A palaeolimnological study of the anthropogenic impact on dissolved organic carbon in South Swedish lakes

Author

Summary, in English

During the past three decades, increases have been observed in dissolved organic carbon (DOC) concentrations and colour in the surface waters of lakes and rivers in parts of Europe and North America, raising concern about the effects on the quality of aquatic environments with consequences for biodiversity, resource availability and recreational use. Various hypotheses have been put forward to explain the recent increases in DOC concentration and numerous studies have been published linking them to declining anthropogenic atmospherically deposited sulphur. Others have argued that increases in DOC content are a consequence of changes in climate, land use or land management practices.

The work presented in this thesis concentrates on identifying the major forcing mechanisms behind observed increases in DOC concentration in the upland area of southern Sweden during recent decades, by comparing variations in the total organic carbon (TOC) concentration in lake water inferred from lake sediments, in response to changes in land use, sulphur deposition and climate during the past eight centuries. Two small lakes with different catchment properties were selected for the study; one dominated by woodland with abundant peat deposits, and another located nearby with patches of agricultural land in an otherwise mainly forested terrain. A number of palaeolimnological methods were applied to the sediment sequences; decadal-scale variations in TOC concentration in the lakes were reconstructed based on visible-near infrared spectroscopy (VNIRS) of sediment successions, high-resolution (20-y) pollen-based reconstructions of local land use were quantified using the Landscape Reconstruction Algorithm (LRA) and the model Local Vegetation Estimates (LOVE), geochemical records provided further information on environmental changes in the lakes and their catchment areas, and changes in pH in the lakes were inferred from diatom analysis. Comparisons were made with population density data and climate records.

The results obtained with the LRA and LOVE models revealed a dynamic land-use pattern, with agricultural expansion from AD 1500 to the end of the 1800s, when population growth and the related increase in the exploitation of the surrounding land had a major impact on catchment erosion and input of terrestrial inorganic and organic matter to the lakes. Evidence also exists of a period of agricultural expansion around AD 1200-1300, followed by partial abandonment of the landscape, which can probably be attributed to the Black Death pandemic. A transition from traditional to modern land use following the industrial revolution took place during the past century, and a concurrent shift in most of the proxy records at around AD 1900 suggests a marked change in external forcing mechanisms common to both lakes, related to a major decrease in population density and the introduction of modern land use. The results revealed generally high TOC concentrations in the lakes prior to AD 1900, with second-order variations associated mainly with changes in the intensity of agricultural land use. The TOC concentrations in the lakes started to decrease around AD 1900, and unusually low TOC concentrations were recorded in the period AD 1930-1990, followed by a recent increase. The variation in sulphur emissions, with an increase in the early 1900s to a peak around AD 1980 followed by a significant decrease, was probably the main driver of lake-water TOC dynamics during the past century. This demonstrates that declining atmospheric sulphur deposition is the most probable driver of the increase in TOC concentration during the past three decades and that these lakes may be recovering to their naturally high-TOC pre-depositional states. The results also demonstrate regional versus local forcing of environmental change and indicate broadly similar regional sensitivities to anthropogenic impact, although responses were site-specific due to the different properties of the catchment areas. Given the reduction in atmospheric sulphur emission during recent decades, it is likely that previously suppressed or masked effects of changes in land use, land management and climate during the past century will become progressively more important drivers of TOC concentrations in lake water in the future. Long-term records of environmental history on decadal to millennial time scales enabled the assessment of ecosystem variability and responses to past anthropogenic disturbance, and may be a useful tool for the development of future environmental management strategies.

Topic

  • Geology
  • Earth and Related Environmental Sciences

Keywords

  • DOC
  • Brownification
  • Landscape Reconstruction Algorithm
  • Land-use changes
  • Lake sediments
  • Anthropogenic impact

Status

Published

ISBN/ISSN/Other

  • ISSN: 0281-3033
  • ISSN: 0281-3033
  • ISBN: 978-91-86746-93-3

Defence date

1 November 2013

Defence time

13:15

Defence place

Geocentrum IIs föreläsningssal Pangea, Sölvegatan 12, Lund