Critical Hg atmospheric loads to sensitive Lake Mergozzo (Italy)

Lake Mergozzo is a small (1.83 km-2) basin with an average depth of 45 m characterized by low total P content and alkalinity; i.e., potentially sensitive to acidification and Hg pollution. The drainage basin of Lake Mergozzo has a limited extension (about 10 km-2) which, coupled with a very limited discharge of waters into the adjacent Lake Maggiore, makes this system particularly sensitive to Hg atmospheric depositions. Analysis of 30 specimen of perch (Perca fluviatilis) resulted in 40% of them having Hg levels above 0.5 mg kg-1, corresponding to the legal limit for fish consumption in Italy. The US EPA limit of 0.3 mg kg-1 limit for protection of human health and aquatic ecosystems was exceeded in 70% of the fishes while all specimen had Hg levels above the UE environmental quality standard for biota of 0.02 mg kg-1. Mercury concentrations in wet atmospheric depositions measured over one year ranged from 0.5 to 52.3 ng L-1, with a weighed average of 12.8 ng L-1 and an estimated annual load of 27.8 g Hg km-2 yr-1. Critical concentrations of Hg in atmospheric depositions were estimated using models developed by the United Nations Environmental Program and, depending on input parameters, were between 0.1 and 18.3 ng L-1. Measured and critical concentrations being in the same range (and considering Hg concentration in perch), reducing Hg pollution in Lake Mergozzo will require substantial abatements in Hg atmospheric loads to the system.