An empirical approach was taken to examine and test a variety of hypotheses that had been presented or postulated in the literature concerning mercury biogeochemistry in lakes. These hypotheses included: (1) mercury contamination in fish increases with increasing sulphate deposition; (2) mercury exposure in persons eating fish increases with increasing sulphate deposition; (3) mercury contamination in fish is unrelated to natural (non-anthropogenic) Hg sources. These hypotheses were all shown to be false for Ontario lakes, predominantly of the Canadian Shield. Also examined was the spatial association of lake water chemistry with sulphate deposition across Ontario. It was determined that morphometric characteristics of the lakes, as well as watershed buffering capacity, together explained more of the spatial variation in lake water chemistry than did patterns of sulphate deposition across the province. Once these confounding influences were removed, the water chemistry variables most closely reflecting patterns of sulphate deposition were dissolved organic carbon (DOC) and sulphate ion concentration ( (SO$\sb4\sp{2-}$)), not pH or alkalinity. Aluminum ion concentration ( (Al$\sp-$)), which is postulated to increase in lakes as a result of acidic deposition, had no discernible relationship to sulphate deposition. Based on the results of empirical analyses, including path analysis (causal modelling), a complex hypothesis explaining the interrelationships of precipitation, water chemistry, geochemistry, and fish mercury contamination was postulated. Of particular note concerning this model were the following: (1) fish mercury and human mercury exposure decrease as sulphate deposition increases; (2) DOC, not pH or alkalinity, is the water chemistry variable most significantly related to sulphate deposition; (3) DOC, not pH, is the water chemistry variable most significantly associated with fish mercury contamination; (4) causal modelling provided empirical evidence supporting the simultaneous influence of 3 different mechanisms postulated in the literature to control or influence mercury accumulation in freshwater fish.

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