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. Soil erosion and associated sediment transfer are among the major causes ofaquatic ecosystem and surface water quality impairment. Through land use andagricultural practices, human activities modify the soil erosive risk and thecatchment connectivity, becoming a key factor of sediment dynamics. Hence,restoration and management plans of water bodies can only be efficient if thesediment sources and the proportion attributable to different land uses areidentified. According to this aim, we applied two approaches, namely compound-specificisotope analysis (CSIA) of long-chain fatty acids (FAs) and triterpenoidbiomarker analysis, to a eutrophic lake, Lake Baldegg, and its agriculturally usedcatchment (Switzerland). Soils reflecting the five main land uses of thecatchment (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 landand orchards fall in between. The soil signal was compared to the isotopicsignature of a lake sediment sequence covering ca. 130 years (before 1885 to2009). The recent lake samples (1940 to 2009, with the exception of 1964 to1972) fall into the soil isotopic signature polygon and indicate an importantcontribution of the forests, which might be explained by (1) the location ofthe forests on steep slopes, resulting in a higher connectivity of theforests to the lake, and/or (2) potential direct inputs of trees and shrubsgrowing along the rivers feeding the lake and around the lake. However, thelake sediment samples older than 1940 lie outside the source soils' polygon,as a result of FA contribution from a not yet identified source, most likelyproduced by an in situ aquatic source, either algae, bacteria orother microorganisms or an ex-site historic source from wetland soils andplants (e.g. Sphagnum species). Despite the overprint of the yetunknown 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 useand eutrophication history. The investigated highly specific biomarkers werenot detected in the lake sediment, even though they were present in the soils.However, two trimethyltetrahydrochrysenes (TTHCs), natural diageneticproducts of pentacyclic triterpenoids, were found in the lake sediments.Their origin is attributed to the in situ microbial degradation ofsome of the triterpenoids. While the need to apportion sediment sources isespecially crucial in eutrophic systems, our study stresses the importance ofexercising caution with CSIA and triterpenoid biomarkers in such environments,where the active metabolism of bacteria might mask the original terrestrialisotopic signals.

biogeosciences

Plants or bacteria? 130 years of mixed imprints in Lake Baldegg sediments (Switzerland), as revealed by compound-specific isotope analysis (CSIA) and biomarker analysis