Bioavailable heavy metals in estuarine waters as assessed by metal/shell-weight indices in sentinel mussels Mytilus galloprovincialis

Seasonal variations in metal concentrations in soft tissues of marine mussels can be due, to a great extent, to seasonal changes in flesh weight. Consequently, this event results in an unpredictable overestimation of metal bioavailability when the metal concentration in soft tissues is considered as an index of such metal bioavailability. The Abra estuary (Bizkaia, Bay of Biscay, Spain) was used as a natural experimental basin with well-known differences in the levels of metallic pollutants. The level of metal bioavailability was determined at various sites at different times of the year. For this purpose the conventional approach of recording metal concentrations in soft tissues and the approach based on the calculation of metal/shell-weight indices, first proposed by Fischer (1983), were applied. We concluded that seasonal changes in organism condition (Flesh Condition Index) cause unpredictable oscillations in the metal concentration in soft tissues. For this reason the direct use of metal concentration values is not recommended to assess metal bioavailability. The metal/shell-weight indices, however, were not affected by seasonal changes in the flesh weight of sentinel mussels and thus may provide a more realistic indication of metal bioavailability. In the present study, different metal bioavailabilities, assessed by metal/shell-weight indices, were found at different sampling sites, according to the influence of industrial activities. Seasonal trends in metal/shell-weight indices (attributed to different metal inputs at different times) were also recorded. In conclusion, metal/shell-weight indices, characterized by a high sensitivity and a low noise-to-signal ratio, are a simple and reliable tool for assessing metal bioavailability in 'Mussel Watch' monitoring programmes.