Mercury and methylmercury contamination in surficial sediments and clams of a coastal lagoon (Pialassa Baiona, Ravenna, Italy)

The Ramsar site Pialassa Baiona is an intertidal brackish lagoon lying 5 km north of the city of Ravenna and covering a surface area of about 1100 ha. From 1958 to 1976 Pialassa Baiona was heavily impacted by industrial pollution; mercury, polycyclic aromatic hydrocarbons, polymers were among the most important pollutants which nowadays contaminate the sedimentary compartment. Following earlier investigations on total mercury (THg) distribution and bioavailability, this study is focused on mercury speciation in sediments and clams. Methylmercury (MMHg) concentrations were determined in surface sediments (0–5 cm) in different seasons, and compared to THg. Selected sediments were subjected to a sequential extraction procedure to study solid state THg distribution. Operationally, three fractions of inorganic mercury were defined: mobile mercury (Hgm), humic acid complexed mercury (Hgha), and sulphide-bound mercury (Hgs). THg and MMHg concentrations in sediments ranged in the 0.2–250 μg g−1 and 0.13–45 ng g−1 d.w. intervals, respectively. MMHg/THg ratios were higher in summer as well as in sites where the THg burden was lower; the highest MMHg/THg values were observed in Chiaro del Pontazzo, an area subjected to a drastic reduction of salinity. d MMHg concentrations were also determined in tissues of clams (Tapes philippinarum) collected in the lagoon. MMHg in clams felt in the 180–470 ng g−1 interval and accounts for 72–95% of tissue THg.