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

Four short sediment cores from the southwestern Baltic Sea were analysed wi th respect to their content of siliceous microfossils. The aim was to detec t 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 la st century. The study shows that the most significant change in the diatom assemblages occurred 130-140 years ago. The change is recorded in the sedim ents as a shift from periphytic diatom taxa to a predominance of planktonic taxa. This indicates that the photic layer has decreased in depth, probabl y due to eutrophication of the Baltic Sea, which consequently began to affe ct the area investigated approximately AD 1850. To support the results of a changing ratio of periphytic to planktonic diatoms, diatom accumulation ra tes were calculated In general, the diatom accumulation rate data show a de crease in the periphytic accumulation rates and an increase in the plankton ic accumulation rates towards the sediment surface. Some indications of a c older climate are recorded in the sediments approximately 230 years ago. Th ese 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'.