Pore water distribution and benthic fluxes measurements of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea)

The Gulf of Trieste is one of the most mercury-contaminated areas in the Mediterranean Sea. It is characterized by high mercury inputs from the Isonzo River whose tributary, the Idrijca River, drains the mercury mining area of Idrija in Slovenia where extraction activity has taken place for nearly 500 years. This appears, therefore, to be one of the most suitable sites for studying processes that affect Hg cycling in the marine environment and for determining whether sediments might act as secondary sources of mercury species in the water column. Porewater seasonal distributions of total dissolved Hg (HgT) and methylmercury (MeHg) were investigated. Using in situ benthic chambers it was possible to determine benthic fluxes of HgT and MeHg at the water–sediment interface throughout the year. Benthic fluxes were also compared with diffusive fluxes calculated from porewater profiles. The results indicate that, following hypoxic conditions which occurred in late summer in the sea-bottom layer, highest benthic effluxes and porewater concentrations of Hg and MeHg appeared during autumn and winter. This was probably due to the transition from rapid sulphate reduction in late summer to cooler temperatures, higher oxygenation of the bottom water layer, and lower microbial activity which is well suited for Hg transformation, accumulation and flux. A tentative budget based on benthic flux measurements indicates that 75% of HgT is buried in the sediment whereas 25% of HgT, approximately 23% in methylated form, is annually recycled and released at the water–sediment interface.