RESUMO
The profile of 11 trace metals in two commonly used biomonitors (the native oyster Crassostrea palmula and mussel Mytella strigata) from Estero de Urias lagoon, Gulf of California, were studied for six months, covering both dry and wet seasons. Metal concentrations in these two bivalves were compared with concentrations accumulated by Artificial Mussels (AMs) deployed alongside during the same period. Significant temporal variations in Cd, Cr and Mn were observed in both bivalve species and AMs. Temporal changes were observed for Fe in both bivalve species, Pb in oyster only and Cu in both AMs and oysters, revealing seasonal changes in inputs and/or chemical forms of these metals in the lagoon. Significant correlations for Cd, Cr and Cu were found in mussels and oysters, but their Co, Fe, Mn and Zn profiles were very different, despite these two species being taxonomically closely related and often used as biomonitors for metals. Interestingly, Hg and U were detected in AMs but not in oysters and mussels. The difference in metal profile in oysters, mussels and AMs revealed in the present study clearly showed that different biomonitors and AM take up metals differentially from the same environment, and metal profile in a single biomonitor or AM alone therefore, cannot provide a good estimate on metal concentrations in the ambient environment. As such, different biomonitors and AM should be used in metal monitoring, in order to provide a comprehensive picture on metal levels in aquatic ecosystems. Concentrations of Ni and Pb in oysters, and Cr, Fe and Mn in mussels were among the highest reported in coastal waters worldwide. Concentrations of Pb in oysters exceeded legal limits set for bivalve mollusks in EU. Concentrations of Cr in mussels and oysters exceeded or were very close to, respectively, the legal limit for fish, crab-meat, oysters, prawns, and shrimps in Hong Kong. The results indicate a potential public health risk on human consumption of oysters and mussels commonly harvested from the Estero de Urias lagoon, and corresponding pollution control measures are deemed necessary.