Arsenic in Freshwater Systems: Influence of Eutrophication on its

26 Arsenic (As) exists in a variety of chemical forms, and microbial metabolism results in 27 the occurrence of thermodynamically unstable arsenite (As) and methylarsenic compounds 28 in freshwaters (rivers and lakes). The inorganic forms (As and As) and the methylated 29 forms (methylarsonic acid; MMAA and dimethylarsinic acid; DMAA) are the main species 30 of As in freshwaters while the bulk of the total dissolved As is inorganic species. Although 31 the predominant forms of methylarsenic compounds are consistently DMAA followed by 32 MMAA, the DMAA and MMAA species have also been found in freshwaters. Several 33 observations have revealed that phytoplankton activities are responsible for seasonal 34 variations of methylarsenic compounds in freshwaters. Although it was unclear if the 35 occurrences of methylarsenic compounds were from the breakdown of larger molecules or 36 the end-products of phytoplankton biosynthesis, recent studies have revealed that less toxic 37 As-glutathione complexes are intermediates in the biosynthesis of organoarsenic compounds 38 by phytoplankton. Recent studies have also revealed that eutrophication plays an important 39 role in the production, distribution, and cycling of methylarsenic compounds in freshwaters. 40 In this review, recent reports on the influence of eutrophication on distribution, speciation, 41 and bioaccumulation in freshwaters are discussed. 42