Physical and chemical characterization of dissolved arsenic in the South China Sea

Abstract

Arsenic in the marine environment is important because its biogeochemistry is closely linked to the phosphorus cycle. The chemical speciation approaches of dissolved arsenic can identify As(III), As(V), and methylated arsenic species, all of which collectively reveal the involvement of microbial processes in the arsenic cycle. The physical speciation methods, or molecular weight distribution approaches, of dissolved arsenic can disclose the reactivity of arsenic based on the size-reactivity continuum model. We applied a combined physical and chemical characterization method to the dissolved arsenic in seawater from the South China Sea (SCS) to understand arsenic reactivity in the marine biogeochemical cycle. The dissolved inorganic arsenic (DIAs) exhibited typical semi-conservative depth profiles, and the low-molecular-weight (LMW, <1 kDa)-DIAs followed the same trend. The high-molecular-weight (HMW, 1 kDa-0.45 μm)-DIAs was found throughout the water column, with an average concentration of 2.5 nM. As(III) was found in the euphotic zone and mostly remained in the LMW phase. As(V) was therefore the dominant inorganic arsenic species in HMW-DIAs of the SCS. Dissolved organic arsenic (DOAs) exhibited a decreasing trend with depth, with most DOAs restricted to a surface layer of 100 m. Almost half of the DOAs was in the HMW phase, and the HMW-DOAs constituted one-third of the HMW-phase total dissolved arsenic. The occurrence of DOAs in the euphotic zone suggested that arsenic cycling is largely affected by the activities of microbes and plankton organisms. Overall, the biogeochemical cycle of arsenic in the SCS highlights the interactions of physical, chemical and biological factors.