Flooded lignite pits (Tagebaurestseen) in Lusatia, Germany, are acidic (pH 2.5-4) with high concentrations of iron. Mesocosms (total volume 20 l) were set up with water and sediment from a Tagebaurestsee (Koschen, pH 3.1, Fe 20 mg l - 1 , acidity (KB 8 . 2 ) 2.45 mM) to assess the effects of phosphate and organic amendments under natural light and low temperature. Chemical and biological parameters were observed over a 9-month period. Phosphate rock addition (300 mg l - 1 ) resulted in sustained reduction in acidity (H + and Fe 3 + ) in the water column and induced the growth of Chlamydomonas spp. (Chlorophyceae) near the water surface and Lepocinclis teres (Euglenophyceae) in a band above the sediment. Addition of potatoes to mesocosms resulted in the generation of near-anoxic conditions above the sediment, and phosphorus, ammonium and carbon (organic and inorganic) were released as the potatoes decomposed. A pH>6 was attained with 5.1 g (dry weight) of potatoes and pH>8 with 34 g (dry weight). In both mesocosms, more than 90% of total acidity was removed. Algal growth, mainly of small, coccoid Chlorophyceae, Ulothrix sp. (Chlorophyceae) and Eunotia exigua (Bacillariophyceae), was observed throughout the water column.