Mobilization of Manganese and Arsenic under Hypoxia in the Bottom of Lake Biwa

Recently, hypoxia has become serious in Lake Biwa due to progressing of global warming. In this study, we determined Mn and As concentrations and their speciation in lake bottom water and sediments in June and December to elucidate their geochemical behaviors. Lake bottom water and sediment samples were collected from seven stations in the northern basin of Lake Biwa. In sediments, clear enrichments of Mn and As in the surface were observed in all stations. The results of X-ray absorption near-edge structure (XANES) indicated that majority of Mn was Mn2+ in the enrichment layer (0– 2 cm), whereas arsenate was the predominant form of As. Levels of Mn and As in Lake Bottom water in northern basin of Lake Biwa were several to several hundred times higher than average values in the lake. Seasonal variations were limited for both Mn and total As in lake bottom water, whereas the levels of arsenite in winter is generally higher than those in summer. These results suggest that Mn in sediments is released to water column as Mn2+ throughout the year, whereas As level may increase in the future with decreasing oxygen level and subsequent desorption of arsenite from sediments.