Historical landscape utilization and centennial scale changes in lake-water carbon: brownification beyond the perspective of monitoring

Carsten Meyer-Jacob1, Julie Tolu1, Christian Bigler1, Richard Bindler1
1 Department of Ecology and Environmental Science, Umeå University

O 7.5 in Controls of dissolved organic matter fluxes in ecosystems

14.07.2014, 15:15-15:35, H19

Monitoring data have shown rising organic carbon (OC) concentrations in surface waters across large parts of Europe and N America during the past decades, altering water quality, aquatic ecosystem functioning, and terrestrial carbon cycling. Several hypotheses have been proposed to explain this increase, from climate change to changes in anthropogenic forcing, e.g., reduced atmospheric sulphur deposition or alteration of land use practices. To date the key driver or drivers behind this process are still controversial. Further to this, due to the lack of long-term observational data over centuries to millennia it is ambiguous whether this observed increase in OC concentrations is a response only to recent environmental change or whether this represents a return to more natural pre-human-impact conditions.

By using a palaeolimnological approach we reconstructed past lake-water total organic carbon (TOC) concentrations in lakes across the boreal landscape of central Sweden in order to determine long-term TOC trends. Reconstructions are based on a transfer function between visible near-infrared (VNIR) spectra, which are sensitive to changes in the organic matter composition, of surface sediments and the corresponding TOC concentration in the lake-water. Potential driving factors behind changes in TOC concentrations were determined by a multi-proxy analysis of one of the studied lake sediment records extending 10,000 years back in time. Analyses include organic and inorganic geochemistry as well as biological proxies (pollen, diatoms).

Using the dystrophic lake Lång-Älgsjön to exemplify the long-term development, our results show a decrease in lake-water TOC starting gradually from c. 1,700 years ago and accelerating from c. 550 years ago, with an overall decline from ~20 mg L-1 during the mid-Holocene to ~8 mg L-1 in the beginning of the 20th century. This decline has been followed by rising concentrations since the mid-20th century. The decrease in TOC coincides with changes in several proxies indicating catchment disturbance by human activities, such as the occurrence of pollen associated with a more open landscape and cultural plants, increased amounts of charcoal particles, and alterations in the weathering/erosion of soils. The chronology of these changes corresponds to the expansion and decline of the widespread system of summer forest/upland grazing and farming in central Sweden during the 15th century and at the turn of the 20th century, respectively. Frequent grazing and exploitation of forests and mires reduce aboveground vegetation and physically disturb soils. This further affects the carbon cycling by enhancing carbon turnover, reducing the thickness of organic soils and consequently altering the transport of OC from the catchment into lakes.

Our findings suggest that changes in lake-water TOC in central boreal Sweden are strongly associated with historical patterns in land use rather than with variations in climate or sulphur deposition.

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last modified 2014-04-03