Hazardous substances in blue mussels in the north-east Atlantic.

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The take-up by marine organisms of hazardous substances is estimated by monitoring their concentrations in the blue mussel. Mean concentrations of each substance are compared to reference values to produce an Ecological Reference Indicator (ERI), with an ERI under 1 indicating that the pollution levels are relatively safe for the species.

The studies present a mixed picture:

• lindane levels dropped significantly, with the ERI falling steeply from 4.5 in 1990 to under 1 in 1993, where it has stayed since;
• cadmium and zinc levels dropped much more slowly (although from a lower 1990 level than lindane). The cadmium ERI fell from just above 3 to around 2, while the zinc ERI fell from just above to just below 1;
• lead concentrations remained stable, with an ERI hovering around 2, although there was a small decrease in the last two years;
• mercury levels did drop slightly (from almost 4 to under 3) across the 1990-1996 period. However this disguises a sharp rise in the first few years.

It is as yet unclear why the reductions in emissions of all of these substances have not resulted in clearer falls in ERI values for lead and mercury. Part of the problem may in fact be the inherent nature of the monitoring system, as the number, sex and age of mussels collected from one year to the next cannot always be made consistent, and as there are uncertainties on the representativeness of the samples, on differences in bio-availability of the substances, on seasonable variability of the concentrations in mussels