Trace element concentrations in freshwater mussels and macrophytes as related to those in their environment

This study was primarily designed to contribute to the debate "Do accumulator species reflect the element contamination level of their environment?" This research was carried out: 1) to know the distribution of 15 trace elements and calcium in shell and soft tissues of three species of freshwater mussels and macrophytes; 2) to compare the accumulation capacity of each trace element by mussels and by eight species of macrophytes and 3) to test the relationships between the metal concentrations in the mussels and macrophytes and those in water and sediments. The variability of element residues in the mussels is the major limit to accumulator monitoring. The most important causes are: seasonal cycle, physical environment and biological factors such as the size, age and growth rate. This research was designed to eliminate the consequence of variability deriving from the season and the environment. To this end the mussels and macrophytes were collected at the same time from the same habitat: Ranco Bay, Lago Maggiore, Northern Italy. In addition, the element concentrations in more size-classes of the most abundant mussel species (Unio pictorum and Dreissena polymorpha) were measured. Trace elements were analyzed by Inductive Coupled Plasma-Mass Spectrometry (ICP-MS). By arranging the data in sequences of decreasing element concentrations in the organisms as well as in water and sediments, we were able to compare the accumulating ability of the tested species and evaluate their capacity to reflect environmental availability. Neither the sequences in the shell nor those in the tissues were similar to the sequence in the water. The differences between the sequences of the mussel tissues and those of the sediments were less striking than those between shells and sediments. Similar results were obtained by macrophytes. In conclusion, the results of this study (which mimics the monitoring practice) prove that bioaccumulators cannot be used to evaluate the pollutant levels of the environment at the time of collection, since no relationship between metal concentrations in the species and those in the water was found and the relationship with the sediments was very weak. Bioaccumulators can be regarded as a useful tool in long-term studies to follow pollutant variations in the same environment or when substantial differences in pollutant concentrations in different environments were found. This monitoring method yields reliable results to detect new pollutants contaminating the environment, such as artificial radioisotopes, or to follow year to year variations by analyzing pollutant concentrations in the shell layers.