Metal bioavailability assessment in 'mussel-watch' programmes by automated image analysis of autometallographical black silver deposits

The extent of autometallographical black silver deposits (BSD) was quantified by automated image analysis in digestive lysosomes of the digestive gland of mussels collected from 2 estuaries of the coast of Biscay for 1 yr. Additionally, metal composition of digestive lysosomes was characterised by X-ray microprobe analysis. Zn levels were the most variable among the metals analysed by AAS (Cd, Cr, Cu, Ni, Zn, Pb, Fe). A logarithmic regression model explained the changes in volunle density (VD) of BSD by changes in the Znkhell-wt index recorded in mussels from different sites of the coast. Similar regression models were obtained between the VD of BSD and metal/shell-wt ~ndices resulting from pooling the 7 metals (pmol metal g-' dry shell weight). In conclusion, metal bioavailability can be estimated by analysing the VD of BSD in the digestive lysosomes of sent~nel mussels. This index may allow workers to discard chemical analysis of biological samples when low traces of metals are present in the tissues, since the method proposed herein provides a quick and costeffective alternative to routine chemical analyses in biomonitoring programmes. Only when values of VD of BSD reach the plateau of the function would a more accurate chemical analysis be required. This approach does not require special facilities or specialised technicians and would reduce the time and cost of routine metal determination in water pollution monitoring programmes.