2018 in Paranaguá Estuarine System, it was possible to identify three zones according to their salinity confirmed by ANOVA (p < 0,05) and Tukey test (Fig. 2). Corroborating with Lana et al. (2001) and Amorim et al. (2020), we can classify these zones in the upper mesohaline sector, the central polyhaline sector and the lower euryhaline sector.
Metal in oysters
Heavy metals reach aquatic ecosystems naturally, through geochemical weathering or through domestic and industrial anthropogenic activities (Ramesh and Damodhram, 2013). Estuaries are even more exposed to the accumulation of these pollutants, originating from varied sources such as atmospheric diffusion, deposition of solids and soil leaching, mainly in agricultural areas, industrial effluent emissions, sewage collection, garbage collection and mining waste (Barletta et al., 2019), and enabling the accumulation of various metals in marine organisms either cultivated or extracted in these environments (Borak and Hosgood 2007; Mok et al. 2010).
The salinity gradients in Paranaguá are clearly affected by discharges from rivers at the innermost points (#1 and #2) and ocean waters near the estuary outlet (#5), in addition to the central zone (#3 and #4), between the two extremes. This salinity, in turn, especially in events of great variation, can cause losses in growth and survival of aquatic organisms, such as bivalves, affecting their physiological processes (Taylor et al., 2004; Albuquerque et al., 2012; Carregosa, et al., 2014). Thus, the accumulation of heavy metals (Al, As, Mn, Ni, Cu, Zn and Cd) in the tissues of the mollusks (mg.kg− 1, wet weight) may have been affected in the sampling stations (Fig. 3) by, among other variables, the salinity gradient.
There is great importance to understand the dynamics of the environment and the accumulation of metals in oysters, in this region, due to the common access of residents to these mollusks through fishing and/or aquaculture activities, which becomes a risk to the final consumer. The data obtained in the present study were compared with some places around the world (Table 2). According to this data comparison, Paranaguá Estuarine System can be considered an area with controlled impact since the registered data are lower than those of most areas shown in Table 2.
Table 2 – Average concentrations of heavy metals (mg.kg-1 wet weight) in the soft tissue of bivalve mollusks, collected from different coastal areas of the world.
Reference
|
Species
|
Location
|
Al
|
Cd
|
Zn
|
Mn
|
Cu
|
Ni
|
As
|
Brazilian legislation
|
-
|
-
|
-
|
1
|
50
|
-
|
30
|
5
|
1
|
Present study
|
Crassostrea gasar
|
PES/PR, Brazil.
|
20.22
|
0.16
|
250.3
|
1.84
|
3.80
|
0.33
|
0.96
|
Sokolowski et al. (2004) *
|
Perna perna
|
Gulf of Aden, Yemen
|
-
|
4.31
|
30.2
|
2.5
|
17.76
|
-
|
-
|
Rojas de Astudillo (2005)
|
Crassostrea rhizophorae
|
Venezuela
|
-
|
0.43 ± 0.02
|
488 ± 22
|
-
|
14.6 ± 0.5
|
0.17 ± 0.01
|
-
|
Tureck et al. (2006)
|
Crassostrea gigas
|
Santa Catarina, Brazil
|
-
|
<1 – 3.08
|
53.1 – 184.6
|
-
|
1.27 – 53.62
|
<1 – 7.59
|
0.17 – 2.58
|
Castello (2010) *
|
Crassostrea rhizophorae
|
PES/PR, Brazil
|
-
|
0.05
|
474.3
|
-
|
57.06
|
0.55
|
1.35
|
De Souza et al. (2011) *
|
Crassostrea rhizophorae
|
Bahia, Brazil
|
34.8 ± 0.56
|
-
|
220 ± 2.33
|
3.26 ± 0.5
|
4.90 ± 0.05
|
-
|
0.96 ± 0.06
|
Lino et al. (2016) *
|
Perna perna
|
Rio de Janeiro, Brazil
|
-
|
<0.005 – 0.12
|
5.68 – 11.2
|
0.51 – 6.90
|
0.51 – 551.7
|
<0.24 – 0.69
|
-
|
Campolim et al. (2017) *
|
Perna perna
|
Santos Bay, Brazil
|
172 - 301
|
0.04 – 0.15
|
10.1 – 29.3
|
0.86 – 1.65
|
0.69 – 1.38
|
0.78 – 1.98
|
-
|
Wang et al. (2017) *
|
Bathymodiolus platifrons
|
China
|
137.9
|
1793
|
463.8
|
2000
|
579.3
|
-
|
986.2
|
Suami et al. (2019)
|
Egeria Congica
|
Congo
|
-
|
0.14
|
112.6
|
17.46
|
16.28
|
0.49
|
-
|
Vieira et al. (2021)
|
Perna perna
|
Vitória, Brazil
|
50 - 700
|
-
|
500 - 970
|
11.5 - 24
|
13.5 - 32
|
-
|
2 – 4.5
|
* Articles with concentrations converted to wet weight according to Ricciardi and Bourget (1998).
Aluminum, which has no reference in Brazilian legislation, showed a higher concentration at sampling station # 1, a low salinity site, next to the Port of Antonina. Kadar et al. (
2002) described that this metal is commonly found in rivers and lakes, mainly in particulate form. So, it is expected higher incorporation of Al resulted from higher feeding rates of bioavailable particles. On average, Aluminum's results presented minors concentrations than some described in the literature (Campolim et al.,
2017; Wang et al.
2017). Also, the moderate correlation of r = -0.59 between Al and salinity (Fig.
4) stands out, reinforcing the fact that places with lower salinity may present higher Al concentrations in organisms filter feeders.
Manganese presented concentrations between 1 and 3 mg. Kg− 1 (Fig. 3). Although it also has no reference in Brazilian legislation, the Mn values presented can be considered low since studies presented by the United States Environmental Protection Agency (USEPA, 2003) showed concentrations in fruits and vegetables around 40–50 mg. Kg-1, without being considered toxic to consumers, in addition to mentioning that they are elements present in human diet. There was little variation between the oyster collection stations for Mn and, consequently, low correlation with salinity (r = -0.27) (Fig. 4). In the same sense, De Souza et al. (2011), in works with the native oyster on the northern coast of Brazil, also had low concentrations and low variation of this element.
Arsenic levels were above the legal limit in 3 sampling stations, ranging from 0.8 to 1.2 mg. Kg− 1. Castello (2010) also had similar results for As in native oysters in the same Paranaguá Estuarine System, which suggests the need for monitoring actions on site since this metal can cause several problems to human health, which may eventually result in death (Pereira et al., 2009).
Arsenic has already been listed as an environmental threat in several situations, above all because it does not have an essential function for the various organisms present in the aquatic environment (Kim, 2014; Freitas et al., 2018). Despite the greater bioavailability of this metal for mollusks being in organic form, its greatest toxicity is found in the form of inorganic arsenite, acting on the development and survival of species (Chackraborty et al., 2012).
As also demonstrated greater bioavailability at higher salinities with a high correlation value between parameters (r = 0.96) (Fig. 4). This factor can be explained by the fact that As is available as an anion and without competition in catchment locations (McLusky et al., 1986). However, it is noteworthy that its toxicity will not necessarily be linked to this increase in salinity (Bryant et al., 1979).
Cadmium, despite all sampling stations being below the limit of Brazilian legislation, had a similar behavior to As. Its concentrations varied between 0.17 and 0.25 mg.kg− 1 and obtained a positive correlation with salinity (r = 0.67). Even with low records in the present study, it is important to note that this element can be considered as one of the most toxic metals in the environment (Theede et al. 1979). However, its toxicity can also vary between organisms and, in general, there is no influence of salinity on this theme (Hall, 1994).
In this context, the increase in salinity can increase the presence of cations in water such as Ca2+ and Mg2+, also increasing competition with metals in the form of cations and making these metallic elements less bioavailable for the organisms present in these places (Saglam et al. 2013), especially copper and zinc (Millward and Liu, 2003). Thus, Pearson's correlation becomes a tool for observation and confirmation of this process in which it was identified that the concentrations of Cu (r = -0.70) and Zn (r = -0.87) (Fig. 4) were reduced in places with higher salinities.
Copper had concentrations varying between 3.5 and 10.7 mg.kg− 1 that is below the limit established in Brazilian legislation of 30.0 mg.kg− 1. Despite having a fundamental role in the development of several living beings, including human beings, Cu can bring environmental and human health problems when it is in large quantities, especially linked to the gastrointestinal system (Gamakaranage et al., 2011). Considering that Cu is part of the nutritional composition of oysters, along with zinc, this metal can also be monitored using biomarkers such as metallothionein in bivalves, a protein that manifests itself from chronic and/or acute exposure to certain elements (Viarengo et al., 1997).
Zn showed patterns like copper, as they undergo similar processes. However, it had high concentrations, varying between 153.7 and 516.3 mg.kg− 1, and with values above the limit of Brazilian legislation (50 mg.kg− 1) in all sampled locations. Zn is also naturally present in the composition of oysters (Sandstrom, 1997), which is a possible cause of its high concentration in this and in other several works (Rojas de Astudillo, 2005; Castello, 2010; De Souza et al., 2011; Wang et al., 2017). The toxicity of Zn can vary between organisms, but patterns of greater toxicity were observed at lower salinities for aquatic organisms (Bryant et al. 1985), making salinity and its correlation exposed here (r = -0.87) as even more important factors. Studies also link the expression of metallothionein to exposure to zinc in aquatic organisms (Viarengo et al., 1997; Rebelo et al., 2003) as an alternative to identify exposure to metals.
The nickel concentration in oysters ranged from 0.1 to 0.8 mg.kg− 1, all below the limit of the legislation (5.0 mg.kg− 1). Its behavior in relation to salinity was inversely proportional (r = -0.41), although moderate, following the pattern of most metals evaluated here. It should be emphasized that dissolved nickel has a strong tendency to adsorb to suspended solids (Turner et al., 1998), and should be the main source of this element for filtering aquatic organisms, such as oysters. The fact that higher concentrations of Ni were recorded at station #1, an extreme location close to the drainage of rivers, supports the hypothesis of flocculation and subsequent ingestion of particulate Ni by filtering organisms.
In this context, Kumar et al. (2015) also found higher concentrations of dissolved metals (Cd, Cu and Mn) in places with low salinity such as interior estuaries. According to Weltens et al. (2000), this can be explained by the tendency of metals to remain in colloid form in these environments, increasing the buoyancy of the particles and the resulting bioavailability for specific organisms, such as bivalves. This colloidal shape can vary in size with larger ones being more bioavailable and, consequently, bring a higher concentration of certain metals (Pan and Wang, 2002).
Estimated Daily Intakes (EDI)
EDI values were higher in station #1 for most elements (Fig. 5), except for Cu, Cd and As. Cd and As were higher in station #5, corroborating with the correlation results between metal concentration and salinity (Fig. 4). EDI for metals Al, Cu, Mn, Ni and Cd were all below the ORD, both for ALM and HLM (Fig. 5), which means that the average or high consumption of mussels would not offer significant health risks regarding these elements.
For Zn, EDI value overcame ORD (300 µg/day/kg) for HLM in the station #2, but very close to the reference value (307 µg/day/kg), demonstrating the need for observation for this metal especially at that station, but not signifying high risk, however. Zinc is an essential micronutrient in human diet assisting on various metabolic processes, and it is important to remember that its metal is also naturally present in the composition of oysters (Sandstrom, 1997).
The highest concern was posed by arsenic which surpassed ORD (0.3 µg/day/kg) in all stations for HLM consumers and in three stations (#3, #4 and #5) for ALM consumers, even in the situations where the metalloid did not surpass the limits established by Brazilian legislation.
Target hazard Quotient (THQ) and Hazard Index (HI)
For Al, Zn, Mn, Cu, Ni and Cd, mean values of THQ were < 1 for both ALM and HLM consumers (Fig. 6). Maximum values of THQ for these elements presented maximum values much lower than 1, suggesting that the ingestion of mussels was not supposed to cause any adverse effects. Zinc again had high value in station #1, above the expected amounts, but it wouldn't be worrying in analysis separately.
Arsenic, however, presented very elevated values of THQ, much higher than 1, especially for HLM consumers (Fig. 6). The highest values were registered in station #5, again highlighting the values of higher concentration of the metalloid in places of high salinity that could be a source of the As bioavailable for biota, especially for filter-feeding organisms such as oysters. Health risks associated to Crassostrea gasar ingestions are considered elevated regarding arsenic concentrations.
The Hazard Index, that considers the cumulative effects of the ingestion of potentially toxic elements, was much higher than unit in all stations both for ALM and HLM consumers. Zinc and, principally, Arsenic contributed to the elevated HI for ALM and HLM since both elements presented high THQ. The HI index did not show great differences between the stations observed, and the values, in general, were more influenced by As both for ALM and for HLM.
The results of the Hazard Index suggested a risk of health problems due to the consumption of oysters in all seasons; however, the outdoor seasons, closer to Vitória Bay, presented slightly higher risks which may be due to the greater bioavailability of trace metals and arsenic resulting from the effects of flocculation.
The only element for which Target Cancer Risk (TRC) was calculated was arsenic, since USEPA (2010) has an established cancer oral slope. The method established by USEPA considers the risk of cancer negligible when TCR < 10 − 6 and TCR values between 10 − 6 and 10 − 4 are considered acceptable. The threshold value for TCR then, concerning arsenic, is 10 − 4 (Shaheen et al., 2016; Antoine et al., 2017).
The risk of cancer for As varied between 0.0003 and 0.0005 for ALM consumers and from 0.0009 and 0.001 for HLM consumers (Fig. 7), all above the threshold value of 10− 4. Results of TCR leave no doubt that consumers of oysters originated from the study area are exposed to the metalloid pollution with carcinogenic risks as the main consequence. It is important to highlight though that the calculation of TCR was made with total arsenic concentrations, and calculations made with inorganic arsenic only, which is the form that produces toxic effects, results could be lower. However, it is important to highlight that the calculation of TCR was made with total arsenic concentration, and calculation only made with inorganic arsenic, which is the form that produces toxic effects, could result in lower values.