Diatom analyses of sediment from Himmerfjärden estuary, southern archipelago of Stockholm : has the water discharge from a constructed sewage treatment plant led to eutrophication?

A sediment core from Himmerfjärden estuary, south of Stockholm, was examined to detect records of eutrophication on the site since the opening of the sewage treatment plant Himmerfjärdsverket in 1974. The core was analysed with respect to the diatom record and lithology. Four macrofossil that were found in the sediment were dated using 14C-dating. This study aims to detect changes in the environment of Himmerfjärden by using the diatom stratigraphy record. The results have been interpreted and discussed regarding natural environmental and climate change and/or anthropogenic impact, and detected changes will be associated with the history of the sampling site. The results show that the lowermost zone started to deposit around 1300-1490 cal yr BP and the homogeneous sediment indicates that the area was not suffering from hypoxia at that time. There is a successive transition towards more distinct lamination further up in the core which show that the environment in Himmerfjärden have changed and become hypoxic. This may have to do with factors such as the opening of heavily trafficked Södertälje Canal, and also the increased nutrient input from Himmerfjärdsverket. This study could be a part of the process of working towards a “good environmental status” in the Baltic Sea. However, continued and improved work is needed for further and more accurate interpretations