Behavior of arsenic in the coastal area of the Changjiang (Yangtze River) Estuary: Influences of water mass mixing, the spring bloom and hypoxia

Abstract

The biogeochemical cycle of arsenic in the aquatic environment has received scientific attention because of the complex forms and toxicity of this element. Previous studies have shown the frequent occurrence of hypoxia in the coastal area of the Changjiang (Yangtze River) Estuary. In an especially dry year (2006), three cruises were conducted (June, August and October) in this area to investigate the impact of hypoxia on total dissolved inorganic arsenic (TDIAs: [TDIAs]=[AsO43−]+[AsO33−]), dissolved organic arsenic (DOAs) and total arsenic in surface sediments. The distribution of TDIAs in the study area was similar among the three field studies, with high concentrations being found in the coastal area and in near bottom waters offshore in the East China Sea (ECS). TDIAs was scavenged in the surface layer in June. The distribution of DOAs was opposite that of TDIAs, especially in the region having high concentrations of chlorophyll-a (Chl-a). The DOAs/TDAs (total dissolved arsenic: [TDAs]=[DOAs]+[TDIAs]) ratio decreased from June to October. The concentrations of total arsenic in surface sediments decreased gradually from the Changjiang Estuary to the central ECS shelf. The behavior of TDIAs was non-conservative in the coastal area of the Changjiang Estuary. Biological mediation and hypoxia are the main factors influencing the internal cycling of arsenic within the study area. The DOAs/TDAs ratio was negatively correlated with apparent oxygen utilization (r=0.71, p<0.05, and n=33) in the hypoxic zone, which suggests that the decomposition of organic matter may have a significant effect on the internal transformations of various arsenic species in the coastal area of the Changjiang Estuary. Sediment-water incubation experiments conducted under various redox conditions showed that hypoxia resulted in the release of arsenic from the sediments into the water; this may contribute to the high TDIAs concentrations in bottom waters in August. With increased toxicity and residence time of inorganic arsenic species during hypoxia, the potential damage to the ecosystem and marine environment should receive more attention.