The Gulf of Trieste and adjacent Grado and Marano Lagoon (northern
Adriatic Sea) are considered as an important “natural bioreactor” that
can increase sedimentary methylmercury (MeHg) production by mobilizing
the extensive mercury (Hg) pool as a result of historical (500
years) Hg mining and smelting activities in Idrija (NW Slovenia), the
world’s second largest Hg mine. Even though the mine was definitely
closed in 1995 the Soča/Isonzo River system still delivers about 1.5
t of Hg to the gulf annually and the MeHg is elevated in this marine
environment. Research conducted by late M.E. Hines showed that
Hg is readily methylated and demethylated in marine, lagoonary and
freshwater sediments but the relative activities varied greatly with locality.
Methylation activity increased from freshwater to the marine
sites, conversely the highest demethylation was found in estuarine and
lagoon sites. The methylation/demethylation ratios were consequently
low in coastal sites but increased ib the gulf. Subsequent investigations
in the southern, less contaminated (“pristine”) sediments of the gulf
revealed that MeHg is also continuously produced especially in autumn
in parallel with higher bottom water temperature and lower oxygen
content. Sulphate reduction was found to control both processes. Hg
methylation in sediments of coastal lagoon was controlled by rapid
demethylation and the Hg bioavailability was affected by Hg adsorption
and precipitation, In offshore marine sites, the sulphide produced
by sulphate reduction lowers Hg methylation. Methylation of dissolved
Hg, nearly 100% colloidally bonded, in the seawater column of the
gulf was not detected (showing a pronounced Hg reduction potential)
suggesting that sediments are the principal methylation site and the
source of MeHg to the water column. It is likely that demethylation
and reduction prevent the excessive accumulation of MeHg in the food
webs of the Gulf of Trieste.
