Redox elements as indicators of biological processes in the Southern Ocean

Production 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). Surface samples were collected approximately every degree of latitude along the meridional transect 14130' E from 42 to 55 S. In addition, representative vertical profiles were collected at: 4206' S, 14153' E (Subtropical Convergence Zone - STCZ), 4643' S, 14159' E (Subantarctic Zone - SAZ), 5104' S, 14336' E (Subantarctic Front - SAF) and 5344' S, 14142' E (northern branch of the Polar Frontal Zone - PFZ). As(V) was the dominant arsenic species in both vertical profiles and surface waters along the transect. It was also the only species observed at depths greater than 600 m. Production of the reduced arsenic species (As(III), MMA, DMA) was 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 (less than 0.04 nM). Greatest conversion of arsenic 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 production was broadly associated with water temperature and measures of biological production, slightly different trends in methyl arsenic production were found in the SAZ and PF. North of the Subantarctic Front (SAF) methyl arsenic production was well correlated with water temperature, while south of the front no such relation existed. In addition, the ratio of 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 SAZ all contribute to the concentrations of biologically produced arsenic species in this region being amongst the lowest reported for oceanic waters.

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