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SOURCES OF ENVIRONMENTAL MERCURY As a direct result of human activities, mercury levels in river sediments have increased fourfold since precultural times, and twofold to fivefold in sediment cores from lakes and estuaries (Das et al. 1982). During the past 100 years, it has been estimated that more than 500,000 metric tons of Hg entered the atmosphere, hydrosphere, and surface soils, with eventual deposition in subsurface soils and sediments (Das et al. 1982). Several activities that contribute significantly to the global input of Hg include the combustion of fossil fuels; mining and reprocessing of gold, copper, and lead; operation of chloralkali plants; and disposal of batteries and fluorescent lamps (NAS 1978; Das et al. 1982). The atmosphere plays an important role in the mobilization of Hg; 25% to 30% of the total atmospheric Hg burden is of anthropogenic origin (NAS 1978). In the United States, mercury consumption rose from 1,305 metric tons in 1959 to 2,359 tons in 1969 (Table 1). The major use of mercury has been as a cathode in the electrolytic preparation of chlorine and caustic (Nriagu 1979). In 1968 this use accounted for about 33% of the total U.S. demand for Hg (EPA 1980). Of recent U.S. mercury consumption, electrical apparatus have accounted for about 27%; industrial and control instruments, such as switches, thermometers, and barometers, and general laboratory appliances, 14%; antifouling and mildew-proofing paints, 12%; Hg formulations to control fungal diseases of seeds, bulbs, and vegetables, 5%; and dental amalgams, pulp and paper manufacturers, pharmaceuticals, metallurgy and mining, and catalysts, 9% (EPA 1980). Mercury, however, is no longer registered for use in antifouling paints, or for the control of fungal diseases of bulbs (EPA 1980). Mercury from natural sources enters the biosphere directly as a gas, in lava (from terrestrial and oceanic volcanic activity), in solution, or in particulate form; cinnabar (HgS), for example, is a common mineral in hot spring deposits and a major natural source of mercury (Das et al. 1982). The global cycle of Hg involves degassing of the element from the Earth's crust and evaporation from natural bodies of water, atmospheric transport (mainly in the form of Hg vapor), and deposition of Hg back onto land and water. Oceanic effluxes of Hg are tied to equatorial upwelling and phytoplankton activity and may significantly affect the global cycling of this metal. If volatilization of Hg is proportional to primary production in the world's oceans, oceanic phytoplankton activity represents about 36% of the yearly Hg 4