Compound specific stable isotopes and specific biomarkers to trace sediment origin and connectivity of sediment source areas to freshwater systems: case of the Baldegg Lake catchment (CH)
Slope destabilization and associated sediment transfer are one of the major causes of aquatic ecosystems and surface waters quality impairment. Through land uses and agricultural practices, human activities modify the soils erosive risk and the catchments sedimentary connectivity, becoming a key factor of sediment dynamics. Hence, restoration and management plans of water bodies can only be efficient if the sediment sources and their respective contributions, and thus the proportion attributable to different land uses and agricultural practices, are identified. Sediments can be traced, from their source to their deposition zone, using fingerprinting tools.
1. Compound Specific Isotope Analysis (CSIA)
Classic fingerprinting techniques (e.g. magnetism, elemental composition, sediment color, radionuclides, δ15Nbulk) are not adapted at providing information about the contribution of different land uses to erosion. A new technique, using the compound specific stable isotope (CSSI) signatures of inherent organic biomarkers in the soil (in this project: fatty acids, FAs), is developed since a few years (Gibbs, 2008; Blake et al., 2012; Hancock et Revill, 2013; Cooper et al., 2015; Alewell et al., 2016; Reiffarth et al., 2016). Although all plants produce the same FAs, the carbon stable isotopic signature (δ13C) of those biomarkers is different for each plant species. Because of their polar nature, FAs are easily leached from the plant or the decaying plant material and become tightly bound to soil particles. Coupled to a stable isotope mixing model, CSIA can thus help to discriminate and apportion the source soil contribution from different land-uses.
2. Highly specific biomarkers
To overcome the difficulty in distinguishing some land use types from their isotopic signature (e.g. pastures and some arable lands; Alewell et al., 2016), the CSIA approach can be strengthened by the use of highly specific biomarkers. Some triterpenes were indeed recently validated as family- or even species-specific (e.g. some triterpenyl acetates for Asteraceae; some sesqui-, di- and triterpenoids for conifers, methoxyserratenes for Pinaceae; pentacyclic triterpenes methyl ethers (PTMEs) for Gramineae, including Cerealia; Lavrieux et al., 2011; Otto and Wilde, 2001; Le Milbeau et al., 2013; Ohmoto et al., 1970; Jacob et al., 2008; respectively). Mostly recently developed for paleo-environmental studies, the strong potential of highly specific biomarkers as sediment fingerprinting tools remains under-exploited.
The molecular approach is supported by a modified sediment connectivity index (IC) based on the approach by Borselli et al. (2008) and Cavalli et al. (2013). Sediment connectivity is defined as the degree of linkage which controls sediment fluxes throughout landscape, and, in particular, between sediment sources and downstream areas and finally the freshwater system (Cavalli et al., 2013). The identification of connectivity patterns allows for an estimation of the contribution of a given part of the catchment as sediment source, and it defines sediment transfer paths.
4. Study site and sampling
The Baldegg Lake catchment (Canton Lucern, Switzerland) was chosen to apply this combined approach of geochemical fingerprints and sediment connectivity modelling. Almost 80% of the catchment area is used intensively for agriculture, while forests and urban areas cover less than 20%. The lake is eutrophic since the end of the 19th century, despite several attempts to restore the water quality (including an artificial oxygenation system since 1982).
Each land-use geochemical fingerprint is defined from the analysis of plants and soils. The dynamic of sediment input to the freshwater system is assessed on a short and long time scale, using (1) river suspended sediments sampled during high flow conditions and (2) a lake sediment core covering the last decades.