Nereid polychaetes as bioaccumulators of heavy metals from sediments in the lagoon of Venice

The use of benthic organisms as indicators of pollutant bioavailability in sediments is object of a growing concern. Many species are able to survive in metal-contaminated sediments, since they possess detoxification mechanisms involving metallothioneins or other binding proteins. In this context, polychaetes belonging to the Nereididae family are of considerable interest for their well-known ecological significance in marine and estuarine environments. The role of deposit-feeders played by this taxon in trophic webs makes it a keystone of contaminants transfer. In the framework of the Corila’s “metabolism” project, this study was aimed at investigating the potential role played by the nereid polychaetes Hediste diversicolor O.F. Müller as bioaccumulator of metals from sediments in the lagoon of Venice. The choice of this species was motivated by its characteristics of euryecity and semelparity, by its long life cycle and by its ecological importance. For deposit-feeders like nereid polychaetes, metal uptake may occur both via body wall, directly from seawater, and via gut, from interstitial water and from metal-enriched pabulum. Polychaetes and sediment samples were collected in five intertidal and one subtidal sites along a 35 km-long transect in the salt marsh fringe of the lagoon. The sites were characterized by different levels and typology of metal contamination and were selected on the basis of the distribution and population dynamics of the species. Metal concentrations were measured in both animal and sediment samples; moreover, metal partitioning among the various geochemical phases that compose sediments was also determined, in order to achieve additional information on metal bioavailability. A correct treatment of the biological specimens, before carrying out analytical determinations, was recognized to have a fundamental importance, especially in the purging phase; in fact, the retention of sediment particles in the digestive tract of nereids could lead to an overestimation of the metal levels in the tissues. The results showed that the relationships between total metal concentrations in the sediments and those in the worms are rather complex. Sediment composition in the lagoon of Venice is highly variable and thus an evaluation of metal contamination in terms of bioavailability may result extremely difficult. In this context, the relative abundances of the various metal-binding components of sediment may have a strong influence on metal bioavailability and should be taken in to careful consideration.