Combining novel (Py-GC-MS) emerging (FTIRS) and established (XRF; Pb isotope) methods to trace the transport and fate of organic matter and trace metals (As, Co, Cu, Ni, Pb and Zn) in a whole-lake basin

Julie Tolu1, Johan Rydberg2, Lorenz Gerber3, Carsten Meyer-Jacob1, Bindler Richard1
1 Department of Ecology and Environmental Sciences, Umeå University
2 Department of Geochemistry, Technical University of Braunschweig, 38106 Braunschweig, Germany
3 Umeå Plant Science Center, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden

O 8.6 in Trace element and metal biogeochemistry

14.07.2014, 15:35-15:55, H20

The governing assumption for the within-lake distribution of organic matter (OM) and trace metals is based on the generalized model of sediment focusing, which is mainly a function of slope, fetch and water depth. Sediment focusing describes the preferential transport of fine-grained material, such as OM and associated trace metals, to the deeper areas of a lake. Recent studies indicated that spatial variability in the total concentrations of OM and toxic metals is not well explained by sediment focusing alone1,2. Other factors such as complex basin morphology and prevailing wind direction can strongly influence the spatial distribution1-3.

In this study, we combine a novel method (based on pyrolysis-gas chromatography-mass spectrometry) for characterizing OM composition (extended set -~200- of biomarkers of plant type, algae productivity, bacterial activity) together with emerging (Fourier Transformed InfraRed Spectroscopy) and established (X-Ray Fluorescence Spectroscopy; Pb isotopes) methods. The aim is to better understand the transport processes and fate of trace metals in a whole-lake basin considering not only slope, water depth, basin morphology and prevailing wind direction, but also sediment geochemistry and organic composition. We analyzed surface sediments from 45 sites in Härsvatten, a small (0.18 km2) acidified lake consisting of 3 basins separated by shallow areas1.

Our results show that in the deeper basin (24 m; steep slopes), OM and trace metal distributions were well-explained by sediment focusing. However, in the two others basins (10 and 12 m), OM and trace metal distributions were more homogeneous and not related to water depth. This could be explained with help of the Py-GC-MS data, which indicates a high presence of aquatic macrophytes and more algal growth in the shallower areas of these two basins compared to the southern ones. Aquatic macrophytes act as sediment traps and result in accumulation of light, fine-grained, material rich in OM and trace metals (especially As, Ni and Zn) also at shallow locations. For the shallow areas between the three deep basins (<3 m) the accumulation of some trace metals (As, Cu, Co and Pb) was favored by the presence of Fe- and Mn-hydroxides. The isolated littoral shallow zones were characterized by OM from forest litter which seems to immobilize Zn and Ni.

This study gives a better understanding of trace metal fate in aquatic ecosystem and its relation to OM composition, which might have important implications when assessing metal transfer to aquatic foodwebs and calculating metal pollution mass balances.  

1Bindler et al. (2001) Limnol Oceanogr 46: 178–188; 2Rydberg et al. (2012) J Geophys Res G: Biogeosciences 117: 1-13 ; 3Abril et al. (2004) J Environ Radioactiv 72: 145-152

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