Use of whole mussels and mussel gills in metal pollution biomonitoring

The use of marine bivalves in metal pollution monitoring is undoubtedly one of the best tools to evaluate metal pollution in coastal areas worldwide. However, since bivalves integrate metal pollution from both the dissolved and the particulate phases, it cannot be assured that metal bioaccumulation in their tissues will predict the risk posed by metals to sensitive organisms such as invertebrate larvae or microalgae, which are known to be mainly affected by the free metal ion fraction. This study aims to test the usefulness of mussel gills–the primary exchange surfaces with dissolved metals–in reflecting dissolved metal bioavailability in the field. For that, we analyzed metal concentrations in environmental samples (whole soft tissues of mussels, mussel gills, sediment, and particulate and dissolved fractions in the water column) collected from 7 sampling sites during a multi-year investigative monitoring survey in the Ría de Vigo. The results showed a different pattern of Cu and Zn accumulation in gills compared to whole soft tissues, and these differences are attributable to the faster response of gills to changes in dissolved metal concentrations and to the regulation of essential metals at the whole body level. In the case of Pb, by contrast, concentrations in gills were very similar to those in the whole body. The use of mussel gills in metal pollution biomonitoring is a promising tool for the detection of changes in bioavailable metals in the environment, especially for essential metals such as Cu and Zn.