In fresh waters (e.g., lake water), 94–99% of inorganic Hg and 72–97% of organic Hg are complexed by dissolved humic matter [2], whereas, in seawater, the proportion of Hg bound to humic matter is very low due to high chloride-ion concentrations that stabilize Hg species in solution by ionic interactions. Furthermore, the proportion of organic Hg in marine waters is typically less than 5% of the total Hg concentration [3] (although Mediterranean waters show an ‘‘Hg anomaly’’ with an organic Hg content of up to 30% of total Hg [4]), whereas, in freshwater systems, the organic Hg fraction is typically 30% of the total Hg, which is in the range 1–5 ng Hg/L [5].