Recent history of sedimentation and biotic communities in Lake Pyhäjärvi,SW Finland |
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| Authors: | M Räsänen V-P Salonen J Salo M Walls J Sarvala |
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| Institution: | (1) Department of Quaternary Geology, University of Turku, SF-20500 Turku, Finland;(2) Geological Survey of Finland, SF-02150 Espoo, Finland;(3) Laboratory of Ecological Zoology, Department of Biology, University of Turku, SF-20500 Turku, Finland |
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| Abstract: | The sedimentary record covering the last 150 years was studied in the productive clear water lake Pyhäjärvi in SW Finland. The lake has faced significant human-induced changes: (1) the water level was lowered by almost 2 metres in the early 1850s; (2) planktivorous coregonid fishes were successively introduced, commencing in 1908; and (3) nutrient input from intensified agriculture has increased during this century.Sediments were sampled from the 25 m deep depression of the otherwise shallow lake by freeze-corer and were date by 210Pb-chronology and pollen stratigraphy. According to litho-, chemo-, 210Pb- and pollen stratigraphies, the sedimentary sequence consists of five different sedimentary facies, each representing a different depositional environment resulting both from the lowering of water level and different stages of final deposition. The sediments in the depression are believed to have been deposited orderly, but, as a result of resuspension, they have a substantial littoral sediment input.After the lowering of the lake level, oxygen content in the depression is believed to have decreased on the basis of black coloration (sulfides) of the sediment from 1870–1880 onwards. The oxygen deficit worsened after the 1940s when e.g. iron, zinc, calcium and phosphorus were increasingly liberated into the water body. In contrast, diatoms, chironomids and cladoceran communities were notably stable, with the most important biotic changes being: 1) the decreased body size of the cladoceran Bosmina coregoni, apparently due to intensive selective predation by the introduced whitefish; and 2) increased abundances of the diatom Fragilaria crotonensis, and the profundal chironomid Chironomus plumosus f. semirectus after the 1950s, suggesting an increase in the trophic status of the lake. Eutrophication was probably in response to increased nutrient supply due to intensified cultivation and use of industrial fertilizers in the lake's drainage. |
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| Keywords: | paleoecology lake sediments stratigraphy taphonomy human activity Holocene Finland |
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