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To assess the risk of aluminium (Al) toxicity during the restoration of the eutrophic lake Tiefwarensee by hypolimnetic addition of NaAl(OH)(4)-solution (aluminate) the generally limnological monitoring was accompanied by fractionation of Al in water and using Al accumulation on fish gills as bioindicator. The concentration of reactive Al species in the alkaline water (pH 8) peaked at 2mgL(-1) in parts of the anoxic hypolimnion and was 0.088+/-0.053mgL(-1) (n=70) in the epilimnion during the five years of treatment. During an Al treatment cycle in summer 2003, perches showed significant Al accumulation on gills (100microg Al g(-1) dw) whereas roaches, breams and silver carps remained unaffected. Thus, the Al toxicity towards several fish species seems to be low, although the concentration of reactive Al in the lake water increased by a factor of 2. However, high Al toxicity due to lake treatment with aluminate could not be excluded, as high Al-gill concentration was observed. An Al balance two years after the treatment indicates complete export of the added Al into the sediment., (Copyright 2010 Elsevier B.V. All rights reserved.)