This study examines changes in lake-water total organic carbon (LW-TOC) and lake sediment geochemistry in two lakes, Stångtjärnen and Holtjärnen in (Dalarna, Sweden), during the Holocene and the role of the historic forest grazing and farming (fäbod-system). The aims of the study were to: 1. Discern the effects of natural processes on the lake’s biogeochemistry in different position in the landscape. 2. Identify the effects and differences in intensity of historic land-use on the lakes. A multi-proxy study was conducted encompassing multi-element (15) geochemistry, biogenic silica, LW-TOC, chlorophyll a and published pollen records. The first lake, Stångtjärnen, is shaped and influenced by surrounding mires, which developed shortly after deglaciation and stabilized the LW-TOC at 19 mg L-1 throughout most of the Holocene, while Holtjärnen, a small upland lake, changed from a productive lake (BSi: 35 %), low humic (LW-TOC: 8 mg L-1) to a less productive (BSi: 4 %) more humic lake (LW-TOC: 12 mg L-1) in 7300 BP. The intensification of agricultural land-use (e.g. hay-making) in Stångtjärnen reduced the concentrations of organic associated elements (Br, Cl) and LW-TOC and increased lithogenic elements (K, Ti), while Holtjärnen showed less anthropogenic influence. The comparison between the two lakes displayed the intensive influence of land-use on the Stångtjärnen catchment, showcased by changes in the sediment geochemistry, vegetation composition and the extent of the forest-grazing system in a landscape perspective. In response to the changes of the Holocene, Stångtjärnen’s mires became the main influence, while Holtjärnen was more sensitive to changes.

Long-term dynamics of lake-water total organic carbon (LW-TOC) concentrations in freshwater lakes provide an important perspective on the recent increases in LW-TOC observed in many of these systems and may assist with the identification of natural and anthropogenic drivers of change. This study examines how LW-TOC in Dragsjön, a lake situated in an area with a long history of anthropogenic land use, has changed in response to natural and anthropogenic perturbations throughout the Holocene. To provide a better understanding of the processes involved, a multi-proxy study was conducted and included multi-element geochemistry (17 major and trace elements), biogenic silica, organic matter (OM) content and composition, and the faecal biomarker “coprostanol”. The direct biomarker for anthropogenic presence, “coprostanol”, and a detailed characterisation of OM composition are for the first time applied for tracing changes in LW-TOC. Natural processes contributed to stable LW-TOC concentrations in Dragsjön for most part of the Holocene. Humans were present in the catchment from AD 100 as indicated by coprostanol, but did not begin to affect LW-TOC until c. AD 1500. In the last 500 years LW-TOC steadily declined from 17 to 10 mg L-1 in response to anthropogenic alterations to the terrestrial biomass balance. The increase in LW-TOC during the last 70 years likely represents a recovery from anthropogenic disturbance rather than a baseline shift in response to any of the number of proposed recent stressors. The faecal biomarker coprostanol and OM composition provided information essential for identifying and characterising the effects of anthropogenic disturbance.

Varved lake sediments were used to provide information on how a landscape is affected by disturbances of different scales, from global (i.e. climatic) to local (i.e. fires), as well as anthropogenic activities. Geochemical and pollen data, biogenic silica (bSi), lake-water total organic carbon (LWTOC) and chlorophyll a were used as proxies to infer past changes in lake-conditions. The goal was to evaluate the response to scale different disturbances and how it differs among sites. By comparing different lake records, it became possible to isolate the climatic signal from the effect of soil development and vegetation establishment, and differences emerging from different catchment characteristics. Climatic trends were reconstructed based on the pollen and geochemical data. The sediment records were then compared to identify the effect of each disturbance on individual lakes. One of the most prominent event observed was the immigration of spruce at about 3000 BP which considerably affected sedimentation trends. The presence of spruce within the catchment appears to promote the input of fine-grained material to the lakes. The timing and intensification of anthropogenic activities was established and it was possible to differentiate the effects of human disturbance from changes caused by natural processes such as climate or landscape vegetation cover changes. The results show that farming practices started earlier in more southern locations and that this timing is site-dependent. Two phases were identified, corresponding to the start of slash and burn farming and later to the expansion of agricultural practices, with a more profound transformation of the landscape.

Due to the current increase in lake water total organic carbon (LW-TOC), many studies have investigated the long-term dynamics using lake sediments. However, these have mainly been focused on Sweden’s south and south-central parts. Thus, this study aimed to investigate how LW-TOC concentrations in northern freshwater systems have changed over the recent few centuries and what effect potential trends or patterns may have on mercury (Hg). A multi-proxy study was conducted that included geochemical characteristics, inferred LW-TOC, chlorophyll a, and land-use observations acting as a supplement to modern land-use changes. The results show that LW-TOC concentrations display tendencies to a decreasing trend in the deepest part of the sediment, which could be related to traditional land-use activities. This is followed by a more rapid decrease reaching equally low LW-TOC concentrations as in southern and south-central Sweden, here suggested being caused by catchment disturbances superimposed by acid deposition. During the recent decades, LW-TOC displayed increasing concentrations, possibly due to the combined effect of the reduction of the abovementioned factors, but also likely to be superimposed by an increase in temperature similar to what has been shown for Canadian lakes. These results align broadly with lakes in southern and south-central Sweden, although with potentially different forcings in the earliest part of the sediment. This contemporary increase in LW-TOC is accompanied by an increase in Hg in two lakes, with both LW-TOC and Chl-a show possible relations.