As one of the largest brackish water bodies in the world, the Baltic Sea is surrounded by a coastal population of over 85 million and has a drainage area four times larger than the basin itself. The sea is highly vulnerable to hypoxia due to the circulation patterns of the basin, which limits the inflow of oxygenated bottom-waters from the Kattegat Sea. Periods of hypoxia have characterized the basin for the last 8000 years and it has been argued that hypoxia is a natural feature of the Baltic Sea system controlled by climate. Others have, however, argued that periods of hypoxia can be linked to increased anthropogenic impacts, such as deforestation, population increase, and increased land-use. In this study, we examine if the degree of anthropogenic impact in a terrestrial coastal setting during the last 3000 years can be correlated to increased and decreased periods of hypoxia in the Baltic Sea. In 2011 and 2013, sediment cores were retrieved from Lake Storsjön and Lake Färskesjön in southern Sweden. Subsamples of 1 cm3 were collected from the cores at an interval of 1 cm for pollen analysis, charcoal analysis, loss on ignition, and landscape reconstruction modeling (REVEALS).
The degree of anthropogenic landscape openness was higher for Lake Färskesjön than for Lake Storsjön, reaching 60% and 40%, respectively. Degree of impact was also shorter for Lake Storsjön. At Lake Färskesjön, an expansion of pastures occurred between 2300–1500 cal. years BP and 1500–1000 cal. years BP was characterized by reduced farming. Gradual expansion of pasture and cropland occurred between 1000–350 cal. years BP, only disrupted by a short-lasting reduction in cropland by half around 700 cal. years BP. An expansion of heathland followed the reduction between 700–600 cal. years BP. The open land area increased to 60% after 350 cal. years BP, followed by a doubling of cropland and pasture from 200–100 cal. years BP. After 100 cal. years BP, cropland decreased by one-third and woodland expanded to cover half of the landscape. The landscape reconstruction model quantifies the land-use changes, making a comparison to changes in the marine coastal ecosystem possible. Analyzed marine sediments indicates that the last 1000 years of land-use changes in the area had an effect on diatom assemblages, likewise alterations in wetland-area over the last 200 years had an effect on the coastal ecosystem in the Baltic Sea.