12 Distribution of Cd, Pb, As and Hg in Oyster Tissue, Sediment and Water. . .
145
period (Ferreira et al. 2005; Ahmad et al. 2010). It may be that some Cd metals
present in the environment were dispersed due to intensive rains and strong water
currents. Being a sedentary organism, oysters do not have the ability to move
away and search for food. The strong water currents during monsoon season thus
became a major contributory factor that hinders the availability and accumulation of
any particulate matter to oysters. However, other research findings are completely
opposite to the findings to this study, where Cd uptake increased significantly with
decreasing salinity (Mackay et al. 1975; Ke and Wang 2001).
Although relatively low (upper limit: 0.007 mg/kg), mean As concentrations
(0.01 mg/kg) were detected in oyster tissues. Results of this study were several hundred times lower than the average value obtained for the total arsenic concentration
(15.17 mg/kg) in oysters in Taiwan during rainy seasons (Hsiung and Huang 2006).
At present, the Philippines does not have standard limit set for As concentration
in oyster tissues. However, when comparing this with the established standards
of Australia and New Zealand (AUSAID 2007) which is 1.0 mg/kg for oysters,
the values detected in the Philippines are comparatively lower. The concentration
differences of As collected during the pre-monsoon and monsoon season were not
significant (p > 0.05). The capacity of oysters to accumulate As in their system and
the availability and abundance of As in oyster habitat can be among the factors that
contributed to the levels detected.
Only one station (station 4) during the pre-monsoon season yielded a Pb value
(0.55 mg/kg) for oyster tissue within our methodology’s Limit of Detection (LOD)
of 0.50 mg/kg. Incidentally, under the Philippine standards (BFAR 2001c), this
value is just beyond the acceptable limit (0.50 mg/kg) that is fit for human
consumption. This observation supports the fact that Pb does not usually bioaccumulate in most organisms except for filter-feeding organisms like mussels and
oysters as they often possess special metal binding proteins that remove metals from
general distribution in their organs (WHO 1989, 1995).
Mercury was not detected in oyster tissue samples within the machine’s LOD
(0.04 mg/kg). This basically implies that oysters in Lingayen Gulf are relatively
safe from hazards due to Hg. However, caution should be employed in location
specific areas (stations 2 and 12) with notably high concentration of Hg in the
sediment because this could possibly be absorbed by oysters and retained in their
system. As discussed by Shulkin et al. (2003), oysters have the capacity to retain low
concentration of heavy metals making them suitable indicators of low to moderate
contamination.
Heavy Metals in Sediments
Sediment samples were analyzed and found to be contaminated with Pb
(nd-33.90 mg/kg), As (nd-6.50 mg/kg) and Hg (nd-0.30 mg/kg). Although Cd
was not detected. The magnitude of distribution was described as Pb > As > Hg and
was found to be significantly higher in the monsoon season than in the pre-monsoon
season (p < 0.05) (Table 12.2 and Fig. 12.3).
145
period (Ferreira et al. 2005; Ahmad et al. 2010). It may be that some Cd metals
present in the environment were dispersed due to intensive rains and strong water
currents. Being a sedentary organism, oysters do not have the ability to move
away and search for food. The strong water currents during monsoon season thus
became a major contributory factor that hinders the availability and accumulation of
any particulate matter to oysters. However, other research findings are completely
opposite to the findings to this study, where Cd uptake increased significantly with
decreasing salinity (Mackay et al. 1975; Ke and Wang 2001).
Although relatively low (upper limit: 0.007 mg/kg), mean As concentrations
(0.01 mg/kg) were detected in oyster tissues. Results of this study were several hundred times lower than the average value obtained for the total arsenic concentration
(15.17 mg/kg) in oysters in Taiwan during rainy seasons (Hsiung and Huang 2006).
At present, the Philippines does not have standard limit set for As concentration
in oyster tissues. However, when comparing this with the established standards
of Australia and New Zealand (AUSAID 2007) which is 1.0 mg/kg for oysters,
the values detected in the Philippines are comparatively lower. The concentration
differences of As collected during the pre-monsoon and monsoon season were not
significant (p > 0.05). The capacity of oysters to accumulate As in their system and
the availability and abundance of As in oyster habitat can be among the factors that
contributed to the levels detected.
Only one station (station 4) during the pre-monsoon season yielded a Pb value
(0.55 mg/kg) for oyster tissue within our methodology’s Limit of Detection (LOD)
of 0.50 mg/kg. Incidentally, under the Philippine standards (BFAR 2001c), this
value is just beyond the acceptable limit (0.50 mg/kg) that is fit for human
consumption. This observation supports the fact that Pb does not usually bioaccumulate in most organisms except for filter-feeding organisms like mussels and
oysters as they often possess special metal binding proteins that remove metals from
general distribution in their organs (WHO 1989, 1995).
Mercury was not detected in oyster tissue samples within the machine’s LOD
(0.04 mg/kg). This basically implies that oysters in Lingayen Gulf are relatively
safe from hazards due to Hg. However, caution should be employed in location
specific areas (stations 2 and 12) with notably high concentration of Hg in the
sediment because this could possibly be absorbed by oysters and retained in their
system. As discussed by Shulkin et al. (2003), oysters have the capacity to retain low
concentration of heavy metals making them suitable indicators of low to moderate
contamination.
Heavy Metals in Sediments
Sediment samples were analyzed and found to be contaminated with Pb
(nd-33.90 mg/kg), As (nd-6.50 mg/kg) and Hg (nd-0.30 mg/kg). Although Cd
was not detected. The magnitude of distribution was described as Pb > As > Hg and
was found to be significantly higher in the monsoon season than in the pre-monsoon
season (p < 0.05) (Table 12.2 and Fig. 12.3).