VERTICAL AND ANNUAL DISTRIBUTION OF FERRIC AND FERROUS IRON IN ACIDICMINING LAKES

The investigated coal mining lakes (ML Ill, ML 117, and ML 107) in the Lusatian lignite mining district are extremely acidic. The concentrat ions of iron in these geogenic acidified lakes are orders of magnitude higher than in acidic bog lakes and softwater lakes acidified by atmo spheric deposition. For the most part ferric iron was the predominant species by a Fe(III) to Fe(II) ratio of more than 10. Density stratifi cation of the water column leads to vertical concentration gradients o f ferrous iron. Extremely high concentrations of ferrous iron were fou nd in the anoxic layers above the sediment of ML 111 and ML 107. High concentrations of Fe(II) were correlated with high concentrations of c arbon dioxide. A microbial potential to reduce ferric iron was found i n the sediments. In ML 111 and ML 117 the concentrations of Fe(II) in the epilimnion were markedly higher than in the oxic hypolimnetic laye rs. It can be suggested that the occurence of ferrous iron in the epil imnion is of photochemical origin, as described for softwater lakes. M inimum concentrations of dissolved organic carbon in the epilimnion co uld be correlated to the increase of ferrous iron concentrations. Espe cially in springtime and summer the concentrations of Fe(II) in the ep ilimnion were higher than in the oxic hypolimnetic layer below.