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Riparian zone and stream water DOC concentration and quality in relation to land use

Liisa Ukonmaanaho1, Martin Forsius2, Lauri Arvola3, Markus Hartman1, Mike Starr3
1 Finnish Forest Research Institute
2 Finnish Environment Institute
3 University of Helsinki

P 7.18 in Controls of dissolved organic matter fluxes in ecosystems

Poster Session 1 on Monday, 16:30-18:30

Increasing trends in dissolved organic carbon (DOC) concentrations in surface waters have been recently reported in northern Europe and North America. Many different processes have been proposed to explain increased DOC concentrations, e.g. recovery from acidification, increased forest production, climate warming and land use change. The loss of terrestrial organic carbon (OC) to rivers has been estimated to be about 10% of the net ecosystem production on land. OC transport is closely connected to land use, and is largely controlled by hydrological processes. In our study we were particularly interested to determine how concentrations and characteristics of DOC differ among different land use/riparian zones.

The study was carried during 2009-11 in the riparian zone of 5 land use sites: birch and spruce forest, drained and pristine peatland and cultivated field. The sites, except spruce forest, were located in the Löyttyoja catchment (8.0 km2), south Finland. The undisturbed old-growth spruce forest was located in the Valkea-Kotinen catchment (0.3 km2) 20 km north from Löyttyoja. Open area deposition (BD) was collected from Valkea-Kotinen and represents deposition at all studied sites. Throughfall (TF) was collected from the forested sites, soil water (SW) from mineral soil riparian zone, ground water (GW) from peat riparian zones, stream water, and spring water from the Löyttyoja catchment and lake water from Lake Pääjärvi, into which the stream flows. DOC concentrations were determined using a Shimadzu TOC-5000A analyzer. Absorbances at 465 nm (E4) and 665 nm (E6) were measured using Shimadzu UV Spectrophotometer and E4/E6 ratios (reflecting the proportion of fulvic to humic acids) calculated. In addition, concentrations of major nutrients were determined. We present the results for the samples taken during May-November in 2011.

DOC concentrations in the riparian zone and stream water varied greatly between the different land use sites. Highest concentrations were associated with the sites peatlands (drained and pristine peatland, spruce forest). Peatlands are a principal source of DOC in boreal fluvial environments. TF DOC concentrations were similar at both forest sites, while SW and GW DOC concentrations varied with distance from stream. Lowest average DOC concentrations were in BD and spring water, as expected. E4/E6 ratios varied between 1.3 and 20.8, the lowest in BD and highest in GW of the pristine peatland. There was a positive correlation (r=0.84) between DOC concentrations and E4/E6 ratios. E4/E6 ratios higher than 8 indicate the dominance of fulvic acids while ratios lower than 5 indicate the dominance of humic acids (greater degree of humification). As with DOC concentrations, the highest E4/E6 ratios (dominance of fulvic acids) were associated with peatland riparian waters. The lowest E4/E6 ratios were associated with BD, spring water, and mineral soil riparian zone and lake waters (indicating dominance of humic acids).

Letzte Änderung 03.04.2014