Nereid polychaetes as bioaccumulators of heavy metals from sediments in the Venetian Lagoon

Three species of nereid polychaetes, i.e. Hediste diversicolor O.F. Müller, Neanthes succinea Frey & Leuckart and Perinereis cultrifera Grube, have been studied as possible bioaccumulators of heavy metals (Cd, Cu, Pb and Zn) from sediments in the Venetian Lagoon. The choice of this family of polychaetes was motivated by its characteristics of euryecity and semelparity, by its long life cycle and by its ecological importance. Sampling was performed along a known gradient of sediment contamination and on the basis of available information on the distribution and on the population dynamics of the considered species. A correct treatment of the biological specimens was of fundamental importance, especially in the purging phase. In fact, the retention of sediment particles in the digestive tract leads to an overestimation of the levels of contaminants in the tissues; for this reason a specific purging method (6 days in sand and salt water followed by 24 hours in salt water) was employed. The tissue samples were digested in a microwave oven with HNO3 12.4M. Regarding sediments, it was decided to use a cold acid digestion with HCl 1M, because it furnished a sufficiently accurate estimate of the fraction of potentially bioavailable metals. The analyses of all samples were performed by atomic absorption. The results were used to calculate the Bioaccumulation Factor (BAF) as [Me]tissue/[Me]sediment. At the same time, the age of the polychaete population was estimated by mesuring the length of the jaws for each animal. H. diversicolor and N. succinea showed similar bioaccumulation models of Cu, Cd and Pb, with tissue concentrations reflecting the sediment gradient. In the case of Pb and Cd, maximum concentration levels were often reached in animals from areas characterised by very different degrees of sediment contamination. This was not verified for Cu, suggesting the possibility of an accumulation mechanism. The results obtained for Pb and Cd could be explained by a strategy of partial regulation of their intracellular concentrations. At this stage of the study further evidences are needed to confirm the real accumulation of Cu; this metal is reported in literature to be stored in granules inside the epidermal cells and in the nephridia as an accumulation mechanism. The results for P. cultrifera are the most interesting and worthy of further study. This species is able to store quantities of Cu and Cd larger than those found in sediments and much larger than those found in the tissues of H. diversicolor and N. succinea, suggesting a different accumulation strategy. The case of Zn deserves a separate discussion for all three species. In fact, it is clear that the physiological reaction of the animals to Zn concentration in the environment is very different, because of its role in the formation of the jaws of nereid polychaetes. Owing to the large environmental variability in the Venetian Lagoon, it is not surprising that some data differ from the general trends; an attempt was made to explain the different behavioural models found in certain zones by analysing a few factors known to influence the bioavailability of heavy metals (grain-size distribution, iron and organic matter content). The results confirmed those reported in the literature and underlined the importance of studying the factors influencing the bioavailability of heavy metals.