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(III), 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(III) 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.