Micro‐stratigraphy of sediment laminations deposited in a chemically stratifying eutrophic lake during the years 1913–1976

Abstract

A sequence of laminated sediment, 30 cm long, deposited in the deepest part of an eutrophic lake, Lovojärvi, in southern Finland, was analyzed microscopically. Seasonal changes in sedimentation were found to be well retained, due to the absence of bottom animals and the rather high sedimentation rate. Delimitation of the annual layers was based primarily on the repeated cyclic pattern observed in the stratigraphy of the planktonic diatoms. The annual character of these diatom cycles was established through their agreement with information obtained during three earlier one‐year studies on the plaktonic diatom community. In the lower part of the sequence studied the laminated structure appeared quite regular, the annual layers being composed of pairs of a whitish, diatom‐rich “summer” layer, and a dark brown, more organic “cold season” layer, but in the upper part the laminations were more variable, due to allochthonous inputs caused by human activity in the drainage area. Several thick clay layers which could be dated back to known constructional events on the lake shore aided the interpretation of this sequence, within which prominent alterations in the planktonic diatom flora were also observed. The gradual increase in the proportions of acidophilous diatoms during the past decades was interpreted as indicating a slight lowering in the trophic status of the lake, primarily due to the termination of flax soaking, which had been going on by the lake shore for centuries.