The distribution of arsenic compounds in the ocean: Biological activity in the surface zone and removal processes in the deep zone

The vertical profies of inorganic arsenic [As(III)+As(V)], monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) were investigated at four sampling stations in the Pacific Ocean and a sampling station in the southern Tasman Sea. In addition, the concentrations of those compounds in surface waters of the Pacific Ocean and Tasman Sea have been determined. The vertical profiles of inorganic arsenic showed the low concentrations in both the surface and deep/bottom zones. The depleted concentrations in the surface zone varied from 1000 to 1700 ng dm^-3 and that in the deep/bottom zone varied from 1300 to 2050 ng dm^-3. The maximum concentrations that varied from 1500 to 2450 ng dm^-3 were usually observed at a depth of about 2000 m. Both MMAA and DMAA were observed throughout the water column at sampling stations in the north-western and equatorial regions of the Pacific Ocean. At the sampling station in the central northern Pacific gyre, DMAA was the only methylated arsenic compound observed throughout the water column. On the contrary, at the sampling station in the southern Tasman Sea, the only detected methylated arsenic compound throughout the water column was MMAA. Their vertical profiles showed maximum concentrations in the surface water which abruptly dropped with depth from 0 to 200 m. The concentration in the surface water was close to 10ng dm^-3 for MMAA and varied from 27 to 185 ng dm^-3 for DMAA. At depths greater than 100 m, MMAA and DMAA were at comparable concentrations which varied from 0.7 to 14 ng dm^-3. The low inorganic arsenic concentration in the surface zone was due to biological activity. This activity resulted in the uptake of As(V) and subsequent reduction and methylation to MMAA and DMAA. DMAA was the main predominant arsenic compound resulting from biological activity in surface waters. The low inorganic arsenic concentrations in the deep and bottom zones were likely to be caused by the adsorption of dissolved inorganic arsenic onto sinking particulates rich in iron and manganese oxides.