The biogeochemical cycling of mercury. Deposition of Hg to the ocean is primarily as Hg(II). A substantial amount (roughly 1/3) of the deposited Hg is returned to the atmosphere following reduction of Hg(II) to Hg(0). Bioaccumulation of Hg occurs mostly through the retention of monomethylHg (MMHg) in biota. Most of the Hg entering the ocean is not methylHg, however, and complex biological and abiological processes result in the formation of MMHg and dimethylHg (DMHg).