Plants or bacteria? 130 years of mixed imprints in Lake Baldegg sediments (Switzerland), as revealed by compound-specific isotope analysis (CSIA) and biomarker analysis
Authors: Lavrieux Marlène , Birkholz Axel , Meusburger Katrin , Wiesenberg Guido L. B. , Gilli Adrian , Stamm Christian , Alewell Christine 
Published: May 2019

Abstract. Soil erosion and associated sediment transfer are among the major causes of
aquatic ecosystem and surface water quality impairment. Through land use and
agricultural practices, human activities modify the soil erosive risk and the
catchment 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 the proportion attributable to different land uses are
identified. According to this aim, we applied two approaches, namely compound-specific
isotope analysis (CSIA) of long-chain fatty acids (FAs) and triterpenoid
biomarker analysis, to a eutrophic lake, Lake Baldegg, and its agriculturally used
catchment (Switzerland). Soils reflecting the five main land uses of the
catchment (arable lands, temporary and permanent grasslands, mixed forests,
orchards) were subjected to CSIA. The compound-specific stable isotope
δ13C signatures clearly discriminate between potential grasslands
(permanent and temporary) and forest sources. Signatures of agricultural land
and orchards fall in between. The soil signal was compared to the isotopic
signature of a lake sediment sequence covering ca. 130 years (before 1885 to
2009). The recent lake samples (1940 to 2009, with the exception of 1964 to
1972) fall into the soil isotopic signature polygon and indicate an important
contribution of the forests, which might be explained by (1) the location of
the forests on steep slopes, resulting in a higher connectivity of the
forests to the lake, and/or (2) potential direct inputs of trees and shrubs
growing along the rivers feeding the lake and around the lake. However, the
lake sediment samples older than 1940 lie outside the source soils' polygon,
as a result of FA contribution from a not yet identified source, most likely
produced by an in situ aquatic source, either algae, bacteria or
other microorganisms or an ex-site historic source from wetland soils and
plants (e.g. Sphagnum species). Despite the overprint of the yet
unknown source on the historic isotopic signal of the lake sediments,
land use and catchment history are clearly reflected in the CSIA results,
with isotopic shifts being synchronous with changes in the catchment, land use
and eutrophication history. The investigated highly specific biomarkers were
not detected in the lake sediment, even though they were present in the soils.
However, two trimethyltetrahydrochrysenes (TTHCs), natural diagenetic
products of pentacyclic triterpenoids, were found in the lake sediments.
Their origin is attributed to the in situ microbial degradation of
some of the triterpenoids. While the need to apportion sediment sources is
especially crucial in eutrophic systems, our study stresses the importance of
exercising caution with CSIA and triterpenoid biomarkers in such environments,
where the active metabolism of bacteria might mask the original terrestrial
isotopic signals.