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Environmental changes of the last three centuries indicated by siliceous microfossil records from the southwestern Baltic Sea

Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden

Graham Shimmield

Dunstaffnage Marine Laboratory, Oban, Argyll PA37 4AD, UK

Timothy Brand

Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK

Four short sediment cores from the southwestern Baltic Sea were analysed with respect to their content of siliceous microfossils. The aim was to detect and date changes in the composition of the diatom flora and to link these changes to variations in the anthropogenic load of nutrients during the last century. The study shows that the most significant change in the diatom assemblages occurred 130-140 years ago. The change is recorded in the sediments as a shift from periphytic diatom taxa to a predominance of planktonic taxa. This indicates that the photic layer has decreased in depth, probably due to eutrophication of the Baltic Sea, which consequently began to affect the area investigated approximately AD 1850. To support the results of a changing ratio of periphytic to planktonic diatoms, diatom accumulation rates were calculated. In general, the diatom accumulation rate data show a decrease in the periphytic accumulation rates and an increase in the planktonic accumulation rates towards the sediment surface. Some indications of a colder climate are recorded in the sediments approximately 230 years ago. These results are in accordance with the record of maximum extent of sea ice in the same area and are suggested to represent a late stage in the ‘Little Ice Age’.

Key Words: Baltic Sea • diatoms • 210Pb • siliceous microfossils • eutrophication • climate change • human impact

The Holocene, Vol. 9, No. 1, 25-38 (1999)
DOI: 10.1191/095968399676523977


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