The Acidic Lignite Pit-lakes of Germany-microcosm Experiments on Acidity Removal through Controlled Eutrophication

Pit lakes in the Lausitz lignite mining district of Germany are diverse in size and morphometry. Many are extremely acidic with pH ≤ 3 and high in iron concentrations. Productivity in most of the mining lakes is generally low and they support only simple food webs; a few of the acidic lakes, with high nutrient concentrations, are highly productive. State planners hope to develop the lake region for recreation. To that end, various methods are being investigated to remove acidity from the lake waters. The microcosm experiment described here employs controlled eutrophication to enhance element cycling and sedimentbound alkalinity generating processes. Sixty litre microcosms with water and sediment from Lake Grünewalde (pH 3.0, Fe 14 mg L, acidity (KB8.3) 2 mmol L and phosphorus 4 μg L) were set up under laboratory conditions. Addition of nutrients (organic carbon and phosphorus) led to dramatic increases in primary production associated with blooms of green algae (Chlamydomonas sp.) and diatoms (Eunotia exigua) but no substantial removal of acidity. Generation of temporary anaerobic conditions through addition of potatoes led to removal of 85 % of acidity, all detectable dissolved iron, and an increase in pH from 3 to 7. These conditions were maintained with this treatment for the remainder of the 8 month observation period following removal of iron and protons from the water column. Additional