An enclosure of 30 m diameter was covered by a floating foil, and whey was repeatedly added to the water column to stimulate microbial respiration. A suspension of whey was successfully mixed into the enclosure by means of a boat motor. Whey was completely dissolved and subsequently consumed by microbial respiration in the water column. This resulted in oxygen consumption leading to anoxic conditions. About 10 mmol m⁻² d⁻¹ oxygen permanently entered the enclosure from the atmosphere, while a minor amount of oxygen was produced by primary production. By careful monitoring and repeated additions, it was possible to keep the bottom of the enclosure permanently anoxic, even during mixing periods in autumn and spring. Fe³⁺, however, was not reduced significantly. A laboratory experiment revealed that microbial iron reduction was inhibited by both low concentrations of organic substrates and low temperature. Since Fe^III is a potential oxidizing agent, it is questionable, if the stability of metal-sulfides in acidic mine pit lakes can be increased by the addition of complex organic substrates.