Arsenic cycling in eutrophic Lake Greifen, Switzerland: Influence of seasonal redox processes

The arsenic cycle was investigated over 1.5 yr in a eutrophic Swiss lake with a seasonally anoxic hypolimnion. Arsenate, As(V), was predominant throughout the entire water column when the lake was well-mixed in winter. During summer stratification, arsenate disappeared in the epilimnion and was replaced by arsenite, As@), but in the anoxic hypolimnion As(V) remained the dominant species. Reduction of As(V) in the hypolimnion occurred only late in the stagnation phase, in the presence of sulfide. The dominant redox reactions of inorganic As in the epilimnion were biologically mediated reduction by phytoplankton in summer and oxidation of As(W) by Mn oxides in fall. Approximately 50% of total dissolved As in the epilimnion and hypolimnion was refractory to the direct hydride generation method. The composition of particles, collected regularly in sediment traps, indicated that Fe oxides were the main scavengers for inorganic As. Nevertheless sedimentation was only a minor sink of inorganic As. The occurrence, chemistry, and toxicology of arsenic in natural waters have been studied for many years (Andreae 197 8,1979; Peterson and Carpenter 1983; Anderson and Bruland 199 1). Dissolved As has been found in inorganic forms such as arsenate, As(V), or arsenite, As(III), or in organic forms such as dimethylarsinic acid, DMAA, or monomethylarsenic acid, MMAA. Numerous other dissolved organic As species occur in natural waters and new compounds are still being discovered (Bettencourt and Andreae 199 1). From thermodynamic considerations, As(V) is expected to be the dominant species in oxic Acknowledgments We are indebted to S. Rapsomanikis for discussions and the use of his laboratory and equipment to determine MMAA and DMAA in water samples of Lake Greifen. H. Ambiihl is acknowledged for providing biological data. We thank D. Kistler for his role in the fieldwork and C. M&ler and U. Miiller for analytical work. This study was supported by Schweizerischer Nationalfonds, project 20-5607.88. Walters, and As(II1) should predominate under reducing conditions in the presence of Fe(II) or S(-II). Investigations of As speciation in oceanic water have demonstrated the dominance of As(V), but methylated and reduced species were found in significant concentrations in photic zones where they correlated well with photosynthetic activities (Andreae 197 8, 1979; Andreaea and Froelich 1984). Similar distributions of As species have been found in freshwater systems (Andreae 1978; Anderson and Bruland 199 1). Biogeochemical processes controlling the inorganic speciation of As have been studied by Peterson and Carpenter (198 3) in an intermittently anoxic fjord and by Seyler antd Martin (1989) in a permanently stratified lake. Increasing As(II1) concentrations were found below the 02-H2S boundary in these cases. This study is part of a larger project examining how redox conditions influence trace elements (Fe, Mn, As, Cr, Zn) in a eutrophic lake (Sigg et al. 199 1). Here, we focus on speciation of dissolved inorganic As in relationship to biological and chemical factors controlling redox chemistry over the annual cycle of the lake. We examine also how Mn and Fe oxides influence the cycling of As in the water collumn. Cycles of trace elements were studied in highly eutrophic, monomictic Lake Greifen, w’hich lies on the Swiss plateau. The surface area of the lake is 8.5 km2 and the volume 150 x lo6 m3. Its average depth is 17.7 m (max depth, 32.2 m) and water residence time is 1.1 yr. The dissolved phosphate concentration in the mixed water column during overturn was 3.5 pm01 literl. Chlorophyll a was lo-40 mg n--3 during algae blooms in 1990 and 199 1 (IL Ambiihl pers. comm.). To focus on the seasonal variations, we took water samples from the deepest part of the lake