Porewater distribution and benthic flux 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 Me diterranean Sea. It is characterized by high mercury inputs from the Isonzo River whose tributary, the Idrijca River, drains the mercury mining area o f Idrija in Slovenia where extraction activity has taken place for nearly 5 00 years. This appears, therefore, to be one of the most suitable sites for studying processes that affect Hg cycling in the marine environment and fo r determining whether sediments might act as secondary sources of mercury s pecies in the water column. Porewater seasonal distributions of total disso lved Hg (Hg-T) and methylmercury (MeHg) were investigated. Using in situ be nthic chambers it was possible to determine benthic fluxes of Hg, and MeHg at the water-sediment interface throughout the year. Benthic fluxes were al so compared with diffusive fluxes calculated from porewater profiles. The r esults indicate that, following hypoxic conditions which occurred in late s ummer in the sea-bottom layer, highest benthic effluxes and porewater conce ntrations of Hg and MeHg appeared during autumn and winter. This was probab ly due to the transition from rapid sulphate reduction in late summer to co oler 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 Hg-T is buried in the sediment whereas 25% of Hg-T, approximate ly 23% in methylated form, is annually recycled and released at the water-s ediment interface. (C) 1999 Academic Press.