EUTROPHICATION OF MONDSEE (UPPER AUSTRIA) AS INDICATED BY THE DIATOM STRATIGRAPHY OF A SEDIMENT CORE

Abstract

The population dynamics of dominant planktonic diatoms during the eutrophication of Mondsee, Austria, are described by analysis of a 36 cm in situ frozen sediment core from the profundal region using tape peel preparation techniques; the results are compared with data from plankton investigations. The development of Melosira italica ssp. subarctica and Stephanodiscus neoastraea at the expense of St.alpinus and Cyclotella bodanica indicates the transitional state from oligo- to mesotrophy in the 1930's. The rising level of trophy is indicated by a sudden appearance of Fragilaria crotonensis (flared type) during the 1950s. Allochthonous load caused by road constructions and flood lead to low diatom values and a high sedimentation rate (1958—68 10 mm.yr-1) in the late 1950s and early 60s. The eutrophic state of Mondsee, beginning in 1968, is characterized by the onset of a marked light/dark lamination which corresponds to the mass expansion of Tabellaria flocculosa var. asteronelloides. The annual character of the laminations can be verified in part by the seasonal cycles of the diatoms. As a rule light-coloured layers rich in biogenic calcite correspond to diatom blooms of the lake in spring until the onset of stratification; brownish layers to summer/autumn; and the dark ones to winter. The average sedimentation rate of the section 1968–85 is 6.6 mm.yr-1. It can be subdivided by oscillations in abundance between Fragilaria crotonensis (rod type), Tabellaria flocculosa var. asterionelloides, Stephanodiscus parvus, Cyclotella comta, on the one hand, indicating nutrient enrichment especially during the years 1970–72 and 76–78; and Melosira italica ssp. subarctica and Stephanodiscus neoastraea on the other hand. After a decline in diatom abundance (1979/80) in 1981, about 7 years after the installation of a sewage treatment plant, the onset of the re-oligotrophication is reflected by the diatom population structure: Melosira islandica replaces M.italica ssp. subarctica; Stephanodiscus minutulus shows rising percentages at the expense of St. neoastraea, while Tabellaria becomes less important.