Holocene environmental changes as recorded in sediments from Kaggfjärden, southern archipelago of Stockholm, using diatom stratigraphy

Sammanfattning: Four sediment cores were sampled in Kaggfjärden, located in the southern archipelago of Stockholm. Two of the cores were analyzed with respect to the diatom record and lithology. Terrestrial macrofossils were dated using Carbon-14 dating. The aim of the study was to detect and date changes in the environment of Kaggfjärden by using the diatom stratigraphical record. These environmental changes were then interpreted in terms of climate change and/or anthropogenic impact. The results show that the lowermost part of the core was deposited approximately 6300 cal yr BP, thus from the Littorina Sea, a warmer and more marine phase in the Baltic Sea history. The diatom record in these layers is dominated by the typical Littorina species, Pseudosolenia calcar-avis (Schultzee). Both the diatom record and the lithology show that e.g. erosion or non-deposition caused a hiatus in the sediment core; several thousands of years are missing. The following levels were deposited during the 15th century and indicate an environment with greater visibility and less distribution of hypoxia than today. This coincides with the so called “Little Ice Age”. The uppermost part of the core consists of laminated sediments, and by counting the laminations it is clear that the area has suffered from hypoxia for about 110 years. This is interpreted as a result of the big changes in the agricultural landscape that occurred around that time. Further up in the core a change in the diatom assemblage from being dominated by benthic taxa, to planktonic, indicates more turbid water and poor visibility. This is probably a result of the opening of the nearby sewage treatment plant, Himmerfjärdsverket, in the mid 1970´s. The upper layers indicate no improvement in the water quality, rather a more turbid and overloaded situation. This study could be a part of the process of determining what “good environmental status” in the Baltic Sea means, and therefore be at help when monitoring future environmental work in the area.