Comparison between the mercury contamination in populations of Unio pictorum mancus (Mollusca, Bivalvia) from two lakes of different trophic state: the oligo-mesotrophic Lake Maggiore and the eutrophic Lake Candia

This research was designed to evaluate the influence of mussel (Unio pictorum mancus) size on mercury concentration in mussel soft tissues. It also evaluated the relationships between calcium, mercury and selenium in the mussel tissues. The mussel populations from the deep, meso-oligotrophic Lake Maggiore and the shallow, eutrophic Lake Candia were analyzed. The mean concentration of mercury in the tissues was 0.075 mg kg-1 in Lake Maggiore mussels and 0.033 mg kg-1 in Lake Candia mussels. The local mercury content in the mussel biomass per surface unit was about eleven-fold higher in Lake Maggiore (5.99 μg Hg m-2) than in Lake Candia (0.55 μg Hg m-2). This wide difference was due firstly to the larger population density (46.86 vs 6.93 ind m-2) and secondly to the higher individual content (0.128 vs 0.079 μg Hg ind-1) in Lake Maggiore than in Lake Candia. These differences in population density and individual mercury content are the result of both biological and environmental characteristics. Lake trophy and water renewal affect population density and individual growth rate, and ultimately determine their impact on biogeochemical cycles by regulating the mussel standing stock. The relationships between mercury, calcium and selenium were taken into account.