Assessing the history of trace metal (Cu, Zn,Pb) contamination in the North Sea through laser ablation ICP-MS of horse mussel Modiolus modiolus shells

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine the elemental composition of the incremental growth record in the shells of longevous, (similar to 35 yr old) horse mussels Modiolus modiolus. Concentrations of the contaminant metals Cu, Pb and Zn were determined from shells collected in 1984 from 2 sites in the southern North Sea, an 'impacted dump' site and a 'control' site distant from known point source inputs. The age of each shell was determined from the pattern of annual growth lines present in thin shell sections, and each growth Line was assigned a date, thus allowing annual concentrations of these elements to be determined. Using LA-ICP-MS, concentrations of the metals were determined in 5 replicate laser ablations (size similar to 100 x 200 mum), from individual summer and winter growth lines along the length of each of 3 shells from the dump site and 3 shells from the control site. A significant effect of age (year) on Cu, Pb and Zn concentrations in the shells from both sites was apparent, indicating changing concentrations with time. Significant seasonal (summer/winter) effects were only obvious in the concentrations of Pb and Zn in the shells; a non-significant difference in seasonal incorporation of Cu was observed. Concentrations of all 3 metals were significantly elevated in the dump site shells compared with the control site shells. Pearson correlation coefficients, determined for pairs of metals in the shells from both sites, indicated a higher number of significant positive correlations in the concentrations amongst the metals in the dump site shells than in the control shells during certain periods of the mussels' life. Levels of Cu, Zn and Pb were significantly higher in the dump site shells between 1968 and 1974 than in the period from 1975 to 1979. In the control site shells, although Pb and Zn significantly declined during the same periods, the concentrations were substantially and significantly lower than in the dump site shells. The implementation of the Dumping at Sea Act (in 1974) and the subsequent decline in dumping in the North Sea may have resulted in a decrease in the concentration of metals in the shells from the impacted dump site.