Arsenic Biogeochemistry Affected by Eutrophication in Lake Biwa, Japan

Abstract

The seasonal variations of arsenic species in lake water were studied in the mesotrophic northern and eutrophic southern basins of Lake Biwa in Japan. Within the euphotic zone, arsenite [As(III)] increased in spring and fall, and dimethylarsinic acid [DMAA(V)] became the dominant form in summer. Measurable concentrations of monomethylarsonic acid [MMAA(V)] and trivalent methylarsenic species [monomethylarsonous acid, MMAA(III), and dimethylarsinous acid, DMAA(III)] also appeared, although they were always minor fractions. The total arsenic concentration in the euphotic zones remained constant in the northern basin throughout the year. However, it was increased by 2−4 times in the southern basin in summer. The enhancement was caused by the increase of As(V), which was accompanied by the increase of iron, manganese, and phosphorus. The concentration of methylarsenicals per chlorophyll a was lower in the southern basin. These results indicate that the variations of arsenic species in lake water largely depend on biological processes, such as the metabolism of phytoplankton, decomposition of organic matter by bacteria, and microbial reduction of iron and manganese oxides in sediments. Moreover, they show that eutrophication affects the concentration and speciation of arsenic in the lake water.