Meridional distribution of arsenic species in the subantarctic zone of the Southern Ocean, south of Australia

Distribution of the arsenic species total inorganic arsenic [As(V+III)], arsenite [As(III)], monomethyl arsenic (MMA), and dimethyl arsenic (DMA) was studied in the Subantarctic Zone (SAZ) of the Southern Ocean, south of Australia, during the austral autumn (March 1998). As(V) was the dominant arsenic species in both vertical profiles and surface waters along the meridional transect 42°-55° S, 141°30′E. It was also the only species observed at depths >600 m. Concentrations of the reduced arsenic species (As(III), MMA, and DMA) were low in these waters compared with other oceanic sites with similar concentrations of chlorophyll a. As(III) concentrations could not be reliably quantified at any sites (<0.04 nM). The greatest conversion of As(V) to "biological" species was found at the surface in the Subtropical Convergence Zone (2.5%) and decreased heading southward to 1% in the Polar Front (PF). While the decline in methyl arsenic concentrations was broadly associated with water temperature and measures of biological production, slightly different trends were found in the SAZ and PF. North of the Subantarctic Front (SAF), methyl arsenic concentrations were well correlated with water temperature, while south of the front, no such relation existed. In addition, the ratio DMA/MMA increased south of the SAF, associated with a change in the microalgal community composition. Low water temperature, phosphate-replete conditions, and low biological productivity in the Southern Ocean all contribute to the concentrations of biologically produced arsenic species in this region being among the lowest reported for oceanic waters.