Palaeolimnological evidence of forestry practices disturbing small lakes in Finland

The effects of forestry practices on lake water quality were traced in the sediments of six small lakes in Finland by studying the physical and chemical properties and the diatom composition of the sediments. Weighted averaging and a calibration data set of 89 lakes was used to reconstruct lake water pH, conductivity and concentration of total phosphorus on the basis of the sedimentary diatom assemblages. Palaeolimnological changes that can be connected with forest ditchings and fertilizations were detected, and a sudden eutrophication is shown in one case. The effects of clear cuttings seem to be dependent on the subsequent soil treatments (e.g. harrowing or screefing). Some changes in the diatom floras suggest an increase of humic matter in lake water. The links between forestry operations and Aulacoseira distans var. tenella (Nygaard) R. Ross as well as Asterionella ralfsii var. americana Körn. are discussed.