Implications of land use on the carbon cycle - Impacts of long-term human activities on terrestrial organic matter input to aquatic ecosystems in southern Sweden

In this thesis, we carried out multi-proxy analyses to four sediment sequences from different environmental settings in southern Sweden to explore the variability in organic matter transport between terrestrial and aquatic environments in response to long-term land-use changes. The pollen-based Landscape Reconstruction Algorithm was applied to quantitatively reconstruct the catchment-scale landscape dynamics. Lignin phenols (biomarker) were used to trace the terrestrial organic matter preserved in the sediments. Bulk geochemistry including TOC, C/N ratio and BSi was applied for a general assessment of the proportions of terrestrial and aquatic organic matter deposition and estimation of aquatic production. Ti within the XRF data set was used to assess soil erosion.
The study on two sediment sequences from a small forest lake (Lake Skottenesjön) in southwestern Sweden shows that the terrestrial organic matter delivery is sensitive to local land-use variations in the catchment in the past 1000 years. Elevated soil erosion and increased terrestrial organic matter deposition were recorded during intensive wood harvest in the 18th and 19th centuries. No significant change in terrestrial organic matter delivery was observed during the farmland expansion between the 12th and mid-14th century. Export of terrestrial organic and minerogenic matter to the lake was much higher during the period of modern forestry in the 20th century than the period of minor forest disturbance in the 11th century.
A similar multi-proxy study was conducted on the sediment sequences from a large lake (Lake Storsjön) and a fjord-like inlet of Baltic Sea (Gåsfjärden) on
the east coast of Sweden. The two sites are within the same catchment and connected by a river system. The results show that the composition of lignin-derived organic matter deposited in Gåsfjärden is less sensitive to the variation of vegetation cover than in Storsjön, which is likely due to the sortation and
alteration on the terrestrial organic matter during the transportation from the inland to the coast. The concentration of lignin is much lower in Gåsfjärden than Storsjön as the organic matter deposition is dominated by aquatic material in Gåsfjärden. Furthermore, Gåsfjärden receives less degraded terrestrial organic matter, likely because the organic matter liable to degradation has been lost during transportation to the sea.
This study highlights the potential of the combined use of lignin phenols and pollen-based quantitative land-cover reconstructions for investigating long-term changes in terrestrial organic matter input to aquatic ecosystems. The findings of the study provide a better understanding of human impacts on organic carbon cycling from a long-term perspective, which is fundamental for the development of environmental management strategies.